# Fritz H. Schweingruber Annett Börner

# The Plant Stem

A Microscopic Aspect

The Plant Stem

Fritz H. Schweingruber • Annett Börner

The Plant Stem A Microscopic Aspect

Fritz H. Schweingruber Institute for Forest, Snow and Landscape Research – WSL Birmensdorf, Switzerland

Annett Börner Melrose Park South Australia, Australia

ISBN 978-3-319-73523-8 ISBN 978 -3-319-73524-5 (eBook) https://doi.org/10.1007/978-3-319-73524-5

Library of Congress Control Number: 2018944431

© The Editor(s) (if applicable) and The Author(s) 2018. This book is an open access publication.

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# Table of contents





# Abbreviations


# 1. Introduction

Terrestrial life forms made their move on land about 400 million years ago. Plants crossed the barrier between life in water to life in the atmosphere. With the invention of stable stems, plants overcame hydrological and mechanical problems. The construction of plant stems is the focus of this book. It demonstrates that nature created a framework in which plant stems evolved—annual herbs as well as century-old, 100 m-tall trees, from tropical to arctic environments.

The book offers a very wide view of stem anatomy. Chapter 2 explains simple anatomical preparation techniques. The six following chapters present basic, cell-based anatomical traits. Two chapters deal with taxonomically related anatomical stem characteristics in living and fossil plants. Anatomical structures ^OPJO HYL YLSH[LK[V ZOVY[ HUKSVUN[LYT L\_[LYUHSPUÅ\LUJLZ all over the globe are intensively discussed. The general part of the book ends with a section about wood decay and wood conservation.

One major objective of this book is to show that nature principally does not distinguish between plant stems of different growth forms, e.g. between small herbs and very tall trees. The following two microscopic cross-sections demonstrate that the basic stem construction of vascular plants, such as ferns, monocotyledons and dicotyledons, consists of the pith and the cortex, the xylem and phloem, and often a periderm.

Why a new book about plant anatomy? Isn't it repeating knowledge already previously demonstrated by great botanists? That is partially correct. However, this book has different goals.


#### *Principal stem construction of most dicotyledonous vascular plants*

Sections stained with Astrablue/Safranin. Red-stained cell walls indicate an intensive SPNUPÄJH[PVUIS\LZ[HPULKJLSS^HSSZHW\YLS`JLSS\SVZPJJVTWVZP[PVU

**1.1** Main root of the annual, 5 cm-tall, dicotyledonous alpine herb *Polygonum plebeium.*

**1.2** Fifteen-year-old twig of the 10 m-tall subalpine coniferous tree *Pinus mugo.*

an anatomical training. Each concept in this book is introduced by presenting well-known objects in macroscopic images before explaining their microscopic structures, e.g. an orange, followed by microscopic details of oil ducts, or an *Arabidopsis* plant followed by its anatomical stem structure. Basic anatomical knowledge is presented so it can be understood by readers with different academic training.

Secondly, this book addresses a worldwide multilingual auditorium—even when the knowledge of the English language might be limited. International dendroanatomical training classes have shown that pictures overcome many language problems. This book is therefore extensively illustrated, and introductory texts are kept short. Photographs are presented where possible instead of abstract drawings, and the images are captioned and labeled in an easily understandable manner.

The book builds a bridge between basic and detailed anatomical and physiological studies. Most concepts are of common knowledge, and can already be found in many botanical textbooks, e.g. Beck 2010, Bresinsky *et al.* (Strasburger) 2008, Carlquist 2001, Crivellaro & Schweingruber 2015, Cutler *et al.* 2008, Eschrich 1995, Evert 2006, Fahn 1990, Fink 1999, Herendeen *et al.* 1999, Mauseth 1988, Nabors 2004, Schweingruber 2007 and Taylor *et al.* 2009. However, the here presented color photographs of stained microscopic slides enhance pre- ]PV\Z RUV^SLKNL HIV\[SPNUPÄJH[PVU HUKPU JVUZLX\LUJL[OL relations between anatomy, physiology and plant stability. All microscopic slides have been made recently with modern sledge microtomes, and have been analyzed with light microscopes.

:PUJLTVZ[VM[OLZJPLU[PÄJJVU[LU[PZVMJVTTVURUV^SLKNL citations of sources occur only sparsely in the text. The reader JHUÄUK H Z\TTHY`VM ZV\YJLZ HUK YLJVTTLUKLK YLHKPUN H[ the end of the book.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 2. Preparation techniques – Making anatomical structures visible

As the title of this book suggests, the main objective of the book is to explain anatomical structures on the basis of microscopic slides. Since comparability is one of its major goals, most micro-photographs are based on recently prepared microscopic slides. Gärtner & Schweingruber 2013 explain sample design and preparation techniques in detail.

Taxonomically and ecologically labeled fresh material is col-SLJ[LKPU[OL ÄLSK HUK Z[VYLKPU  L[OHUVS ILMVYL ZLJ[PVUing. The heart of the laboratory work is the newly developed sledge microtome (Gärtner *et al*HTVKPÄLKJVW`VM[OL Reichert sledge microtome. New are its low weight, two very Z[HISLRUPMLN\PKLZHUK[OLISHKLOVSKLY^OPJOPZTVKPÄLKMVY disposable blades. These innovations made it possible to section the majority of stems without prior embedding.

Almost all sections are stained for a few minutes with a one to one mixture of Astrablue/Safranin. Staining and dehydration ^P[O L[OHUVSHIZVS\[LL[OHUVSHUK\_`SLULVJJ\YZKPYLJ[S` on the glass slide. Permanent slides, lasting for more than 100 years, are embedded in Canada balsam. Photographs were made under transmitting, normal and polarized light with an Olympus BX51 microscope. Slides are conventionally stored in preparation boxes. A digital databank allows queries by any taxonomical, morphological, geographical or anatomical characteristic of each slide.

**2.1** 7SHZ[PJ IHN JVU[HPUPUN  L[OHUVS ^P[O H sample of a herb stem. The label contains information about the taxon and its morphology, ecology and the sampling date.

Switzerland.

**2.3** Holder with chemicals for preparation of permanent slides. It contains dyes, ethanol and xylene and corresponding pipettes.

**2.4** Principle of staining and dehydrating the cut sample on the glass slide. **2.5** Flattening of microscopic sections with magnets on an iron plate.

**2.6** Section of *Vitis vinifera*, stained with Astrablue/Safranin, photographed under normal and polarized light.

**2.7** Old-fashioned, but safe and handy storage of labeled microscopic slides in prefabricated cardboard boxes.

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# 3. Morphology of the plant body – Tried and tested for 400 million years

;OLÄYZ[WSHU[ZVUSHUKHWWLHYLKZVTLTPSSPVU`LHYZHNVPU the geological periods of the late Silurian and early Devonian. The morphological structure of these spore plants is similar: The tiny leaf-less plants consist of roots, stems and branches. These plants represent the initial point for the evolution of a vast taxonomic and morphological diversity over the next 250 million years. At the beginning of the Early Cretaceous, approximately 140 million years ago, seed plants developed a variety of growth forms, and occupied most terrestrial habitats on earth. The image below demonstrates that many different growth forms exist in ecologically similar ecotones.

*Various growth forms in an ecotone*

**3.1** Riparian zone of the Danube in Slovakia.

Life forms and growth forms are principally synonymous terms. The image on the following page shows that the currently existing plant forms are based on a principle that was invented 400 million years ago. It also demonstrates that plant age does not JVYYLZWVUK ^P[O [OL TVYWOVSVNPJHS JSHZZPÄJH[PVU" WLYLUUPHS and annual plants can have the same size.

A few annual (blue) and perennial (red) growth forms in Fig. 3.2 on the following page relate to branching and plant height:


# 3.1 Growth forms and life forms



# ͛Ǥ͚Ƥ

;OLWYPUJPWHSZ[Y\J[\YLVMZ[LTZPZKLÄULKPU[OPZZLJ[PVUNLUerally following Junikka 1994 and Crivellaro & Schweingruber 2015.

*Bark* is a general term, which includes all tissues outside of the wood (*Borke* in German).

*Inner bark*JVSSVX\PHSVYWOSVLTZJPLU[PÄJ[LYTPUJS\KLZ the living part of the bark.

*Outer bark* JVSSVX\PHS VY YO`[PKVTL ZJPLU[PÄJ [LYT includes all dead parts outside the phellogen.


*Cambial zone* contains the initial meristem, the xylem and phloem mother cells.

*Cork* is the colloquial term for phellem.

*Cortex* is a product of the primary meristem. It is located between the epidermis and the phloem. Rays are absent in the cortex.

*Heartwood* is the dead, non-conducting part of the xylem. *Periderm* consists of phellogen, phelloderm and phellem.

*Pith* is the central parenchymatic part of shoots.


*Sapwood* is the living and water-conducting part of the xylem.


#### *Macroscopic aspect of old bark*

outer bark

periderm

**3.3** Old bark with dilatation in *Betula alba*. dilatation

**3.4** Old bark with dilatation in *Juglans regia*.

rhytidome xylem cortex without rays phloem without rays 100 μm

**3.5** Young bark of *Juniperus communis*.

*Microscopic aspect of young bark*

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The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 4. Cellular composition of the plant body

Single cells can only be observed through microscopes. The cell is anatomically and physiologically complex. It principally consists of protoplasts, which contain various organelles, the vacuole and the cell wall. The following section introduces elements that are visible under normal and polarized light without special microscopic equipment.

# 4.1 The individual cell

Shown here are different cell types, cell walls, nuclei and plastids and ergastic substances (non-protoplasm material such as crystals, resins, tannins etc.).

;OL MVSSV^PUN ÄN\YL ZJOLTH[PJHSS` ZOV^Z HSS JVTWVULU[Z Nuclei, plastids, vacuoles and cell walls are recognizable by light microscopy.

*Plastids*

**4.1** 5\JSLPPUTLYPZ[LTH[PJJLSSZ^P[O\USPNUPÄLKJLSS^HSSZPU*Viscum album*. **4.2** 5\JSLPPUHK\S[JLSSZPUH\_`SLTYH`^P[OSPNUPÄLK[OPJR^HSSZPU*Picea abies*.

**4.3** :[HYJONYHPUZPUSHYNL\USPNUPÄLK cells of *Solanum tuberosum*, polarized light.

**4.4** Chloroplasts in peripheral cells of a needle of *Pinus nigra.*

**4.5** Tannins in heartwood cells of the dwarf shrub *Globularia cordifolia*.

*Ergastic substances*

**4.6** Calcium oxalate crystals in enlarged parenchyma cells of the herb *Gypsophila repens*, polarized light.

# 4.2 Meristematic initials – The source of new cells

Meristematic cells are anatomically undifferentiated and capable of dividing. Meristematic initials are living cells and have L\_JS\ZP]LS` \USPNUPÄLK [OPU WYPTHY` ^HSSZ Primary meristematic initials are arranged in the tip of longitudinal shoots and roots (vegetation point). Secondary meristematic initials are arranged around the shoot and are the initial part of the cambium. Tertiary meristematic initials are a product of parenchyma cells in the bark. They represent the cork cambium (phellogen).

Primary meristems produce the cortex and the pith in stems. Secondary meristems (cambium) are a product of primary meristems and occur around stems where they produce the xylem and phloem. Tertiary meristems are a product of living parenchyma cells in the cortex, the phloem, and rarely in the xylem, where they produce mainly cork cells. Meristematic cells have no pits.

#### *Primary meristems in shoots & roots Embryonic cells in shoot tips, primary meristem*

**4.7** Primary meristems in a sapling of *Carpinus betulus*.

**4.8** Location of a primary meristem in a shoot tip *Acer pseudoplatanus*.

**4.9** Microscopic aspect of embryonic cells in the primary meristem in a shoot of *Larix decidua*.

#### *Embryonic cells in root tips, primary meristem*

100 μm

embryonic cell

**4.10** Macrosopic aspect of the location of a primary meristem in a root tip of *Allium ursinum*.

**4.11** Embryonic cells in the primary meristem in a root of *Allium ursinum*.

25 μm

#### *Juvenile cells in secondary and tertiary meristems*

**4.12** Cells with nuclei in the tertiary meristem (phellogen) of *Paeonia suffruticosa.*

**4.13** Location of the secondary and tertiary meristems in *Pinus mugo*.

**4.14** Juvenile cells with nuclei in secondary meristem and adjacent xylem/phloem in *Viscum album*. phloem xylem

# 4.3 The cuticula – Protection against dehydration

Cells exposed to the air, mostly epidermis cells, protect internal plant cells from dehydration. The cuticle at the external surface of the epidermis is an effective transpiration protection layer. 3LHM Z\YMHJLZ ^P[O ÅH[ HUK JVTWHJ[ J\[PJSLZ HYL NSVZZ` HUK those with rippled cuticles are matte.

Cuticles mainly consist of pectin and cutin. The soluble extract polymerizes after the full development of the organs. Cuticles occur on leaves and young stems without periderm in all taxonomic units of vascular plants. They are absent in roots.

Cuticles are generally absent in water plants, they are thin in plants growing in shadowy conditions, and thick in plants at dry sites. The structure of the surfaces is homogeneous, layered or even granular. Chemically related to cutin are waxes, suberin and sporopollenin.

**4.15** Leaves with a glossy surface OH]L H[OPJR HUK ÅH[ J\[PJ\SHSPRL leaves of *4HNUVSPHNYHUKPÅVYH*.

**4.16** Stem of a water plant (*Potamogeton coloratus*) without cuticula.

*Macroscopic aspect Thin and thick cuticles*

**4.17** Stem of *Ipomaea tricolor* on a shadowy site, with a thin cuticula.

**4.18** Leaf with a thick, unstructured cuticula of *Zamia* sp. on a very dry site.

**4.19** Rippled cuticula of an annual twig of *Ephedra viridis*.

**4.20** Thick, rippled cuticula on the underside of a leaf of *Buxus sempervirens* on a dry site.

#### *Rippled cuticles Structured cuticulae*

**4.21** Layered cuticula on a young stem of the mistletoe *Viscum album*.

**4.22** Granulated cuticula on a leaf of *Welwitschia mirabilis*.

# 4.4 Epidermis – The skin of plants

The epidermis covers the products of primary meristems of most plants. Examples of stems and leaves from ferns, conifers, monocotyledonous and dicotyledonous plants are shown. Epidermis cells form a uniseriate layer of generally isodiametric cells at the periphery of primary plant bodies. Epidermis cells protect internal tissues from dehydration. Local cell wall expansion and cell division form bulliform cells or a variety of trichoms and hairs with special functions.

The epidermis of green plants is punctuated with stomata, allowing gas exchange between the atmosphere and the plant tissue. Since epidermis cell walls are transparent, photosynthesis is possible in all cells below it even when the vacuoles are ÄSSLK^P[OYLKZ[HPULKanthocyanins (pigments).

*Macroscopic aspect of terrestrial plants Epidermis cells in a red leaf*

**4.23** All leaves and young twigs of perennial plants are covered with an epidermis, like here in *4HNUVSPHNYHUKPÅVYH*

**4.24** Red leaves appear red because the LWPKLYTPZ JLSSZ HYLÄSSLK^P[O[OL YLKWPNment anthocyanin, like here in *Berberis* sp.

**4.25** =HJ\VSLZHYLÄSSLK^P[O[OLYLKWPNTLU[HU[OVJ`HUPU in the epidermis of a fruit of *Euonymus europaeus*.

**4.26** Very thin-walled epidermis in the water plant *Elodea canadensis.*

**4.27** 7HY[PHSS` SPNUPÄLK LWPdermis in the swamp plant *Scheuchzeria palustris.*

*Anatomy of a thin-walled epidermis*

**4.28** ,\_[LYUHSS`SPNUPÄLKLWPdermis of the herb *Adoxa moschatellina* on a wet site*.*

#### *Water plants Terrestrial plants*

**4.29** Externally thick-walled, \USPNUPÄLKLWPKLYTPZ@V\UN shoot of *Asparagus* sp. on a dry site.

**4.30** Very thin-walled epidermis in a *Sphagnum* sp.

#### *Anatomy of a thick-walled epidermis*

25 μm 25 μm

**4.31** Externally thick-walled and SPNUPÄLK LWPKLYTPZVM[OLOLYI*Psilotum nudum* on a wet site.

**4.32** Very thick-walled epidermis in *Zamia* sp.

**4.33** ;OPJR^HSSLKSPNUPÄLKLWPKLYmis on a needle of the conifer *Pinus nigra*.

**4.34** Sunken stoma in the epidermis of *Elymus farctus*.

*Anatomy of elongated epidermis cells Glandulous excretion elements*

**4.35** Locally enlarged epidermis cells in *Asparagus scoparius*.

on a shoot of the monocotyledonous *Carex glareosa*.

*Trichoms*

**4.37** Excretion in *Lysimachia vulgaris*. **4.36** )\SSPMVYT SPNUPÄLK LWPKLYTPZ **4.38** Excretion in *Salvia pratensis.*

**4.39** Unicellular hairs in *Lilium martagon*.

**4.40** Bicellular hairs in *Scrophularia peregrina*.

**4.41** Multicellular hairs in *Ptilostemon chamaepeuce*.

**4.42** Stem of the moss *Thuidium tamariscinum*.

# 4.5 Collenchyma – Local peripheral stability

Collenchyma functions as a stabilizing element in edges and ridges of herbaceous plant stems of various genera in dicotyledonous and monocotyledonous plants. They are a product of primary meristems and occur in the cortex of stems. Collenchyma cells are similar to parenchyma cells but are normally longer and have pointed axial ends. Characteristic are the partially thickened primary cell walls. The walls contain cellulose, and a large amount of pectin, which is indicated by blue to purple staining with Astrablue/Safranin. The appearance largely varies: some cells are only slightly and some intensively thickened (lamellar collenchyma) and some only in the angles (angular collenchyma). At least some cells contain protoplasts and nuclei.

**4.43** *Stachys sylvatica*, a Lamiaceae with quadrangular stems.

**4.46** *Rumex alpinus*, a Polygonaceae with longitudinally ribbed annual shoots.

**4.44** The quadrangular stem of *Stachys sylvatica* is stabilized by collenchyma in the cortex.

**4.47** The ridges in the stems of *Rumex alpinus* are stabilized by collenchyma in the cortex.

*Collenchyma is rare in monocotyledonous plants*

outside by an epidermis and towards the inside by parenchyma cells. Thick edges characterize the collenchyma.

**4.48** Living collenchyma cells with intensively thickened angles (angular collenchyma). Most cells contain protoplasts. In a few of them, nuclei are visible.

**4.49** *Tamus communis*, a monocotyledonous plant, belonging to the Dioscoreaceae, with lightly-ribbed annual liana-like shoots.

**4.50** Morphologically hardly differentiated collenchyma in *Tamus communis*. Only the different reaction to the Astrablue/Safranin staining (purple coloring) indicates the collenchyma.

# *Collenchyma occurs mainly in the cortex of dicotyledonous plants*

16 *Ch 4. Cellular composition of the plant body*

# 4.6 Parenchyma cells – Storage and repair

Humans would not be able to exist without plant parenchyma cells because their cell contents, especially carbohydrates, are essential components of our diet. Parenchyma cells are present in all plants. They are mostly small isodiametric or slightly elongated cells without pointed ends. Cell walls of parenchyma JLSSZHYL[OPUVY[OPU[V[PJR^HSSLKSPNUPÄLKVY\USPNUPÄLKHUK perforated with simple pits. Parenchyma cells primarily function as storage cells. Living parenchyma cells are totipotent. They have the potential to regenerate new cell types or entire plants under suitable environmental conditions. They play an essential role in regeneration processes after injury (see Chapter 10).

7HYLUJO`THJLSSZHYLWHY[VMZ[LTZYVV[ZSLH]LZÅV^LYZMY\P[Z and seeds. Abundant amounts of parenchyma cells occur in thickened belowground organs. The life span of parenchyma cells of perennial plants is normally very long. Ray-parenchyma cells can live for more than 100 years in the sapwood of conifers or more than 200 years in dwarf shrubs of the arctic.

# *Parenchyma cells occur in all terrestrial plants*

**4.52** (WWYV\_VM[OL \_`SLTPU[YLLZ HYLWHYLU chyma cells. Parenchyma cells in the phloem can change their mode of behavior and form new shoots.

**4.53** Scar on the stem of *Acer pseudoplatanus*. Living parenchyma cells repair wounds and protect the stem against destructive organisms.

produce new shoots.

**4.51** Potatoes (*Solanum tuberosum*) consist mainly of parenchyma cells. They are able to

**4.54** Parenchyma cells are TVZ[S` ÄSSLK ^P[O Z[HYJO grains in the rhizome of *Anemone nemorosa*, polarized light.

**4.59** Water-storing parenchyma cells in the succu-

lent *Sedum acre*.

**4.55** Isodimetric parenchyma cells in the stem of the moss *Polytrichum commune*.

**4.60** Very small parenchyma cells between air-conducting spaces in the pith of swamp plant *Scirpus radicans*.

#### *Shape of parenchyma cells*

**4.56** Parenchyma cells in the xylem of *Sonchus leptophyllus*. Axially sectioned cells are round, radially sectioned cells are elongated.

**4.61** Parenchyma cells surround air canals in the cortex of the swamp plant *Orontium aquaticum*.

**4.57** Simple pits are characteristic for parenchyma cells. Ray cells in the bark of *Paeonia suffruticosa*.

**4.62** Parenchyma cells alternate with sieve cells in the bark of conifers such as *Abies alba*.

**4.58** Irregularly formed paren chyma cells, the callus cells, appear after wounding in *Picea abies*.

**4.63** Long-living parenchymatic ray cells with nuclei (blue) in *Picea abies*.

# 4.7 Fibers and tracheids – Stabilisation and water conduction

4HU` ^VVKLU WYVK\J[Z LN ILHTZ IVHYKZ ÄYL^VVK HUK paper, are substantial basics of our daily life. The main function of ÄILYZPZstabilization, while tracheids are stabilizing and conduct water.

Fibers and tracheids are long cells with elongated, pointed tips. Tips are a result of post-cambial axial elongation. Fibers or tracheids occur in all growth forms and on all sites in spore plants (ferns, horsetails), in conifers, in monocotyledonous and dicotyledonous plants. They are part of annual and perennial stems VMYVV[ZHUKSLH]LZ-PILYZOH]LTVYLVYSLZZ[OPJRSPNUPÄLKJLSS walls. Characteristic is the presence of secondary walls. Pits in ÄILYZHYLTVZ[S`simple or slightly bordered. Pits in tracheids are bordered (see Chapter 5.2, Fig. 5.19). Fibers occur in the xylem, phloem and cortex, tracheids only in the xylem.

*Use of Ƥ*

**4.64** Fibers are omnipresent in human life in the form of wood products.

**4.65** :OVY[ ÄILYZ ¶ 200 μm) in shrubs, such as *Buxus sempervirens.*

*Fibers occur in all vascular plants*

ÄILYZ

**4.66** 3VUN ÄILYZ ¶ >1000 μm) in trees, such as *Fagus sylvatica*.

**4.67** Long (1,000–3,000 μm), KLSPNUPÄLK [YHJOLPKZ \ZLK for pulp) in *Pinus sylvestris*.

**4.68** Fibers in stems of dicotyledonous dwarf shrubs like *Sibbaldia procumbens*.

**4.69** Fibers in culms and leaves of monocotyledonous herbs like *Festuca alpina* (top) or *Festuca erecta*.

#### *Fibers occur in all parts of plants*

**4.71** Fibers in a shoot of *Ranunculus lanuginosus*.

**4.72** Fibers in a root of *Ledum palustre*. **4.73** Fibers in a leaf of *Carex sempervirens*.

**4.70** Fibers occur in stems, roots and leaves. *Cas-*

*tanea sativa*.

**4.74** Fibers in a shoot (cortex) of *Cucumis sativus*. **4.76** Fibers in a culm of *Sesleria coerulea*.

*Fibers occur in all parts of stems*

**4.75** Fibers in the bark of *Chrysothamnus parryi*.

**4.77** Thick-walled tracheids in *Larix decidua*. **4.78** ;OPJR^HSSLKÄILYZPU*Eryobotrya japonica*.

*ǦƤ*

**4.79** ;OPU^HSSLKÄILYZPU*Salix foetida*.

# 4.8 Sclereids in the bark – Extraordinary cell-wall thickening

*Sclereids in stems*

We notice sclereids when eating pears: The granules remaining between your teeth are sclereids. Sclereids are absent in the xylem but frequent in the phloem, cortex and pith of trees and herbs. They also occur in fruits and nut shells, and rarely in leaves. Characteristic for sclereids are the irregularly formed cells with thick, secondary walls, distinct simple pits and often distinct growth layers. A special case are star-like sclereids in the aerenchyma of the water lily (*Nuphar* sp). Sclereids in the bark normally occur in the non-conducting (adult) phloem and in rays. Sclereids originate from parenchyma cells with a shortterm accelerated growth of secondary walls.

250 μm

**4.81** Many sclereids in a ray and the cortex in *Fagus sylvatica*, polarized light.

**4.80** Hard bark of *Fagus sylvatica*. **4.82** Flexible and soft culms and

**4.83** Star-like sclereids in air-conducting channels (aerenchyma) in 100 μm

*Nuphar lutea*, polarized light.

*Sclereids in fruits*

**4.85** Sclereids in the peripheral part of the fruit of *Mespilus germanica*, polarized light).

**4.84** Fruit of *Mespilus germanica*. **4.86** Fruits of the hazelnut (*Corylus avellana*).

**4.87** Sclereids in the shell of a hazelnut.

# *Single and groups of sclereids* layers

**4.88** Group of sclereids in the bark of *Picea abies*. **4.89** Isolated sclereids in the fruit of *Mespilus germanica*.

**4.90** Isolated sclereids with distinct layers in a leaf of *Welwitschia mirabilis*.

Plant life on terrestrial sites would not exist without the water conducting ]LZZLSZ/V^L]LYP[PZNLULYHSS`KPMÄJ\S[[VZLL]LZsels with bare eyes, because their diameter is usually below the resolution of human eyes. Vessels consist of vessel elements. Fully developed vessel elements are dead and more or less LSVUNH[LK;OL`HYLJVTWVZLKVMSPNUPÄLKJLSS^HSSZperforation plates at their distal ends and pits at the longitudinal walls. Lig-UPÄJH[PVUVM[OL^HSSZVM]LZZLSLSLTLU[ZWYL]LU[ZJLSSJVSSHWZL

Vessels occur in the xylem of most spore plants (ferns, horsetails, lycopods) and most monocotyledons and dicotyledons, except in conifers and few others where they are replaced by tracheids. Vessels occur in all growth forms. Diameters are usually large 4.9 Vessels – Water conduction

in lianas (>200–500 μm), smaller in trees (50–200 μm) and small in herbs (20–50 μm). The length of vessel elements varies from <100 μm in small herbs to >1,500 μm in trees. Vessel elements are normally short in ring-porous and long in diffuseporous tree species.

Vessels conduct water mainly in axial direction from the root to the leaves. Lateral perforations (bordered pits) also indicate a lateral water transport. Vessels exist in the xylem of all plant organs in various dimensions. Vessels are normally larger in roots than in stems, and even smaller in leaves.

For a discussion of cell-wall structures see Chapter 5.1.

#### *Macroscopic aspect Anatomy of vessel elements*

**4.91** Vessels in the earlywood of ring-porous species (e.g. *Quercus* sp.) are visible to the bare eye.

**4.92** Long vessel element (500 μm) with helical thickenings in *Tilia* sp.

*Vessels occur in all vascular plants*

# bordered pits perforation plate

**4.93** Short and wide vessel in the earlywood of *Fraxinus excelsior*. Reprinted from Greguss 1945.

**4.94** Lycopods: clubmoss *Lycopodium clavatum*.

**4.95** Monocotyledons: stem of palm *Phoenix dactylifera*.

**4.96** Dicotyledons: stem of apple tree *Malus domestica*.

#### *Vessels occur in all growth forms*

**4.97** Very large vessel diameters (550 μm) in lianas *Celastrus* sp. (top) and *Thunbergia* sp. (bottom).

**4.98** Large vessel diameter (80 μm) in the dicotyledonous tree *Fagus sylvatica*.

#### *Vessels occur in most parts of plants*

**4.99** Very small vessel diameters (20 μm) in the 5 cm-tall annual herb *Erophila verna*.

**4.100** Diameter of earlywood vessels in the stem (top) is 70 μm, in the root (bottom) 250 μm in *Prunus amygdalus*.

**4.101** Small vessel diameters (20 μm) in a vascular bundle of a thick leaf of *Clusea rosea*.

# 4.10 Cork cells – Defense against organisms, heat and cold

Cork is a perfect insulation material. We use it to seal wine IV[[SLZ HUK[VPUZ\SH[L^HSSZ HUK ÅVVYZ4VZ[ VM[OLPUK\Z[YPally used cork is a product of cork oak (*Quercus suber*). Plants insulate their stems against extreme environmental conditions SPRLPU[LUZLYHKPH[PVUISHZ[PUNI`ZHUKHUKPJLJY`Z[HSZÄYLVY ÅVVKPUN\*VYRJLSSZWOLSSLTVJJ\YV\[ZPKLVM[OLWOSVLTHUK cortex of most taxa and growth forms in conifers and dicotyledons. They are rare in monocotyledonous plants, and occur there only in trees. Cork cell walls consist of suberin, which is KPMÄJ\S[[Vdecompose for many fungi. Cork cells of any form can be thin- or thick-walled.

Cork formation occurs usually on the outside of stems. However, a few species produce cork rhythmically within the xylem. Small, long-lived herbs compartmentalize their center by forming cork internally within the stem.

**4.102** Conifers: Layered bark of *Pinus sylvestris*.

**4.103** Dicotyledonous plants: Layered bark of the tree *Quercus suber*.

**4.104** Dicotyledonous plants: Layered bark of a small cushion of the alpine *Saxifraga oppositifolia*.

**4.105** Monocotyledonous plants: Cork cells in the tree *Dracaena serrulata*.

**4.106** A small mantle of thin-walled cork cells surrounds the water-storing cortex of the succulent *Sedum acre*.

**4.107** A large zone of thick-walled, rectangular cork cells surrounds the ZVM[\USPNUPÄLK\_`SLTVM[OLHSWPUL cushion plant *Saxifraga caesia*.

*Thin- and thick-walled cork cells Cork cells within the xylem*

**4.108** Tangential cork layers between compartments of xylem in *Artemisia tridentata*.

**4.109** Cork layers in a stem separate the living from the dead xylem in the center of the stem of *Scorzonera virgata*.

# 4.11 Sieve cells, sieve tubes and companion cells – Conduction of assimilates

Sieve elements are part of the bast. Neolithic settlers knew sieve cells and bast ÄILYZ]LY`^LSSHUK\ZLK[OLIHZ[VMZL]LYHSWSHU[Z to braid baskets and tissues. Today, papier mâché ("chewed paper") is probably the only remaining useful product.

Sieve cells and sieve tubes are a major part of the phloem, which mainly conducts assimilates from leaves to parenchyma cells. Sieve elements are accompanied by parenchyma cells and companion cells. Sieve cells have ZPL]LÄLSKZVU[OLPYSH[ eral sides, while sieve tubes have plates at their distal ends and on their lateral walls. However, the anatomical differentiation PU JYVZZ ZLJ[PVUZ PZ NLULYHSS` KPMÄJ\S[ :PL]L [\ILZ HUK ZPL]L JLSSZHYLSVUN[OPU^HSSLKHUK\USPNUPÄLK(K\S[ZPL]LLSLTLU[Z do not contain nuclei. The metabolism of the sieve elements is maintained by the nuclei in the companion cells. Companion cells are smaller and always adjacent to sieve tubes.

Sieve cells occur in all vascular plants from ferns to dicotyledons. Sieve cells and sieve tubes often collapse following their death. More information is given in Chapter 6.2.8.

**4.110** Neolithic tissues made of *Tilia* bast. Photo: Amt für Städtebau Unterwasserarchäologie, Univ. Zürich.

**4.113** Phloem of the tree *Adansonia digitata*, consisting of sieve tubes, companion cells and parenchyma cells.

#### *Macroscopic aspect of the phloem*

**4.111** Stripped bark of *Populus* ZW )HZ[ ÄILYZ dismantled from the xylem in the cambial zone.

**4.112** Large layered phloem between the xylem and cork in *Juglans regia*.

**4.115** Sieve plates of sieve tubes in *Adansonia digitata*.

**4.116** Sieve elements of the dicotyledonous herb *Bryonia dioeca* (left) with sieve plates at distal ends, and of the conifer *Larix decidua* with lateral sieve plates (right).

**4.117** Sieve cells in the phloem of the fern *Cryptogramma crispa*.

pa pa 50 μm 500 μm 50 μm

**4.118** Sieve cells in the phloem of the conifer *Picea abies*.

si

**4.119** Sieve tube in the phloem of the dicotyledonous *Quercus ilex.*

**4.120** Sieve tubes in a vascular bundle of the monocotyledonous *Arundo donax*.

Today, the best known products of secretory cells are resin condensates, e.g. colophony, amber, incense, and oils in orange skins. The resins are excreted from resin ducts in trees. Fifty years ago, latex as a product of secretory canals was intensively used as gum for tires and chewing gum.

Plant secretory cells are formed either externally, from epidermal tissues, or internally, by primary or secondary meristems. Described in this section are mainly internal secretory cells. They sporadically occur in the whole taxonomic system of terrestrial plants from ferns to dicotyledonous plants. Secretory JLSSZHYL[OPU^HSSLK\USPNUPÄLKJLSSZ:PUNSLJLSSZVJJ\YPU[OL xylem of a few trees and produce oil. Laticifers (latex ducts) form long, uni- or multicellular tubes. Very frequent are ducts that are surrounded by resin-producing, long-lived epithelial and parenchyma cells. Many slime-producing secretory cells are anatomically identical to normal parenchyma cells.

4.12 Secretory cells – Defense

Secretory cells occur around ducts in the pith, xylem, phloem and cortex of stems, roots, leaves and fruits.

*Resin use in art and religion*

**4.121** Amber is a fossilised, condensed resin, produced from various conifers 100 million years ago. It has been regarded as a magic "stone" since Neolithic times. Photo: L. Mons hausen.

**4.122** Frankincense is a product of resin ducts in the bark of the desert tree *Boswellia sacra*. Incense smoke is used for many ritual acts in a number of different religions. Photo: S. Fleckney.

**4.126** Resin ducts in the xylem of the conifer *Pinus* 

*mugo*.

**4.123** Ducts in the skin of oranges (*Citrus sinensis*) produce essential oils.

**4.124** Caoutchouc harvesting on a tree of *Hevea brasiliensis*.

**4.125** Resin in the sapwood of the conifer *Pinus sylvestris*.

*Resin ducts in the xylem of conifers*

**4.127** Living epithelial and parenchyma cells around a resin duct in *Pinus sylvestris*.

**4.128** Resin duct in a ray in *Pinus sylvestris*, tangential section.

**4.129** Living epithelial and parenchyma cells around a resin duct of *Pinus sylvestris*, radial section.

#### *Anatomy of ducts in stems of dicotyledonous plants*

**4.130** Resin ducts in the phloem of the incense tree *Boswellia sacra*.

**4.131** Traumatic "resin" ducts in the xylem of an almond tree (*Prunus amygdalus*).

**4.132** Macroscopic aspect of traumatic "resin" ducts (kino veins) in the xylem of *Eucalyptus obliqua*. Photo: P. Majewski.

**4.133** Microscopic aspect of traumatic "resin" ducts (kino vein) in the xylem of a *Eucalyptus* sp.

**4.134** Resin duct in a needle of the conifer *Picea abies*.

**4.135** Duct in the cortex of the bark of the herb *Petasites paradoxus*.

#### *Anatomy of oil cells Anatomy of laticifers*

**4.138** Enlarged oil cells in the xylem of the tree *Phoebe nanmu*, tangential section. It was the preferred wood for the construction of the Forbidden City in Beijing.

**4.139** Enlarged oil cells in the xylem of *Phoebe nanmu*, radial section.

**4.136** Ducts in the pith of a twig of *Grewia villosa*.

**4.140** Latex-producing laticifers in the phloem and the cortex of the

shrub *Euphorbia armena*.

250 μm

**4.137** Ducts in the shell of a hazelnut (*Corylus avellana*).

**4.141** A single laticifer, consisting of several secretory elements in *Scrophularia dentata*.

# 1 mm 1 mm 50 μm

#### 26 *Ch 4. Cellular composition of the plant body*

# 4.13 Intercellulars and aerenchyma – Air circulation within the plant

The very light-weighted shoots of reed (*Phragmites communis*) and many other grass-like shore plants have been used for the JVUZ[Y\J[PVU VM IVH[Z HUK [OL PUZ\SH[PVU VM YVVMZ HUK ÅVVYZ Hay from wet meadows in the European Alps is used as bedding in cattle stables. These practical uses are based on the hollow stems, and the presence of aerenchyma in the shoots. Intercellulars of any form occur mainly in wetland plants. They guarantee the gas exchange from the stomata in the leaves to all JLSSZ^P[OPU[OLWSHU[3HYNLPU[LYJLSS\SHYZHYLKLÄULKHZHLYLUchyma. Hollow shoots and aerenchyma in water plants allow [OLT[VÅVH[:THSSPU[LYJLSS\SHYZVJJ\YTHPUS`PUSLH]LZWP[O and cortex of plants in all taxonomic units of vascular plants, and aerenchyma with particular structures occur in plants of wet environments, e.g. in swamps and lakes (helophytes and hydrophytes).

Intercellulars are small spaces between round cells. Aerenchyma consists of parenchyma cells surrounding the intercellulars. They form nets, sponge-like tissues in thick stems, star-like groups, channels, irregular and radial spaces and lacunas.

*Use of wetland plants*

**4.142** 9VVÄUN ^P[O YLLK *Phragmites communis*).

**4.143** Stacks of dry, stiff, nutrientpoor grasses and sedges in a wet montane meadow. The material is used instead of cereal straw for bedding in cattle stables. Photo: M. Küchler.

**4.145** Stellate combined cells form HU HPYÄSSLK ZOVV[ JLU[LYPU*Juncus conglomeratus*.

**4.146** (PYÄSSLKYHKPHSZWHJLZPU[OL cortex of a rhizome of *Juncus arcticus*.

**4.147** Large air tubes conduct air from the leaf to the root in *Nymphaea alba*.

**4.144** Hollow pith as the result of extreme stem expansion in *Polygo-*

500 μm

*num amphibium*.

**4.148** Small air tubes conduct air from the leaf to the root in *Potamogeton natans*.

**4.149** Small lacunas within vascular bundles conduct air from the leaf to the roots in *Dacytlis glomerata*.

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# 5. Structure of the cell wall and cell contents

# 5.1 Principal cell-wall structure – Form and stability

The cell wall of seed plants principally consists of several layers: the middle lamella, the primary, secondary and tertiary wall, a concept presented by Evert 2006. Secondary walls consist of THJYVÄIYPSZ;OL` HYL ]PZPISL \UKLY[OLSPNO[TPJYVZJVWL However, the TPJYVÄIYPSZHYLVUS`]PZPISLPUSVUNP[\KPUHSZLJtions of compression wood in conifers (see Chapter 10.4.1, Fig. 10.105) and under the electron microscope. In reality, there is a great chemical and morphological diversity in cell-wall structures (Evert 2006).

;OLTPKKSLSHTLSSHHUK[OLWYPTHY`^HSSZHYLVW[PJHSS`KPMÄJ\S[ to distinguish. Primary walls are thin and consist of irregularly KPZ[YPI\[LK TPJYVÄIYPSZ ^OPJO HYL LTILKKLK PU H TH[YP\_ VM hemicellulose and pectin. Therefore they disappear in polar-PaLKSPNO[;OLSPURZIL[^LLU[OLTPJYVÄIYPSZ HYLUV[WLYTHnent and can be dissolved by enzymes. Therefore, cell-wall expansion is possible.

;OLZLJVUKHY`^HSSZHYLUVYTHSS`[OPJRHUKSPNUPÄLK;OL`Z[HY[ to form after cell expansion is completed. Characteristic at elec- [YVUTPJYVZJVWPJZJHSLHYL[OLOLSPJHSS`VYPLU[LKTHJYVÄIYPSZ consisting of cellulose and hemicellulose. They are embedded in SPNUPU;OLKPYLJ[PVUVM[OLTHJYVÄIYPSZJOHUNLZ^P[OPU the wall but they always remain helically oriented. Due to the crystalline structure of the cellulose, secondary walls are anisotropic and are visible under polarized light. The links between [OLTHJYVÄIYPSZ HYLWLYTHULU["[OLYLMVYL[OL ZPaLVM[OL JLSS remains stable after its formation. Secondary walls provide mechanical support against gravity and turgor pressure.

The tertiary wall is normally thin, and often covered with warts. 4PJYVÄIYPS VYPLU[H[PVU PZ YHUKVT HUK [OL` HYL [OLYLMVYL UV[ visible under polarized light. Layered cellulosic cell walls occur in all taxonomic units of vascular plants, in all habitats, from the desert to the water.

**5.1** Cell walls of vascular plants consist of the middle lamella, the primary, secondary and tertiary wall.

**5.2** Latewood (red) and cambium (blue) of *Larix decidua* shown in transmitting (left) and polarized light (right). Cell ^HSSZPU[OL JHTIPHS aVUL HYLUV[SPNUPÄLK HUK JVUZPZ[VUS` of primary walls. They therefore disappear in polarized light.

**5.3** Latewood of *Picea abies* in formation. Thin tertiary walls (blue) appear immediately after the formation of secondary walls (light).

*Primary walls Secondary walls*

**5.4** 7YPTHY`^HSSZ^P[OV\[SPNUPÄJHtion in the cambium area of a stem of *Corylus avellana*. All cells, except vessels, contain protoplasts.

**5.5** Fully developed primary walls and rudimentary walls in all cells of the water plant *Ranunculus trichophyllus*.

**5.6** Fully developed cell walls with primary, secondary and tertiary walls of tracheids in the latewood of the conifer *Pinus elliottii*.

**5.7** Fully developed cell walls with primary and secondary walls of ÄILYZPU[OL\_`SLTVM*Juglans regia*. Tertiary walls are only visible in the parenchyma cells.

#### *Secondary walls Tertiary walls*

**5.10** Fibers with thin-walled, unlig-UPÄLK[LY[PHY`^HSSZIS\LPU*Stachys sylvatica*.

**5.8** Multilayered cell walls of sclereids in the phloem of *Picea abies* in normal and polarized light.

**5.9** Fully developed cell walls with primary, secondary and tertiary walls VMÄILYZPU[OL\_`SLTVM*Vitis vini fera*. ;LY[PHY`^HSSZHYLUV[SPNUPÄLK

**5.11** Cell walls of MLYUÄILYZ*Lygodium* sp.) are constructed the same as in seed plants. Visible are middle lamella, primary, secondary and tertiary walls.

#### *Cell walls of ferns and fungi*

**5.12** The unstructured cell walls of marine algae (*Fucus serratus*) consist of cellulose and glycoproteins.

**5.13** The unstructured cell walls of the blue-stain fungus *Grosmannia clavigera* consist of chitin.

**5.14** Unstructured cell walls of fungi in the lichen *Usnea barbata*.

# 5.2 Pits – Lateral contact between cells

Cells communicate with each other. Two neighboring cells are anatomically connected by pits. Through their channels, cells communicate physiologically. There are two principal structures; simple pits and bordered pits. However, many transition forms exist.

#### *Simple pits*

Simple pits occur in all vascular plants. They form a channel [OYV\NO[OLZLJVUKHY`^HSSVMULPNOIVYPUNJLSSZ;OL\USPNUPÄLK middle lamella in the pit blocks the channel but is perforated by plasmodesmata (not visible with light microscopes), through ^OPJOZ\NHYZHTPUVHJPKZOVYTVULZHUKU\[YPLU[ZJHUÅV^ Simple pits of different forms occur in axial parenchyma cells, horizontal parenchymatic ray cells, rays and ÄILYZ 0M ZPTWSL pits connect rays and vessels they are called vessel-ray pits.

The form and size of the opening varies. Pits in parenchyma JLSSZHYLYV\UKHUK[OVZLPUÄILYZHYLVM[LUZSP[SPRL;OL`JHU be small (<2 μm) or large (>4 μm) or horizontally or axially enlarged.

**5.15** Structure of simple pits in rays of *Pinus sylvestris*. The middle lamellae HUKWYPTHY`^HSSZHYLUV[SPNUPÄLKIS\LPU`V\UNJLSSZ

**5.16** Cross section of simple pits in sclereids of hazelnut shells (*Corylus avellana*).

**5.17** Cross section of simple pits on ray cells in *Ephedra viridis*.

# *Structure of simple pits*

50 μm

**5.20** Small vessel-ray pits in the xylem of *Betula aetnensis*.

*Size and form of simple pits*

**5.21** Large round vessel-ray pits in the xylem of *Salix arctica*.

**5.18** Top view of round simple pits in cortex-parenchyma cells of *Rosa canina*.

**5.19** Radial view of slit-like simple WP[ZPUÄILYZVM*Magnolia acuminata*.

**5.22** Scalariform vessel-ray pits in the xylem of *Euphorbia calyptrata*.

**5.23** Large, fenestrate ray pits in the xylem of *Pinus sibirica*.

31

#### *Bordered pits*

Bordered pits occur in vessels, and axial and radial tracheids. The difference to simple pits is that secondary walls arch over the pit channel and that the middle lamellae and the primary walls are thickened (torus). Tori in pits of living cells are unligni-ÄLKHUKSH[LYHSS`ÅL\_PISL)VYKLYLKWP[Z^P[O\USPNUPÄLK[VYPHJ[ as valves. If pressure in one cell decreases, e.g. by injury, the torus gets pressed to the wall with the higher pressure (healthy cell). If bordered pits connect vessels they are called intervessel pits. A special case are intervessel pits with warts on their openings, the so-called *vestured pits*.

The form of the outer borders, the size, and the position in a cell wall varies. They can be small (<2 μm) or large (>4 μm), with round or angular outlines, or horizontally enlarged (*scalariform pits*) or net-like (*reticulate pits*). Pits are mostly alternating, rarely they are arranged opposite. Bordered pits in tracheids of conifers are in one, two or multiple rows. All these characteristics are taxonomically relevant.

**5.24** Ray tracheid in *Pinus sylvestris* ^P[O \USPNUPÄLK tori.

**5.25** Tracheid in the dwarf shrub *Sarcococca hookeriana*.

**5.26** Vessels with pits in the mistletoe *Viscum album*, ra dial (top) and cross (bottom) section.

**5.27** Tracheids in *Picea excelsa*, cross section, with \USPNUPÄLK[VYP

**5.28** Vessel in *Sinapidendron frutescens*, with vestured pits.

**5.29** Small in *Betula nana*.

25 μm

**5.31** Scalariform in *Ribes* 

25 μm 25 μm 25 μm 50 μm

**5.30** Large in *Salix alba*.

*alpinum*.

*Arrangement of bordered pits in tracheids of conifers*

**5.32** Alternate in *Reseda suffruticosa*.

**5.33** Opposite in *Platanus* sp.

**5.34** One row in *Pinus banksiana*. **5.35** Two rows in *Araucaria angustifolia*. **5.36** Multiple rows in the carboniferous *Dadoxylon* sp.

#### *Form and arrangement of bordered pits in deciduous plants*

# 5.3 Perforation plates – Axial contact between vessels

Vessels are perforated at their axial ends. The perforation plates are normally oblique positioned at their radial walls. Therefore they can be observed on radial sections.

Three perforation-plate types exist principally:

*Simple perforation plates* are characterized by their large, round to oval opening. The vast majority of species have simple perforations.

*Foraminate perforation plates* with several round openings occur only in the family of Ephedraceae, which stands taxonomically between conifers and dicotyledonous plants.

*Scalariform perforation plates* are characterized by their oblique position and horizontal bars. The number of bars varies from one to more than 30. The thickness of the bars and the size of the plates are also variable. The occurrence of scalari-MVYTWLYMVYH[PVUWSH[LZPZZWLJPLZZWLJPÄJ;OL`THPUS`VJJ\YPU larger plants from shrubs to trees. They are rare in tropical environments and in small plants. Hardly any herb has scalariform perforation plates.

*Aberrant scalariform perforation plates* have been observed in a few species of small plants. They occur mainly in the family of Asteraceae.

It has been suggested that foraminate perforation plates stand at the beginning of an evolutionary process of trees, which ends with simple perforation plates (Bailey 1944).

**5.37** *Fraxinus excelsior.*  Reprinted from Greguss

1945.

*Simple*

**5.38** *Carduus macrocepha-*

*lus*

**5.39** *Salix helvetica*

*Foraminate*

**5.40** *Ephedra distachya* ssp. *helvetica*

*Scalariform*

**5.41** <10 bars, *Paeonia fruticans.*

**5.42** <10 bars, *Buxus sempervirens.*

**5.43** 10–20 bars, *Betula humilis.*

#### *Scalariform*

**5.44** >20 bars, *Viburnum opulus.*

**5.45** >20 bars, *Menyanthes trifoliata.*

#### *Aberrant scalariform*

**5.46** *Bidens tripartita*

# 5.4 Helical thickenings – Special wall thickenings

The secondary wall in ÄILYZtracheids and vessels of many species occurs as annular rings, spirals or nets. Those structures are absent in parenchyma cells of the xylem. The principal function of spiral thickenings might be stabilization but their occurrence in the plant body indicates also an ontogenetic and phylogenetic component. Helical thickenings have occurred in the xylem of all taxonomic units of vascular plants since their move to the land 350 million years ago.

*Annular thickenings* occur exclusively in the protoxylem and metaxylem of vascular bundles. The rings represent an early ontogenetic form of secondary walls.

*Helical thickenings* of various thickness occur in the mature xylem. Helical thickenings are of great taxonomic value ILJH\ZL[OL`HWWLHYVUS`PUZWLJPÄJ[H\_H!


*Helical thickenings are not to be confused with helical cavities in the secondary walls of compression wood in conifers, or with reticulate pits in some dicotyledonous plants.*

*Helical thickenings in the proto- and metaxylem*

**5.47** Cross and radial section of a species of *Rosa*. Helical thickenings occur immediately around the pith.

**5.48** Cross section of a species of *Rosa*. Helical thickenings occur in vessels at the initial point of vascular bundles.

**5.49** Helical thickenings in vessels of *Colocynthis vulgaris*. The gap between the ZWPYHSIHUKZPU[OLÄYZ[]LZsel (protoxylem?) is wider than that in the second vessel (metaxylem).

#### *Helical thickenings in tracheids and vessels*

conifer *Taxus baccata*.

tr v tr

**5.50** Helical thickenings in tracheids in the xylem of the **5.51** Helical thickenings in tracheids and vessels of the dicotyledonous dwarf shrub *Digitalis obscura*.

*Helical thickenings in vessels of dicotyledonous plants*

**5.52** Densely positioned helical thickenings with a ÅH[ HUNSL PU *Adenocarpus viscosus*.

**5.53** Thin helical thickenings in vessels in *Tilia platyphyllos*.

**5.54** Thin helical thickenings in vessels in *Lonicera xylosteum*.

*Do not confuse*

**5.55** Helical cavities in compression-wood tracheids of the conifer *Pinus sylvestris*.

**5.56** Reticulate pits with very wide openings in *Aeonium urbicum*.

# ͝Ǥ͝Ȃƪ

Tyloses occur in vessels. They represent an essential feature in compartmentalized parts of stems at the boundary between heartwood and sapwood, living and dead parts of branches, or in injured parts of the xylem. Tyloses block the axial water transport and defend living tissues against pathogens.

Tyloses are irregularly formed cell walls with various wall thicknesses inside vessels. Their origins are specially formed

barrier zones

cell wall layers inside secondary walls in neighboring parenchyma cells which expand balloon-like through pit openings into the vessels. Such cells are called contact cells. Tyloses are NLULYHSS`\UZ[Y\J[\YLKHUK\USPNUPÄLK/V^L]LY[`SVZLZ^P[O simple WP[ZHSZVL\_PZ[;`SVZLZPUVSKLY[PZZ\LZHYLVM[LUSPNUPÄLK and impregnated with phenols. Nuclei often migrate into the tyloses. Tyloses occur in vessels of the primary and secondary xylem of trees and herbs.

**5.57** Tyloses occur at the heartwood-sapwood boundary of oaks (*Quercus* sp.).

#### *Tyloses in a defense zone*

**5.59** Tyloses in a defense zone (barrier zone) in a root of *Fagus sylvatica*. **5.58** Tyloses inside an injured part of a stem of *Acer pseudoplatanus*.

**5.60** Tyloses in vessels of a defense zone block the water conductance in *Fagus sylvatica*.

**5.61** Selective occurrence of thin- ^HSSLKSPNUPÄLK[`SVZLZPU]LZZLSZVM the arctic dwarf shrub *Salix arctica*.

#### *Anatomy of tyloses Nuclei in tyloses*

**5.62** Tyloses in vessels of the nettle *Urtica dioica*.

**5.63** Tyloses with simple pits in *Maclura pomifera*.

**5.64** Tyloses in an earlywood vessel with nuclei in the climber *Hedera helix*.

# 5.6 Cell contents – Everything inside the cell wall

Living adult cells of vascular plants principally contain a *protoplast* and vacuoles. The protoplast is the living unit of a cell. It is separated by the tonoplast (semipermeable thin biomembrane) from the vacuole. The protoplast contains a liquid (cytoplasm) in which various organelles such as the nucleus with nucleolus, plastids (starch grains, chloroplasts, chromoplasts, leucoplasts), mitochondria, the endoplasmatic reticulum, Golgi vesicles and ribosomes occur. The vacuole contains ergastic substances such as crystals, oil bodies and various types of phenols. Most organelles contain genetic information.

**5.65** Schematic representation of an adult cell. Visible by light microscopy are nuclei, starch grains, chloroplasts, chromoplasts, crystals, oil bodies and phenols.

The cell nuclei are the basic structural and functional unit of organisms. They occur in all cell types and all organs of plants, however, their life span greatly varies.

**5.66** Nuclei in a secondary cambium and rays of the dwarf shrub *Viscum album*, radial section.

**5.67** Small, round, 80-year-old nuclei in ray cells of the arctic dwarf shrub *Rhododendron lapponicum*.

**5.68** Radially elongated nuclei in young ray cells of the conifer *Abies alba*.

*Nuclei in parenchyma cells Ƥ*

**5.69** Protoplast and nuclei in latewood tracheids of the conifer *Pinus sylvestris*, cross section. Only the most recently formed tracheids are living, they contain protoplasts.

**5.70** Nuclei with pointed axial ends PUÄILYZVM[OLOLYI*Anthemis palestina* in the desert, radial section.

# 5.6.2 Plastids – Green, yellow and white bodies

All living cells contain plastids. Chloroplasts are green-pigmented plastids. They contain the green photosynthetically active chlorophyll and the yellow carotenoids. Chromoplasts are yellow-pigmented plastids and contain carotenoid pigments. Leucoplasts are non-pigmented plastids.

Chloroplasts occur in all green parts of plants. They represent a substantial part in leaves (10–200 chloroplasts per cell) but also occur in parenchymatic parts of stems and fruits. Disk-like bodies, the thylacoids, are only visible in electron-microscopic THNUPÄJH[PVUZ

Chromoplasts occur mainly in yellow- and orange-pigmented WHY[ZVMWSHU[ZZ\JOHZPUÅV^LYZMY\P[ZHUKYVV[Z\*OYVTVWSHZ[Z are also a product of aging chloroplasts in leaves.

Leucoplasts do not contain pigments. Some types can develop PU[VJOSVYVWSHZ[ZVYJOYVTVWSHZ[Z\UKLY[OLPUÅ\LUJLVMSPNO[

#### *Chloroplasts make the world green*

**5.71** Green leaves contain chloroplasts. *Hamamelis virginiana*.

**5.72** Green parts of stems contain chloroplasts. *Clematis alpina*.

**5.73** Chloroplasts in the needle of *Pinus nigra*.

**5.74** Chloroplasts and leucoplasts in a variegated leaf of *Cornus sericea*.

**5.75** Aging leaves of *Hosta* sp. lose their chlorophyll, and yellow carotenoids now characterize the aspect.

#### *Chromoplasts make the world colorful*

**5.76** @LSSV^ ÅV^LYZ VM*Hypericum* sp. contain chromoplasts.

**5.77** Orange carrots (*Daucus carota*) contain chromoplasts.

**5.78** Chromoplasts in the petal of *Lonicera tatarica*.

# 5.6.3 Starch grains – Stored energy

Starch is a carbohydrate and plays a fundamental role for life on earth. Without starch from potatoes, manihot or cereals such as wheat, corn, rice and many others, human existence is unimaginable. Starch stores the energy of polysaccharides in a UVUVZTV[PJLMÄJPLU[^H`PUparenchyma cells in all parts of all vascular plants. When sugars are re-synthesized to starch, the grains are called amyloplasts.

Starch grains are easy to recognize under the microscope in polarized light by the characteristic "Maltese cross", or by staining with potassium iodide. Form and size of the grains vary. In most cases they are more or less globular with a central point (hilum). Layers are the result of alternating deposits of different polysaccharides.

*Starch as the base for (human) life Starch grains in tubers of potatos*

**5.79** Flour from cereals.

**5.80** Section of a potato (*Solanum tuberosum*^P[OZ[HYJOÅV^PUNV\[

**5.81** Large starch grains in *Solanum tuberosum*.

**5.82** Starch in *Solanum tuberosum* with the characteristic "Maltese crosses" seen in polarized light.

**5.83** Starch in the pith of a rhizome of the herb *;LSSPTH NYHUKPÅVYH*, polarized light.

**5.84** Small starch grains in rays and axial parenchyma cells in the xylem of a twig of *Fraxinus excelsior*, cross section.

**5.85** Starch stained with potassium iodide in ray cells of the wood of *Abies alba*, radial section.

#### *Starch grains in shoots Starch grains in fruits*

**5.86** Starch in the soft fruit of a banana (*Musa* × *paradisiaca*).

# 5.6.4 Crystals in vacuoles – Regulators and metabolic waste

Crystals are excreted from protoplasts and deposited in vacuoles. Most crystals in cells are calcium oxalates. They have two major functions. Calcium is an essentially required element for plant growth, therefore calcium oxalate crystals often occur in meristematic tissues where calcium ions regulate the transport of organic molecules. Calcium oxalate is an end product of metabolic processes (metabolic waste), therefore calcium oxalates are often deposited in vacuoles of adult cells in all parts of plants, where they remain until cell death. Some plants form special cells, so-called crystal idioblasts, where crystals are deposited.

Calcium oxalate crystallizes in the form of prismatic crystals of various shapes: as druses, raphides and irregular small grains. The distribution and shape of calcium oxalates in tissues is a valuable taxonomic feature.

*Crystals in meristematic tissues Crystal forms*

**5.87** Crystals in the meristematic tissue of a bud of *Acer pseudoplatanus*, polarized light.

**5.88** Elongated crystals (styloids) in the cambial zone of the herb *Hippocrepis comosa*, polarized light.

**5.89** Prismatic crystals in a xylem ray of *Fagus sylvatica*, polarized light.

**5.90** Crystal sand in cortex cells of the tropical *Piper nigrum*, polarized light.

**5.91** Crystal druses in expanded cells of the cortex of the alpine herb *Astrantia major*, polarized light.

**5.92** Elongated crystals (raphides) in idioblasts in the cortex of the liana-like dwarf shrub *Rubia tibetica*, polarized light.

**5.93** Elongated and layered crystal in the rhizome of *Iris sibirica*, polarized light.

*Crystal arrangement in bark*

*Crystal forms Crystals in chambers*

**5.94** Prismatic crystals in a large idioblast (radial section) of the small shrub *Neochamaelea pulverulenta*, polarized light.

**5.95** Irregularly dispersed crystal druses in the phloem of the shrub *Buxus sempervirens*, polarized light.

**5.96** Tangentially arranged crystals in the phloem of the alpine shrub *Ribes alpinum*, polarized light.

**5.97** Radially arranged crystals along large rays in the phloem of the climber *Parthenocissus inserta*, polarized light.

**5.98** Prismatic crystals, arranged HYV\UKÄILY NYV\WZPU[OLWOSVLT of *Salix purpurea*, polarized light.

# 5.6.5 Stained substances within the stem – Defense

Stained substances occur in the phloem, xylem and fruits of all vascular plants. Most obvious are discolorations in stems with heartwood, injuries or subfossil wood.

The term indicates the chemical and anatomical heterogeneity of the amorphic substances. Different terms are used, e.g. organic compounds, organic extractives, tannins (polyphenols), gums, resins and oils. Characteristic for all of them are the colors, which range without staining from yellow to red and dark brown, and with Astrablue/Safranin staining from blue to red and black.

A selective variety of different stained substances are presented, which occur in the xylem in vessels, axial and radial paren-JO`THJLSSZÄILYZHUKJLSSZPU[OLWOSVLT;OLWYVK\JPUNJLSS structures are discussed in Chapter 4.12.

**5.99** Dark-stained heartwood in various Australian wood species.

**5.102** Black-stained substances in vessels of *Acacia longifolia* (Astrablue/Safranin-stained).

*Macroscopic aspect of dark-stained substances*

**5.100** Dark-stained zones in biological defense zones (compartmentalization) of *Fagus sylvatica*.

**5.101** Dark-stained heartwood of a subfossil, waterlogged *Quercus* sp.

*Stained substances in vessels*

*Stained substances in parenchyma cells*

**5.103** Dark-blue-stained substances in vessels and parenchyma cells in the heartwood of *Juglans regia* (Astra blue/Safranin-stained).

**5.104** Dark-blue-stained substances in vessels of a *Paliurus spina-christi*  (Astra blue/Safranin-stained).

**5.105** Black-, red-, blue- and yellowstained substances in vessels and vascular tracheids in *Spartocytisus supranubius* (Astra blue/Safranin-stained).

**5.106** Brownish-stained substances in parenchyma cells in the heartwood of *Juniperus communis* (unstained).

**5.107** Dark-red-stained substances in axial and radial parenchyma cells in a chestnut-blight-affected *Castanea sativa* (Astra blue/Safranin-stained).

**5.108** Brown-stained substances in a large ray of a dead part of the vine *Vitis vinifera* (Astra blue/Safraninstained).

**5.109** Brown-stained substances in WHYLUJO`TH JLSSZ HUKÄILYZPU[OL heartwood of the dwarf shrub *Arctostaphylos uva-ursi* (unstained).

#### *Ƥ*

**5.110** Brown-stained substances in vessels and ÄILYZ PU [OL OLHY[^VVK VM LIVU`*Diospyros* sp. (unstained). Parenchyma cells do not contain dark-stained substances.

**5.111** Red-stained substances in parenchyma cell walls and blue-stained substances in cell lumina of a rhizome of the fern *Osmunda regalis* (Astra blue/Safranin-stained).

#### *Stained substances in decaying wood*

**5.112** Red-stained substances in vessels of a KLSPNUPÄLKKLJH`PUNZ[LTVM[OLK^HYMZOY\I*Arctostaphylos alpina* (Astra blue/Safranin-stained).

**5.113** Brown-stained substances in ray cells of the black heartwood of a subfossil, waterlogged *Quercus* sp. (unstained). Fibers are slightly impregnated by yellowish substances.

**5.114** Black-stained substances in idioblasts in the phloem of the desert shrub *Krameria grayi* (Astra blue/Safranin-stained).

#### *Stained substances in subfossil wood Stained substances in cells of the phloem*

**5.115** Dark-red-stained substances in parenchyma cells of the phloem of the herb *Scorzonera graminifolia* (Astra blue/Safranin-stained).

**5.116** Red-stained substances around groups of sieve tubes in *Cichorium intybus* (Astra blue/ Safranin-stained).

**5.117** Unknown substances in the bark of the herb *Symphytum creticum* ^OPJO YLÅLJ[ PU polarized light.

*Stained substances in / around ducts*

**5.118** Yellow-stained substances, probably oil, around ducts in the phloem of the herb *Heracleum pinnatum* (Astra blue/Safranin-stained).

#### *Stained substances in cells of the phloem*

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# 6. Primary, secondary and tertiary meristems

#### *+LÄUP[PVUZ*

Meristems initiate longitudinal growth on tips of shoots and roots of plant bodies. Meristematic tissues consist of living cells, which produce new cells.

*Primary meristems* on shoot tips (apical meristems) are embryonic tissues, which originate from seeds. They produce the epidermis, the cortex, the leaves and the pith.

*Secondary meristems* originate from primary meristems and produce the xylem and phloem. The whole conducting system is called stele. The arrangement of vascular bundles within the JLU[YHSZ[YHUKKLÄULZ[OL[`WLVMZ[LSLProtostele: one vascular bundle (mosses); plectostele, polystele: several vascular bundles in the center (lycopods); eustele: concentrically arranged isolated or laterally connected vascular bundles in a ring (most dicotyledons). The term stele is used here only as an anatomical characteristic, not in relation to evolutionary stages.

*Tertiary meristems* originate from parenchymatic tissues, which are located within xylem, phloem and cortex.

Structural variation in meristematic products will be discussed in the following chapters.

#### *Production rates*

Production rates and proportions between shoot length, diameters and number of cells within the pith, xylem, phloem, cortex and phellem vary greatly. Most obvious are differences in the

**6.2** Short shoots on a long shoot of the conifer *Larix decidua*.

*Long and short shoots*

**6.3** Short and long shoots in *Fagus sylvatica*.

**6.1** Left: Primary, secondary and tertiary meristems in a twig of *Fraxinus excelsior*. Right: Schematic representation of primary, secondary and tertiary meristems. Reprinted from Schweingruber *et al.* 2008.

xylem (product of the cambium) and the product of the primary and secondary meristems (cortex, phloem, phellem).

*Proportions of xylem, phloem, cortex and phellem*

**6.4** Xylem to bark proportion of 10:1 in the mangrove *Rhizophora mangle*.

**6.5** Xylem to bark proportion of 2:1 in a young stem of *Quercus suber* (cork oak).

**6.6** Herb with a small pith. Pith to xylem and bark proportion of 1:6 in *Schistophyllidium bifurcum*.

portion of 1:10 in *Linum bienne*.

**6.10** Herb with a small cortex. Cortex to phloem proportion of 1:1 in *Bupleurum bladensis*.

**6.11** Herb with a large cortex. Cortex to phloem proportion of 8:1 in *Honkenia peploides*.

portion of 1:1 in *Draba cachartena*.

**6.7** Herb with a large pith. Pith to xylem and bark proportion of 1:0.3 in *Impatiens macroptera.*

**6.12** Herb with a small phellem. Phellem to phloem and cortex proportion of 1:5 in *Thesium arvense*.

**6.13** Herb with a large phellem. Phellem to phloem and cortex proportion of 4:1 in *Saxifraga oppositifolia*.

# 6.1 Primary meristems in apical zones – Initials of longitudinal and radial growth

# 6.1.1 Macroscopic aspect of primary meristems in apical shoots and roots – Grow higher, grow deeper

The origin of primary meristems is in the seed and all meristematic derivates in apical zones are also primary meristems. As soon as the seed germinates, the germs divide into a root and a shoot. Apical meristems occur on roots and shoots, on the primary as well as on all adventitious shoots and roots. In dormant as well as in active periods apical meristems in roots are not protected by buds but often wrapped in a mantel of hyphae (mycorrhiza).

Apical meristems occur in mosses and in all vascular annual and perennial plants.

(WPJHSTLYPZ[LTZPUZOVV[ZÄYZ[MVYTZ[LTZ^P[OSLH]LZ;OLZL meristems often change their TVKLHUKHSZVMVYTÅV^LYZHUK fruits (see also Chapter 10, Fig. 10.1).

**6.14** *Castanea sativa* seed-

ling with a primary shoot and

primary root.

**6.15** *Carpinus betulus* seedling with primary apical meristems.

*Apical meristems on shoots and roots*

**6.16** *Lonicera xylosteum* sapling with apical meristems on shoots and roots.

mycorrhiza

**6.17** Adult grass *Festuca rupestris* with apical meristems in the root zone.

**6.18** Moss *Polytrichum commune* with apical meristems on shoots. Rhizoides are covered by mycorrhiza.

*Apical meristems on adventitious shoots Apical meristems in buds*

**6.19** Injured stem of *Taxus baccata* with adventitious shoots, which contain apical meristems.

**6.20** Adventitious shoot on a *Fagus sylvatica* stem.

**6.21** Terminal shoot of *Acer pseudoplatanus* with a terminal bud and two adventitious buds.

**6.22** Terminal shoot of *Acer pseudoplatanus* with an apical meristem wrapped in undeveloped leaves and bud scales.

# 6.1.2 Apical shoot dynamics – Long and short shoots – Grow fast, grow slow

The aspect of plants is determined by the position of buds, the formation of long and short shoots, and the growth and death dynamic of apical meristems in shoots. The past activity of apical meristems on shoots can be determined by *bud scale scars*. They are overgrown wounds of deleted bud scales after leaf Å\ZOPUN;OL[LYTZZOVY[HUKSVUNZOVV[HYL]HN\LS`KLÄULK

Internodes between bud scale scars indicate an extreme *variability of longitudinal growth*. The long distances between bud scale scars, e.g. in long shoots, make it easy to *determine the age* of twigs. Short distances, or indiscernible bud scale scars at the outside of shoots (e.g. short shoots) hinder macroscopic age determination of shoots. However, microscopic age determination is possible with remaining *pith bridges* in the position of bud scale scars. *Ring counting* in short shoots is normally not reliable.

*abies.*

**6.23** Bud scales in *Picea*  **6.24** Short shoots in *Pinus mugo.*

*Macroscopic aspect of bud scale scars and short shoots*

**6.25** Bud scale scars in *Pinus mugo.*

**6.26** Bud scale scar in *Quercus robur.*

**6.27** Rhizome with short shoots and shoot scars in *7VS`NVU\TVMÄJPUHSL*.

**6.28** Short shoots in *Larix decidua*.

**6.29** Longitudinal section of a short shoot with annual bridges in the pith of *Larix decidua*.

**6.30** Cross section of *Larix decidua* short shoot. Annual rings are absent.

**6.31** Shoots of the arctic dwarf shrub *Cassiope tetragona*.

**6.32** Annual latewood bridge in the latewood of a shoot of *Cassiope tetragona*.

# 6.1.3 Shoot death and metamorphosis – The end of longitudinal growth: Twigs must die

*Normal shoot formation* happens in a period of two to four weeks (e.g. in *Quercus*) or lasts for the whole vegetation period (e.g. in *Populus*). However, the life span of shoots varies between one and more than 1,000 years.

Very soon after the formation of twigs, the *self pruning* process starts. The majority of twigs die and drop after a few years. Only a few dominating shoots remain on the plant for the whole lifetime on the individual. The survivors develop into branches and stems and form the crowns of trees, shrubs and herbs. The crown form is a result of selective death of twigs and branches. The principal stem is the winner of an extensive programmed dying process.

Twig shedding, also called twig abscission or cladaptosis, occurs principally in three forms:

a) Twigs dry out, get affected by fungi and drop even due to slight mechanical disturbances. This is the most common type of twig shedding.


The most common shoot transformation (metamorphosis) is the change from the vegetative to the generative form; from shoot to ÅV^LYMVYTH[PVU

Very common is the transformation of shoots into thorns. In this case the apical meristem loses its replication capacity and changes its mode to an extensive growth of secondary walls.

**6.33** Twigs lose their vitality, die, get affected by fungi and break off. *Corylus avellana*.

**6.34** Twigs break near the base at a predetermined mechanical weak zone in *Salix alba*.

**6.35** Compartmentalized wounds of broken twigs in *Fraxinus excel-*

**6.36** Compartmentalized wound in *Viscum album*.

**6.37** Predetermined breaking zone on a debarked twig of *Quercus robur*.

*cus robur*.

#### *sior*. *Macroscopic aspect of breaking zones*

**6.38** Scar of a broken twig in *Quer-* **6.39** Shed twigs of *Quercus robur*.

**6.40** Shed twig of *Gnetum gnemon*.

#### *Microscopic aspect of breaking zones in Quercus robur*

**6.41** Longitudinal section through a breaking zone. It is characterized I`HWVVYSPNUPÄJH[PVUIS\LaVUL

**6.42** Breaking zone with numerous crystals, polarized light.

**6.43** Anatomical structure below the breaking zone. This structure is typical for oak wood.

**6.44** Anatomical structure of the breaking zone. This structure is very different from the basal twig and characterized by the absence of ÄIYV\ZSH[L^VVKHUKZTHSS]LZZLSZ

#### *ƪǡfruits and thorns*

**6.45** *Carduus macrocephalus*

**6.46** *Sempervivum wulfenii*

Flowers and fruits on short shoots

**6.47** *Gentiana utricularia*

Flowers on long shoots, needles on short shoots

**6.48** *Pinus mugo*

**6.49** *Alnus viridis*

**6.50** *Buxus sempervirens*

**6.51** *Crataegus monogyna* **6.52** *Crataegus monogyna*

Thorns on long shoots

long shoot

short shoot

#### 48 *Ch 6. Primary, secondary and tertiary meristems*

# 6.1.4 Microscopic aspect of apical meristems of shoots and roots – Towards heaven and earth

The principal differences between apical root and shoot growth are shown below. Differences and similarities between apical meristems in roots and shoots are obvious in microscopic sections. This is here demonstrated on some dicotyledonous and monocotyledonous species.

abrasive soil particles. Central shoot cells primarily produce SLHMZI\[HSZVHSSWHY[ZVMÅV^LYZ

Omnipotent cells in the center of the shoot are common for the apex of roots and shoots. Bipolarity is also common, which means meristematic cells produce cells towards two axial directions: geocentric and heliocentric. Central cells of the roots The major difference between the two types is in the zone behind [OL[PW:VVUHM[LY[OLÄYZ[JLSSKPMMLYLU[PH[PVU[OLKPJV[`SLKVUous plant produces a cambial zone which separates the cortex with the initial leaves (leaf primordial) from the central cylinder.

produce the root cup, which determines the trajectory and protect the inner central cells. Root cup cells get sloughed off by

**6.53** Apical meristem of *Elodea canadensis*, a monocotyledonous water plant. Slide: J. Lieder. ground meristem

**6.55** No cambium in the monocotyledonous water plant *Elodea canadensis*. Slide: J. Lieder.

**6.56** Apical root meristems in the monocotyledonous plant *Allium ursinum*.

**6.57** (WPJHS YVV[ TLYPZ[LTZ VM HU \UPKLU[PÄLK dicotyledonous species. Slide: S. Egli.

**6.58** Cambium present in the dicotyledonous *Euphorbia cyparissias*.

# 6.1.5 From primary apical meristem to secondary lateral meristems in *shoots* – From longitudinal to radial growth

The transformation from primary to secondary meristems occurs in apical zones of roots and shoots of dicotyledonous plants. Initial apical meristems in herbs are mostly unprotected, while in trees they are mostly protected by bud scales.

The principles of secondary meristem formation are similar in all shoots of plants, however, in the detail there are many differences. The herb *Euphorbia chamaecyparissias* and the trees *Acer pseudoplatanus* and *Fraxinus excelsior* are discussed here.

In shoots, the formation of leaves in the cortex and the central pith are common. In all examined species, xylem and phloem MVYTH[PVU Z[HY[Z ULHY [OL HWL\_ HUK SPNUPÄJH[PVU VJJ\YZ SH[LY In detail: Cells of the central part of shoots of dicotyledonous plants remain in a parenchymatic, undifferentiated state (the pith). Around the primary meristem a ring of collateral vascular bundles is formed, which consists of protoxylem and protophloem. 3PNUPÄJH[PVUVJJ\YZPU*Euphorbia* 10 mm and in *Acer* and *Fraxinus* 2 mm behind the apex. Vessels of the protoxylem and metaxylem are characterized by annular and helical thickenings. Crystals of various forms are very frequent in *Acer* and *Fraxinus* but are almost absent in *Euphorbia*. Crystals play a role in cell wall formation.

**6.59** A mantel of poorly developed leaves wraps the meristematic apex in *Euphorbia cyparissias*.

**6.60** Bud scales wrap the meristematic apex in *Acer pseudoplatanus*.

*Protected in leaf sheath of bud scales – macroscopic aspect*

**6.61** External bud scales and internal initial leaves protect the meristematic apex in *Acer pseudoplatanus* and *Fraxinus excelsior*.

**6.62** Longitudinal section of *Euphorbia cyparissias*. **6.63** Longitudinal section of *Acer* 

*pseudoplatanus*.

**6.64** Longitudinal section of *Fraxinus excelsior*.

#### *Secondary meristem creates initial xylem and phloem*

annular thickenings

**6.65** Longitudinal section of a vascular bundle in the tip of *Euphorbia cyparissias.*

**6.66** The vessel wall structure changes from helical thickenings in the protoxylem to round bordered pits in the secondary xylem of *Cycas revoluta*.

*Ƥ*

**6.67** Cross section near the tip of a young shoot of *Acer pseudoplatanus*. First vascular bundles are formed but there collateral vascular bundle

PZUVSPNUPÄJH[PVU

**6.68** Longitudinal section of a young shoot of *Euphorbia cyparissias* with initial vas-

cular bundles.

**6.69** Cross section of *Euphorbia cyparissias* 10 mm behind the tip. The xylem of the vascular bundles already contains HML^SPNUPÄLKÄILYZ

**6.70** Vegetation point within a bud of *Acer pseudoplatanus*, polarized light.

*Cell-wall formation and calcium oxalate crystals*

**6.71** Cambial zone of *Acer pseudoplatanus*, polarized light.

**6.72** Cambial zone of *Fraxinus excelsior*, polarized light.

# 6.1.6 From primary apical meristem to secondary lateral meristems in *roots* – From longitudinal to radial growth

Differentiation between shoot and root takes place in the socalled root collar, the zone between the cotyledons and the root which can be found in herbs, shrubs and trees. Shoots are characterized by a pith, while roots have none.

In contrast to the shoot, apex cells of the root differentiate very soon after their formation, xylem towards the inside and phloem towards the outside. Root apex cells behave like a secondary meristem. Therefore the roots of dicotyledonous plants have no pith. However, the width of the transition zone between the WP[OÄSSLKZOVV[HUK[OLWP[OSLZZYVV[]HYPLZIL[^LLUTTHUK 20 cm.

**6.73** Root collar of *Tordylium apulum*.

**6.74** Root collar of *Chenopodium opulifolium*.

**6.75** Shoot, with a pith, of *Euphrasia* sp.

**6.76** Root, without a pith, of *Euphrasia* sp.

#### *Shoot and root in a conifer Shoot and root in a dicotyledonous tree*

with a pith.

500 μm

**6.78** Cross section of a shoot of *Picea abies* 

**6.77** Sapling of *Picea abies.* **6.80** A 30 cm-tall sapling of **6.79** Cross section of a root, 10 cm below the ground, of *Picea abies* without a pith.

*Fagus sylvatica.*

**6.81** Cross section of a shoot with a pith, in the upper part of the germination stem of *Fagus sylvatica*.

**6.82** Cross section of a root without a pith, 10 cm below the ground, of *Fagus sylvatica*.

### 6.1.7 From primary apical meristem in shoots to roots in plants without cambium (*monocotyledons*)

The taxonomic and morphological diversity is enormous within the monocotyledons, be it in shoots, rhizomes or roots. Dramatic anatomical changes occur along the stem axis. Each section is characterized by typical anatomical structures.

Cells of the apical meristem of shoots and rhizomes differentiate very soon after their formation into parenchyma and isolated closed vascular bundles (no cambium). The bundles in [OLÅV^LY Z[HSRculm) are collateral, those in the rhizome in general concentric. Cells of the apical meristem of roots form a central vascular cylinder and a cortex. The cylinder is surrounded by an endodermis and a pericycle. The pericycle occasionally initiates lateral roots. Vascular bundles are located in the central cylinder inside an endodermis.

This is shown here for a few species from different families. However, the anatomical diversity is much larger.

#### **6.83** Young apical meristem in *(ZWHYHN\ZVMÄJPUHSPZ*.

**6.84** Rhizome of *Juncus conglomeratus*.

vascular bundle

**6.85** Rhizomes of *Hedychium gardnerarum*.

**6.86** Polar root of *Plantago maritima*.

#### *Morphological and anatomical stem structure of shoots, rhizomes and roots* rudimentary

vascular bundle

#### *Carex pendula,* Cyperaceae

**6.87** Flower stalk of *Carex pendula*.

*ƪ* and *conglomeratus*, Juncaceae

vascular bundle endodermis pith

**6.88** Cross section of a triangular culm. Vascular bundles are located

at the periphery.

**6.89** Cross section of a rhizome. Concentric vascular bundles are central cylinder 250 μm 250 μm 250 μm

> located in the central cylinder inside of a thick-walled endodermis.

**6.90** Cross section of a root. Vascular bundles are located around a cylinder

**6.94** Cross section of a root of *Juncus conglomeratus*. Vascular bundles are located around a thick-walled JLU[YHS ÄIYPSSVZL JLU[LY;OL JVY[L\_ contains large aerenchymatic tissue.

vascular bundles 500 μm

**6.91** Culms of *1\UJ\ZPUÅL\_\Z*. **6.92** Cross section of a culm of *1\UJ\Z PUÅL\_\Z*. Large and small vascular bundles alternate at the periphery.

**6.93** Cross section of a rhizome of *Juncus conglomeratus*. Vascular bundles are located in the central cylinder inside a thick-walled endodermis.

[OPJR^HSSLKJLU[YHSÄIYPSSVZLJLU[LY cortex

central

endodermis

cork

cortex cortex

#### *Phoenix canariensis*, Palmaceae

**6.95** *Phoenix canariensis*

**6.96** Cross section of a vegetation point from where palm syrup is harvested.

**6.97** Cross section of a root. Vascular bundles are located around a thick- ^HSSLKJLU[YHSÄIYPSSVZLJLU[LY

#### *Structure of vascular bundles in shoots, rhizomes and roots*

**6.98** Closed collateral vascular bundle in a shoot of *Carex pilosa*. The xylem consists of a group of protoxylem and a few lateral metaxylem vessels. The phloem consists of sieve tubes and companion cells. The vessels are surrounded by a layer VMÄILYZ

**6.99** Concentric vascular bundle in a rhizome of *Carex pilosa*. Vessels surround a central group of ZPL]L [\ILZ( ZTHSS ZOLH[O VM ÄILYZ Z\YYV\UKZ the vascular bundle.

metaxylem 250 μm

vessel

**6.100** Closed collateral vascular bundles in a root of *Carex pendula*. The closed collateral vascular bundles are located inside of a thick-walled endodermis.

**6.101** Closed collateral vascular bundle in a shoot of *Juncus arcticus*. The xylem consists of a group of protoxylem and a few lateral metaxylem vessels.

ÄILYZ ÄILYZ ÄILYZ vessel sieve tube

**6.102** Concentric vascular bundle of a rhizome of *Juncus arcticus*. Vessels surround a central group of sieve tubes and companion cells. A sheath of [OPJR^HSSLKÄILYZZ\YYV\UKZ[OL]HZJ\SHYI\UKSL

xylem phloem vessel xylem phloem endodermis

**6.103** Separated xylem and phloem inside a thick-walled endodermis in a root of *Juncus conglomeratus*.

# 6.1.8 From primary apical meristem in shoots to roots in vascular *spore plants*

There is a great taxonomic and morphologic variety within the vascular spore plants, e.g. the lycopods, spikemosses, horsetails and MLYUZ 3P[[SL ]HYPH[PVUVJJ\YZPU[OLÄYZ[[OYLL\UP[ZOV^ ever, it is tremendous within the ferns. Major vascular spore plants have no secondary growth but anatomical changes occur along the stem axis. Products of apical meristem in shoots are leaves, often with sporophytes, and rhizomes. The product of geotropic apical meristem is the root.

The plant size and morphological variability is rather small in spikemoss (*Selaginella*), clubmoss (*Lycopodium*) and in horsetails (*Equisetum*). All types form long shoots and rhizomes with thin roots. In contrast, size and morphological variability is extremely large in ferns. All plant parts have concentric vascular bundles with the xylem in the center. Their bundles are surrounded by a cortex. The form varies from round to long oval. The number of vessels is normally high in *Selaginalla*, clubmosses and ferns. It is reduced to a few vessels in horsetails. This section presents an overview. More details are shown in Chapter 7.

#### *Macroscopic aspect of the whole plant*

**6.104** Perennial prostrate shoots of the spikemoss *Selaginella denticulata*.

**6.105** Fertile annual shoots of the clubmoss *Lycopodium clavatum*.

**6.106** Fertile annual shoots of the horsetail *Equisetum telmateia*.

**6.107** Sterile annual shoots of the horsetail *Equisetum hiemale*.

**6.108** Tree fern *Cyathea cooperi*.

**6.109** Climbing fern *Lygodium* sp.

**6.110** Hemicryptophytic fern *Blechnum spicant*.

**6.111** Hydrophytic fern *Marsilea quadrifolia*.

#### *Anatomical structure of shoots, rhizomes and roots*

**6.112** Shoot of *Selaginella* sp. with three vascular bundles.

**6.113** Shoot of *Lycopodium alpinum*. Irregularly distributed vascular bundles in a central cylinder (stele).

**6.114** Shoot of *Equisetum hiemale*. Circular arranged, round vascular bundles.

**6.115** Root of *Equisetum arvense*. One concentric vascular bundle.

**6.116** Stem cross section of the tree fern *Cyathea cooperi*.

**6.117** One central vascular bundle in the liana-like fern *Lygodium* sp.

**6.118** Fine root of the hemicryptic fern *+Y`VW[LYPZ ÄSP\_THZ* with a single concentric vascular bundle. 50 μm

phloem xylem

◂ **6.119** Basal part of the hemicryptic fern *Polystichum lonchitis*. Irregularly formed vascular bundles are arranged around the pith.

**6.120** Microscopic cross section of a petiole of the hydrophytic fern *Marsilea quadrifolia* with collateral vascular bundles.

**6.121** Round vascular bundle in a leaf of *+Y`VW[LYPZÄSP\_THZ*.

#### *Structure of vascular bundles in shoots, rhizomes and roots*

**6.122** Long oval vascular bundle in a shoot of *Selaginella* sp.

**6.123** Long oval vascular bundle in a stem of the tree fern *Cyathea cooperi*. endodermis endodermis endodermis xylem xylem phloem phloem

**6.124** Round vascular bundle with reduced xylem in the hemicryptophytic horsetail *Equisetum hiemale*, polarized light.

# 6.1.9 Pericycle and endodermis – Separation of central cylinder and cortex

Cortex and central cylinder (stele) in roots and rhizomes of monocotyledonous and dicotyledonous plants are separated by a pericycle and an endodermis. The pericycle is the outermost layer of the stele and the endodermis is the innermost cell layer of the cortex. The pericycle is a meristematic relict of the primary root meristem; it keeps its protoplast. Most of the time it is in a dormant state, but if it is in a meristematic mode it produces lateral roots. In young states it also initiates the cork cambium. The endodermis primarily regulates hydrological differences between the central cylinder and the cortex. It maintains the root pressure and protects the central vascular bundles from toxic substances, which occasionally occur in the cortex.

Only optimally developed endodermis and pericycle zones in a few roots are described in textbooks. In reality endodermis and WLYPJ`JSL HYL VM[LU \UYLJVNUPaHISL VY KPMÄJ\S[ [V KPZ[PUN\PZO Also, the anatomy of endodermis varies. Presented here are a few "unproblematic" examples.

In many monocotyledonous plants the cell walls are extremely thick-walled on the inner and lateral sides. Often described but rarely occurring is the endodermis with Casparian strips. The Z[YPWZMVYTHSPNUPÄLKIHUKVMYHKPHSHUK[YHUZ]LYZL^HSSZ

# pericycle new shoots central cylinder cortex

**6.125** Pericycle separates central cylinder and cortex and initiates new lateral shoots in the rhizome in *Triglochin palustris*.

**6.126** Pericycle cells with nuclei in *Triglochin palustris*. cortex

**6.127** Pericycle with nuclei, surrounding a concentric vascular bundle in *Polypodium vulgare.* central cylinder central cylinder central cylinder

*Pericycle and endodermis Location of endodermis*

**6.129** A thick-walled endodermis separates the cortex from the central cylinder in *Carex appropinquata*.

**6.130** Thick-walled endodermis in the rhizome of *Juncus gerardii*. central cylinder cortex

**6.131** Thick-walled endodermis in the shoot of *Potamogeton gramineus*.

**6.132** Location of Casparian strips around vascular bundles in *Equisetum hiemale*.

#### *Structure of endodermis Endodermis with Casparian strips*

**6.128** Distinct pericycle and endodermis in *Eleocharis* 

*palustre.*

50 μm

**6.133** Endodermis of *Equisetum hiemale* with Casparian strips around vascular bundles.

Casparian strip cortexendodermisvascular bundle25 μm

**6.134** Endodermis of *Equisetum hiemale* with Casparian strips, polarized light.

# 6.2 Secondary and tertiary meristems and radial growth – Cambium and cork cambium

# 6.2.1 Macroscopic aspect of radial growth and xylem coloration

### – Stems get thicker

Stem thickening occurs through the lateral secondary meristem. This is the cambium, which is located between the xylem and phloem. In most conifers and dicotyledonous plants the cambium forms a mantle around the xylem. Plants with successive cambia (several active cambia) are a special case.

Years after wood formation, the inner part of the stem loses its conducting capacity. This is the moment when the stem differentiates into sapwood and heartwood. The peripheral sapwood conducts water and contains living parenchyma cells. In contrast, the heartwood does not conduct water and all cells are dead. Here, parenchyma cells often contain phenolic substances which play an important part in biological defense mechanisms. The width of the sapwood is generally proportional to the transpiring leaf area: the more leaves in the tree crown, the larger the sapwood.

A few groups of stem cross sections can be differentiated macroscopically.

#### *Species with colored heartwood*


#### *Species without colored heartwood*


Irregularly shaped discolorations are related to biological attacks. Different colors, textures and brilliance of heartwood, as well as color differences between heart- and sapwood are IHZPJMLH[\YLZMVYTHJYVZJVWPJ^VVKPKLU[PÄJH[PVU;OPZPZWLYfectly presented in the old *Woodbook* by R.B. Hough, republished in 2002.

The outline of stems varies from round (most trees), to eccen- [YPJSLHUPUN[YLLZ[VÅ\[LKI\[[YLZZLKZ[LTIHZPZHUKZX\HYL Multiple stems occur mainly in perennial herbs. The bark thickness (phloem, cortex, cork) in relation to the xylem is very variable. The texture in transverse and longitudinal sections is ZWLJPLZZWLJPÄJ VY THPUS` YLSH[LK [V [OL Z[Y\J[\YL VM HUU\HS rings and rays.

**6.135** One cambium is located between the central xylem and the peripheral bark in an 11-yearold conifer twig of *Pinus sylvestris*.

*Location of the cambia*

**6.136** One cambium is located between the central xylem and the peripheral bark of a four-yearold arctic herb, *Cerastium arcticum*.

**6.137** Several peripheral cambia form several ÄILY HUK WHYLUJO`TH IHUKZ K\YPUN VUL `LHY This plant of *Haloxylon persicum* is approximately 10–12 years old.

#### *Sapwood and heartwood*

**6.138** A belt of light sapwood surrounds the brown heartwood in the center of the conifer *Pinus sylvestris*.

**6.139** A belt of light sapwood surrounds the dark brown heartwood in the center of the deciduous tree *Rhamnus cathartica*.

**6.140** Heartwood and sapwood are not differentiated by color differences in the deciduous tree *Carpinus betulus*.

#### *Discolorations are defense reactions Biological resistance*

**6.141** Living parts of stems react to injuries with the formation of dark-stained phenolic substances. Compartmentalized overgrown injury in *Acer pseudoplatanus*.

**6.142** "Splash heartwood" (German "Spritzkern") in *Fagus sylvatica* is a sign of bacterial infections.

**6.143** Heartwood of *Pinus sylvestris* is more resistant against fungal infestations than sapwood.

**6.144** Eccentric stem due to compression wood formation in *Picea abies*.

*Outline of stems*

**6.145** Fluted stem of the shrub *Crataegus* sp.

**6.146** Square stem of the tree-like succulent *Euphorbia ingens*.

#### *Bark thickness*

**6.147** Thin bark in relation to the xylem in *Pinus mugo*.

**6.148** A large cork belt and a small phloem surround the xylem in *Quercus suber*.

stele

**6.149** A large cortex surrounds the xylem of the herb *Heracleum pinnatum*.

#### *Bark thickness Multiple stems*

**6.150** An extremely large cortex surrounds a very small xylem in the giant cactus *Carnegia gigantea* (dry cross section).

**6.151** An extremely large aerenchymatic cortex surrounds a very small stele in the water plant *Menyanthes trifoliata*.

**6.152** Stems of small cushion plants like *Saussurea glanduligera* in alpine zones are composed of many small individual stems.

**6.153** The cutting direction of stems in parquet ÅVVYPUN OPNOSPNO[Z [OL HUU\HS YPUN Z[Y\J[\YL VM the wood of *Quercus* sp.

#### *Wood texture*

**6.154** The radial cutting direction shows the Z[Y\J[\YLVM[OLYH`ZPUWHYX\L[ÅVVYPUNVM*Quercus* sp.

**6.155** The section through burls with sleeping buds shows the unusual wood structure in an antique chest, made of a deciduous tree species.

# 6.2.2 Microscopic aspect of radial growth (conifers, dicotyledonous plants and palm ferns) – An overview

#### *Radial growth of conifers and dicotyledonous plants with one cambium*

As soon as the cambium is active it forms secondary tissue: the secondary xylem and the secondary phloem. The xylem is different from the one that was formed by the primary meristem: Tracheids and vessels do not have any annular or thick annular or spiral thickenings. The cambium transfers the single vascular bundles into a continuous ring of xylem and phloem.

#### *Radial growth of some dicotyledonous plants with several (successive) cambia*

Numerous species, especially those in the families of Amaranthaceae and Caryophyllaceae, form and maintain several cam-IPH(ZZVVUHZ[OLÄYZ[JHTIP\TPZMVYTLKP[WYVK\JLZH\_`SLT and a phloem like in all other dicotyledonous plants. However, this stage lasts only for a short time. For growing in thickness, parenchyma cells outside of the phloem get reactivated and form a new cambium, which again produces a xylem and a phloem. This process repeats itself over many years. The lifetime of successive active cambia is limited but their effect is preserved in the anatomical structure of the stem.

#### *Radial growth of a few monocotyledonous plants*

Secondary radial growth occurs in a few families of monocotyledonous plants, e.g. in *Dracaena* sp. and *Yucca* sp. As in the group with successive cambia, parenchyma cells in the primary bark (cortex) get reactivated and form—towards the center—a continuous belt of parenchyma cells around the stem. A few of them remain active and form vascular bundles.

*sylvestris.*

**6.156** One cambium produces the xylem and phloem. One-year-old shoot of the conifer *Pinus*  *Radial growth with one cambium* cambium

**6.157** One cambium produces the xylem and phloem. Three-year-old shoot of the dicotyledon-

initials of vascular bundles

**6.158** The secondary cambium merges the primary vascular bundles into a continuous belt in a twig of *Pinus sylvestris.*

**6.159** The annual dicotyledonous herb *Chenopodium botrys*, Amarantha ceae.

*Radial growth with several cambia (successive cambia)*

ous tree *Alnus glutinosa.*

**6.160** Cross section of the basal stem of the annual dicotyledonous herb *Chenopodium botrys*. Several cambia (blue rings) produce xylem and phloem simultaneously.

**6.161** The monocotyledonous tree *Dra caena draco*, Asparagaceae.

# 6.2.3 Production and enlargement of new cells in the xylem of a thickening stem – The need for more and larger cells

Stem thickening is related to an increase and enlargement of axial elements, like parenchyma cells, tracheids and ÄILYZHUK an increase and enlargement of rays. The number of cells at the periphery is lower in smaller than it is in thicker stems. Due to increased leaf area and plant weight, larger plants need more water-conducting and stabilizing cells than smaller plants.

The process of stem thickening is anatomically expressed by H\_PHS JLSS PUP[PH[PVUZ [YHJOLPKZ ÄILYZ WHYLUJO`TH JLSSZ HUK vessels), new ray initiations and dilating rays. This is underlined by Bailey 1923 who counted 794 tracheids in a one-year-old stem of *Pinus strobus* and 32,000 tracheids in a 60-year-old plant. The thickening process is also accompanied by cell death; cross sections of conifers show the disappearance of cell rows.

With the insertion of new ray cells and the enlargement of primary rays (ray dilatation), radial strength as well as storage capacity increases. The initial point for new cells is located in the cambial zone. New tracheids divide longitudinally. New rays are initiated in living tracheids, which change their mode; instead of longitudinal separation into tracheids, a small ray cell splits off laterally.

**6.163** Initiation (circles) and disappearance (arrows) of tracheids in the young root of a 20 m-tall *Pinus nigra* tree.

*More cells and larger cells*

**6.164** 0UP[PH[PVUHUKLUSHYNLTLU[VMÄILYZPU[OL stem of a 7 cm-tall annual herb *Erophila verna*.

**6.165** Dilatation of large rays by insertion of new ray cells and enlargement of cells in *Rosa pendulina*. 3 cells wide

**6.166** Dividing of tracheids in longitudinal direction (left) and separation of a single ray cell from a tracheid (right) in *Pinus sylvestris*.

### ͞Ǥ͚Ǥ͜ơ*xylem* – The multifunctional stem center

Genetic information, physiological needs and ecological triggers form the background for all anatomical structures. The anatomical expression of many biological and biochemical processes are presented in the following. Basic wood and bark formation processes already existed in late Devonian times (370 million years ago) in conifer-like stem structures. These ancient principles have been transferred to the phylogenetically young angiosperms (140 million years ago).

The following basic processes can be observed in conifers and angiosperms: cell-type differentiation, cell-wall differentiation, nuclei differentiation, cell-wall enlargement, cell-wall thickening and SPNUPÄJH[PVU.LUL[PJPUMVYTH[PVUKL[LYTPULZ[OLNLUeral arrangement and distribution of cell types.

Cambium mother cells form anatomically undifferentiated phloem and xylem mother cells. These three cell types are anatomically combined in the cambial zone.

;OLÄYZ[HUH[VTPJHSL\_WYLZZPVUVM*cell differentiation* appears within the cambial zone. Initial stages of conifers show tracheids, rays and resin ducts in the xylem and sieve cells and parenchyma cells in the phloem. In addition, angiosperms form vessels. In relation to space and physiological needs, some cell types have priority; resin ducts push aside tracheids and rays HUKPUHUNPVZWLYTZ]LZZLSZKPZWSHJLÄILYZHUKYH`Z;OLKPMferentiation of the *nucleus form* takes place along with the celltype differentiation.

*Cambial zone*

**6.167** Cambial zone of the conifer *Picea abies* in the dormant state. Cambium initials, xylem mother cells and phloem mother cells are not anatomically differentiated.

*First-formed xylem cells*

**6.168** Cambial zone of the angiosperm *Ficus carica* in a dormant state. Cambium initials, xylem mother cells and phloem mother cells are not anatomically differentiated.

**6.169** First-formed cells in the earlywood of a conifer. Tracheids and ray cells can be recognized on the xylem side and parenchyma cells on the phloem side.

**6.170** First-formed cells in the earlywood of the conifer *Larix decidua*. Formed are 2–3 rows of tracheids and a large cavity for a resin duct. The duct has the spatial priority.

**6.171** First-formed cells in the earlywood of the angiosperm *Ficus carica*. Formed are JLSSZ VM ÄILYZ HUK H SHYNL ]LZZLS;OL vessel has the spatial priority.

**6.172** Round nuclei in cambial initials, phloem ray cells and axial phloem parenchyma cells; axially elongated nuclei in tracheids and phloem initials; radially elongated nuclei in xylem ray parenchyma cells in the conifer *Picea abies*. cambial zone xylem phloem

The xylem and phloem mother cells already contain the information about their *cellular pathway* before their anatomical expression. The differentiation capacity of anatomically undifferentiated mother cells is very dynamic and changes within short time periods. This is very obvious in angiosperms. In one TVTLU[\_`SLTTV[OLYJLSSZKP]PKLPU[VÄILYZHUKPU[OLUL\_[ PU[V ]LZZLSZ OV^L]LY [OL JOHUNL MYVT ÄILY VY ]LZZLS [V YH` cells is inexistent or rare.

**6.173** Cambial initials periodically determine which cell type has to be MVYTLK:VTL[PTLZ[OLPUP[PHSKPMMLYLU[PH[LZPU[VHÄILYHUKZVTL[PTLZPU[V a vessel. *Fraxinus excelsior*.

**6.174** Cambial initials periodically determine which cell type has to be MVYTLK;OL YHKPHS JVU[PU\H[PVU VM MVYTLK JLSS [`WLZ ÄILYZ ]LZZLSZ HUK parenchyma cells) seems to be chaotic, however, the general pattern is typical for *Viscum album*.

The second phase of radial growth is radial and axial *cell enlargement*. The process takes place in the stage of primary wall formation, however, each cell type has its own expand-PUN JOHYHJ[LYPZ[PJ9HKPHSS` ÄILYZ L\_WHUK WVVYS` I\[SVUNP[\ dinally, they do so extensively (up to nine times), at which the axial ends become wedged. Axial parenchyma cells radially also expand poorly, and longitudinally only slightly; they generally remain in the state of the initials. Ray cells radially expand extensively, while longitudinally hardly at all. Vessels expand in radial, tangential and longitudinal direction.

**6.175** Derivates of xylem mother cells axially LUSHYNLH[KPMMLYLU[YH[LZ0UP[PHSÄILYHUKYH`JLSSZ have the same radial dimension. Tracheids expand slightly; ray cells expand extensively. *Picea abies*.

**6.176** Derivates of xylem mother cells axially enlarge at different rates. Parenchyma cells stay TVYLVYSLZZ[OLPYPUP[PHSSLUN[OOV^L]LYÄILYZ axially expand extensively. *Ulmus laevis*.

**6.177** After elongation, the axially elongated tracheids in *Picea abies* become wedged.

Simultaneously with the wall expansion, *cell-wall differentiation* takes place. This is demonstrated here on bordered pits in conifers and dicotyledonous angiosperms. First, TPJYVÄIYPSZ MVYT H submicrosopic comb-like pattern. Next, the outer border of the bordered pits in tracheids can be observed under a microscope. 0[PUKPJH[LZ[OLÄUHSZPaLVM[OL[YHJOLPK^HSS+PMMLYLU[PH[PVUVM the pits takes place over the course of a few weeks into pits with SPNUPÄLKIVYKLYZHUK\USPNUPÄLKtori. Tori of conifers lignify when they are no longer involved in the water-conducting process. This normally occurs at the sapwood-heartwood boundary.

ray tracheid pit ray parenchyma cell 25 μm 25 μm 25 μm

*ơ*

**6.178** Formation of bordered pits in tracheids of the conifer *Picea abies*-PYZ[\USPNUPÄLKYV\UK small pit borders appear. Development of the pit IVYKLYZVJJ\YZZPT\S[HULV\ZS`^P[OSPNUPÄJH[PVU WP[IVYKLYZSPNUPÄLK initial bordered pit

nucleus

**6.179** ;OL ÄUHS Z[HNL VM [OL KL]LSVWTLU[ ULHY the cambium shows pits in axial tracheids and YH`[YHJOLPKZ^P[OSHYNLIVYKLYZHUK\USPNUPÄLK tori in a radial section of *Pinus sylvestris*.

**6.180** Final stage of the development of bordered pits in a transverse section of *Picea abies*. The IVYKLYZHYLSPNUPÄLK[OL[VYPHYL\USPNUPÄLK

**6.181** ;YHJOLPKWP[Z^P[OPUKPZ[PUJ[\USPNUPÄLK[VYPIS\LHUK YH`[YHJOLPK WP[Z^P[OKPZ[PUJ[\USPNUPÄLK[VYPPUHJYVZZZLJ[PVUSLM[HUKYHKPHSZLJ[PVU (right) of *Drimys piperita*.

**6.182** Intervessel pits with distinct tori and vessel-ray pits with distinct, \USPNUPÄLK[VYPPUHJYVZZZLJ[PVUSLM[HUK[HUNLU[PHSZLJ[PVUYPNO[VM*Viscum album*.

*Cell-wall thickening and SPNUPÄJH[PVU* are the last processes to [HRL WSHJL 3PNUPÄJH[PVU VJJ\YZ PTTLKPH[LS` HM[LY VY K\YPUN the formation of the cellulose matrix. With the formation of the cellulosic matrix of the secondary wall, cell walls become thicker. This process is accompanied by the incrustation of lignin. It starts in primary walls in the corner of cells and expands towards the lumen of the cell. In conifers, all diffuse-porous ZWLJPLZHUKPU[OLSH[L^VVKVMYPUNWVYV\ZZWLJPLZSPNUPÄJH[PVU VJJ\YZMYVU[SPRLILOPUK[OLJHTIP\T;OLSPNUPÄJH[PVUWYVJLZZ is different in the earlywood of ring-porous species because the MVYTH[PVUVMLHYS`^VVK]LZZLSZPZHZ`UJOYVUV\Z3PNUPÄLK]LZ-ZLSZHUK[OLZ\YYV\UKPUNÄILYZVM[OLÄYZ[MVYTLKJLSSZZ[H`ZPKL I`ZPKL^P[OUL^S`MVYTLK\USPNUPÄLK]LZZLSZ

**6.183** 3PNUPÄJH[PVU VM JLSS^HSSZ VM ]LZZLSZÄILYZ HUK YH`ZPU*Salix fragilis*. Ontogenetically young cells near the cambium are thin-walled HUK\USPNUPÄLK

cambial zone

**6.184** Cellulose-matrix formation PZ L\_WYLZZLKI` YLÅLJ[PVUPUWVSHYized light. Cellulose formation starts immediately after cell expansion is completed.

**6.185** \*LSS^HSS [OPJRLUPUN SPNUPÄ-JH[PVUVMJLSS^HSSZVM]LZZLSZÄILYZ and rays in *Buxus sempervirens*. All cells near the cambium are thin- ^HSSLKHUK\USPNUPÄLK;OL[OPJRLU-PUNHUKSPNUPÄJH[PVUWYVJLZZVJJ\YZ within 8–10 cell rows.

**6.186** Cellulose-matrix formation PZ L\_WYLZZLKI` YLÅLJ[PVUPUWVSHYized light. Cellulose formation starts immediately after cell expansion is completed.

**6.187** Cells with protoplasts continuously produce cellulosic matrix and lignin. This process occurs during several months in *Picea abies*.

**6.188** Continuous cell-wall thicken-PUN HUK SPNUPÄJH[PVU PU [OL JVUPMLY *Abies alba.*

**6.189** Continuous cell-wall thicken-PUN HUK SPNUPÄJH[PVU PU [OL KPMM\ZL porous angiosperm *Prunus padus.*

**6.190** \*LSS MVYTH[PVU HUK SPNUPÄcation occurs at different times in *Fraxinus excelsior*.

# 6.2.5 Timing of xylem formation

Ring formation in plants of seasonal climates is principally divided into a dormant and an active phase. Cell division by the cambium and cell growth (enlargement, SPNUPÄJH[PVUHYLWHY[ of the active phase. The beginning of cambial activity is indicated by a large, anatomically undifferentiated cambial zone, while this zone is much smaller during dormancy. Genetic fac- [VYZ KPJ[H[L[OL YO`[OT VM JLSS[`WLMVYTH[PVULNPU[V ÄILYZ or vessels), and environmental factors modify the general principle and regulate the quantity and the size of cells.

The duration and occurrence of a ring-formation period varies\*. It depends on:

Ń *Taxonomy*. For example, in 2001, the cambial activity of *Prunus padus* trees in the lowland of temperate zones began in week 13 (late March), while that of *Juglans regia* trees began in week 23 (early June; Schweingruber & Poschlod 2005). :WLJPLZZWLJPÄJKPMMLYLUJLZPU[PTPUNPUOLYIZJHUILT\JO larger, e.g. *Erophila verna* HU HUU\HS ZTHSS OLYIM\SÄSSZP[Z stem-formation cycle within three weeks in early March, and the small *Euphrasia cuspidata* within three weeks in August.


*Cambium width*

**6.191** Small cambial zone during the dormant period in *Larix decidua*.

**6.196** Large cambial zone in the active period in the earlywood of *Larix decidua*.

**6.192** Ring formation is completed within three weeks in February in the annual herb *Erophila verna*.

*Ƥ*

**6.193** Ring formation occurs in temperate cli-TH[LZ K\YPUN MV\Y [V Ä]L months from April to September. *Acer campestre*.

**6.194** Almost completed ring in the herb *Thlaspi perfoliatum* at the end of February in the Mediterranean zone of Cyprus.

**6.195** Completed last ring in *Arctostaphylos alpina* at the beginning of August on a sunny slope in Greenland.

**6.200** Incomplete last ring in *Dryas octopetala* at the beginning of August in a snow bed in Greenland.

**6.197** Ring formation is completed within four weeks in August in the annual herb *Euphrasia cuspidata*.

**6.198** Ring in formation in the tree *:JOLMÅLYHHI`ZZPUPJH* in a tropical rain forest. Ring formation lasts 11–12 months.

**6.199** Incomplete ring in the herb *Thlaspi perfoliatum* at the end of April in the temperate zone of Switzerland.

\*All examples discussed on this page relate to Northern Hemisphere seasons.

# 6.2.6 ơ*phloem* – The multifunctional stem periphery

Most cell formation processes described for the xylem also occur in the phloem: cell type differentiation, cell-wall differentiation, nucleus differentiation, cell-wall enlargement, cellwall thickening, SPNUPÄJH[PVUHUKMVYTH[PVUVMcrystals are the basic steps.

The following aspects are different than in the xylem (Huber 1961):


phloem by lateral sieve areas; perforation plates in vessels are replaced by axial sieve plates.

Ń The xylem normally forms a dense block of tissue onto which the phloem gets pushed, which results in collapsed sieve elements. As soon as sieve tubes die they collapse due to the higher turgor of neighboring parenchyma cells, the pressure from newly formed cells and/or the strength of the phellem belt. The processes primarily take place in the juvenile stage between the cortex and the xylem and in adult stages between the xylem and the rhytidome (isolated dead tissues formed by the phellogen). (Holdheide 1951)

Shown below are the principal changes during the thickening and aging process for a conifer, and a diffuse-porous and a ringporous angiosperm.

**6.201** Comparison of xylem and phloem rings in *Abies alba*. Xylem/phloem = 7:1, xylem rings dis- [PUJ[WOSVLTYPUNZVUS`KPZ[PUJ[PUÄYZ[ML^`LHYZ

**6.202** Juvenile bark of the conifer *Pinus sylvestris*. Characteristic are the small phloem, a large cortex and a small rhytidome.

**6.203** Adult bark of the conifer *Pinus sylvestris*. Characteristic are the large phloem, an absent cortex and a large rhytidome.

**6.204** Juvenile bark of the deciduous angiosperm *Fagus sylvatica*. Characteristic are a small phloem, a large cortex containing a continu-V\Z ÄILYZJSLYLPK ILS[ HUK H ZTHSS periderm.

**6.205** Adult bark of *Fagus sylvatica*. Characteristic are a large phloem with sclereid groups, a very small cortex with remnants of the juvenile ÄILYZJSLYLPKILS[HUKHZTHSSWLYPderm; rhytidome is absent.

**6.206** Juvenile bark of the deciduous angiosperm *Quercus robur*. Characteristic are a large phloem containing NYV\WZVMÄILYZHSHYNLJVY[L\_^P[OH JVU[PU\V\Z ÄILYsclereid belt and a small periderm; rhytidome is absent.

**6.207** Adult bark of *Quercus robur*. Characteristic are a large phloem, consisting of many bands of groups ÄILYZHUKHML^NYV\WZVMZJSLYLPKZ and a rhytidome; cortex is absent.

#### *Changing formation mode* ray ray

f

si

**6.208** Annual rhythms in *Abies alba* are indicated by a tangential row of early-bark parenchyma cells and several rows of late-bark sieve cells.

**6.209** Regular rhythms in *Juniperus communis* are indicated by thin-walled tangential rows of sieve cells, parenchyma cells (with U\JSLP HUK [OPJR^HSSLK ÄILYZ Annual growth rates are indistinct.

**6.210** Rhythms are indicated in *Sorbus chamaemespilus* by poorly differentiated zones of sieve tubes and parenchyma cells and distinct NYV\WZ VM ÄILYZ -PILYZ WYVIHIS` develop in the second year.

**6.211** Arrhythmic formation of sieve tubes, companion cells and parenchyma cells in *Buxus sempervirens*. Radial rows are not permanent due to aperiodic lateral cell divisions and cell death (circles).

**6.212** The general pattern in *Cotinus coggygria* changes periodically when the cambial mode changes to the production of resin ducts. In later stages, living parenchyma cells produce thick secondary walls (sclereids).

**6.213** Extensive cell enlargements of parenchyma cells in the cortex of *Abies alba*. The enlarged parenchyma cells produce slime.

#### *ơ Cell enlargement, tissue and ray dilatation* phellem dilatation dilated sclerotized ray

**6.214** Lateral cell divisions and cell-wall expansion increase the circumference of the stem wedgelike in *Lavatera acerifolia*.

**6.215** Ray dilatation and sclerotization in *Fagus sylvatica*.

# 

**6.216** Sieve tubes in the cambial zone of *Metasequoia glyptostroboides*. Calcium oxalate crystals seem to play a physiological role in the formation of the primary wall.

**6.217** Left: Adult sieve plates on radial walls in *Larix decidua*.

Right: Sieve plate on the axial end of a sieve tube in *Nelumbo nucifera*.

#### *Ǧơ Collapse of sieve tubes*

si

f

f

si

**6.218** Irregular pattern of collapsed sieve tubes in *Hippophae rhamnoides*. Sieve tubes in the cambial zone are not collapsed.

**6.219** Tangential lines of collapsed sieve tubes in *Laburnum anagyroides*.

# 6.2.7 Formation of tertiary meristems, the cork cambium – A new skin

Tertiary meristems determine the face of tree stems because the MVYTH[PVUTLJOHUPZTZHYLZWLJPLZZWLJPÄJ

#### *Periderms in the bark*

Most plants with secondary growth form a tertiary meristem, which is located somewhere in the bark: the *phellogen*. Toward the inside, the phellogen produces a few long-lived parenchyma cells, the *phelloderm*, and towards the outside, it produces various amounts of short-lived cork cells, the *phellem*. Their walls consist of cutin or suberin. Their origin are living parenchymatic cells. In young shoots, parenchyma cells of the cortex, and in older shoots, parts of the phloem get reactivated to meristems. The number of formed cells is normally much bigger towards the outside than towards the inside. The zone of phellogen, phelloderm and phellem is called *periderm*. All dead phloem

*Morphology of the bark*

**6.220** Juvenile shoots and adult bark in *Prosopis* sp.

**6.221** The phellem in *Acer griseum*

*Ƥ*

**6.224** Bark of *Pinus mugo*.

and cortex parts outside of the phellogen are called *rhytidome*. This formation mode occurs in all growth forms of conifers and dicotyledons.

With continuous stem thickening and the associated tension, the external phellogen and adjacent phloem and cortex parts KPL HUK UVYTHSS` ÅHRL VMM 9O`[PKVTLZ HYL ZWLJPLZZWLJPÄJ ;OLYLMVYL [YLL ZWLJPLZ JHU IL PKLU[PÄLK THJYVZJVWPJHSS` I` their bark: the face of the tree. Godet 2011 presents the bark of central European tree species.

Cork formation is essential for most perennial terrestrial plants because cork layers build a continuous mantle around the plant. It protects the plant lifelong against mechanical and biological damages.

*Size of the cork mantle*

**6.222** Large phellem in *Acer campestre*.

**6.223** Small phellem in *Taxus baccata*.

*Periderms formed in the cortex*

*Periderms formed in the phloem*

**6.225** Bark of *Carpinus betulus*.

**6.226** Bark of *Betula pendula*.

**6.227** Bark of *Pinus mugo*.

**6.228** Bark of *Alnus glutinosa*.

[OPUS`ÅHRLZVMM

70 *Ch 6. Primary, secondary and tertiary meristems*

#### *Periderms form lenticels*

Phellem layers seal the stem. The phellogen locally creates perforations in the young twigs by accelerated cork-cell production: the lenticels. Lenticels occur on young twigs and especially on roots in wet environments. The phellogen locally forms an external tissue with numerous intercellulars, which permit the entrance of air to the cortex.

**6.229** ;^PN ^P[O ÅV^LYZ VM *Forsythia suspensa*.

**6.230** Annual twig of *Acer pseudoplatanus* with lenticels.

phloemxylem 250 μm

**6.231** Lenticel in a twig of *Forsythia suspensa*.

**6.232** Lenticel in a root of *Alnus glutinosa*.

#### *Periderms in stem centers*

A special form of cork formation can occur, mainly in small, long-lived plants of certain families (e.g. Lamiaceae, Rosaceae, Fabaceae or Aceraceae) at high altitudes and northern latitudes. As soon as a plant is unable to maintain the metabolism of the stem as a whole, living parenchyma cells in the xylem get reactivated to form a cork cambium, the products of which separate part of the living tissue towards the inside. This process occurs repeatedly and forms an internal rhytidome inside the stem.

**6.233** *Potentilla nitida*, Rosaceae, a 5 cm-tall alpine plant with a longlived rhizome.

*Periderms in stem centers as protection layers*

**6.236** Rhizome of the alpine herb *Nepeta discolor*, Lamiaceae, with one active and four inactive central periderms.

**6.240** The central periderm of *Epilobium angustifolium*.

**6.237** *Epilobium angustifolium*, Onagraceae, with a long-lived rhizome.

**6.234** Rhizome of *Potentilla nitida* with a re-shaped, round stem. The original central part disappeared and the wood was sealed by a periderm.

living

**6.238** Rhizome of *Epilobium angustifolium* with a small living part and many central dead periderms.

**6.235** The new periderm in *Potentilla nitida* bridged vessel/parenchyma parts and enlarged rays.

**6.239** Living part of the rhizome of *Epilobium angustifolium*, with a central periderm.

#### *Periderms as protection layers*

**6.241** *Saussurea gnaphalodes*, Asteraceae, a 5 cm-tall alpine plant with a long-lived rhizome.

**6.242** Rhizome of *Saussurea glanduligera*, composed of many separated partial rhizomes.

**6.243** Stem separation by central periderms in *Potentilla crantzii*, Rosaceae.

**6.244** Stem compartments separated by secondary periderms in *Potentilla crantzii*.

#### *Periderms as breaking zones for leaves*

Just as important are the accelerated cork-formations at the break-off zones of leaves. Long before the leaves drop, the

**6.245** Leaf scar below a bud of *Acer pseudoplatanus*.

**6.246** Leaf scars in a cabbage stem, *Brassica oleracea*.

**6.247** Leaf of *Castanea sativa*, separated by a periderm.

**6.248** Periderm on a leaf scar in *Castanea sativa*.

# *Periderms as breaking zones for spines*

Spines are products of local periderms. In contrast to all other periderms they differentiate into special forms. Spines occur on stems, e.g. of *Bombax ceiba* (cotton tree), roses and others, and on fruits, e.g. of *Aesculus hippocastanum* (horse chestnut). The spine itself is a product of the hypodermis (cells just below the epidermis) and the breaking zone is the outermost part of the periderm, the phellem.

phellogen becomes active and forms a layer of phellem cells. As soon as the leaves drop, the potential wound is already sealed.

**6.249** Spines on the stem of *Bombax ceiba*, Malvaceae.

**6.250** Spines on a twig of *Rosa arvensis*, Rosaceae.

**6.251** Spine on a twig of *Rosa* 

*arvensis*.

peridermcortex cambium 100 μm

**6.252** Breaking zone of a spine of *Rosa arvensis*.

### *Breaking zones for leaves*

# 6.2.8 Life span and death of cells – Cells must die

In the following, the *programmed cell death* or *apoptosis* is anatomically described. *Genetically predetermined cell death* is behind all phenomena in which plants shed leafs, twigs or fruits. These processes have partially been described in Chapter 6.2.7 "Tertiary meristems". Death is part of any living organism. The physiologically driven dying process is called apoptosis or programmed cell death. The live span of the entire plant body is also genetically predetermined.

*Aging processes*, called *senescence*, lead to the death of central parts of the stem (heartwood formation) or of entire plant bodies. Annual plants sometimes survive for only a few weeks, while perennials live for up to 5,000 years.

*Externally induced cell death* is behind all phenomena in which pathological factors or extreme ecological conditions determine cell death. This dying process is called necrosis or necrobiosis and is described in Chapter 10.6.

#### *Programmed cell death within living parts of plants*

A healthy, functioning plant body is based on a perfectly designed balance of living and dead cells. Genetically induced programs activate enzymes (caspases), which determine the longevity of cells. Meristematic cells of clonal plants theoretically can live forever. The life span of their derivates varies within a time range of a few days up to more than 100 years; in reality only parenchymatic cells have such a long life span. Xylem and phloem mother cells, conducting tissues (tracheids, vessels, cork cells and sieve elements) and sclereids have a short live span. Illustrated in the following is the age mosaic of juvenile and adult tissues in conifers and deciduous angiosperms.

#### *Programmed cell death separates living and dead parts – Heartwood formation*

The macroscopic characteristics of heartwood are described in Chapter 6.2.1, and of heartwood substances in Chapter 5.6.5 (Fig. 5.99–5.118). For deeper insight into heartwood formation processes see Fromm 2013.

**6.253** Juvenile tissues in a nine-year-old twig of *Pinus mugo*. **6.254** Adult tissue in a 60-year-old stem of *Pinus sylvestris*.

**6.255** Juvenile tissue in a one-year-old twig of *Fraxinus excelsior*.

#### *Longevity of meristematic cells in the cambial zone*

**6.257** Conifer *Pinus sylvestris.*

**6.258** Dicotyledonous *Sambucus nigra.*

**6.256** Adult tissue in a 30-year-old stem of *Fraxinus ornus*.

# 6.3 Cambial variants – Phloem elements within the xylem

Within the groups of palm ferns (Cycadopsidae) and dicotyledonous angiosperms species exist in which the cambium does not constantly produce a centripetal xylem and a centrifugal phloem. This group principally contains two formation modes which each include many different subtypes.

*One cambium periodically produces centripetal bark elements* A phloem containing sieve cells, companion cells and parenchyma cells, or cork cells. The normal formation mode (vessels, ÄILYZWHYLUJO`THPZL\_WHUKLK[VLSLTLU[ZVM[OLWOSVLTVY the periderm.

#### *Several circular arranged cambia simultaneously produce xylem and phloem*

This group comes under the term plants with *successive cambia*. Within this unit there are principally two groups:


*Ƥǡǡǡ*

**6.259** The herb *Gaura lindheimeri*, Onagraceae.

**6.260** Tangential rows of groups of sieve tubes in *Gaura lindheimeri*.

**6.261** Group of sieve tubes with companion cells containing nuclei in *Gaura lindheimeri*.

**6.262** The shrub *Simmondsia chinensis*, Simmondsiaceae.

**6.263** Circular rows of groups of sieve tubes in *Simmondsia chinensis*.

**6.264** Groups of sieve tubes and parenchyma JLSSZ ^P[OPU H KLUZL ÄILY]LZZLS [PZZ\L PU *Simmondsia chinensis*.

vessel

#### *One cambium periodically produces xylem and bands of cork*

**6.265** The dwarf shrub *Artemisia tridentata*, Asteraceae.

**6.266** Tangential bands of cork cells between a ]LZZLSÄILY[PZZ\LPU*Artemisia tridentata*.

**6.267** Thin-walled cork cells in the xylem of *Tanacetum millefolium*.

**6.268** The monocotyledonous tree *Dracaena serrulata*, Asparagaceae.

**6.269** Xylem and cortex of *Dracaena serrulata*.

**6.270** Single concentric vascular bundles between rays in *Dracaena serrulata*.

**6.271** Isolated vascular bundles ^P[OPUHKLUZLÄILY[PZZ\LPU*Bassia prostrata*, Amaranthaceae.

vascular bundles phloem xylem ÄILY 250 μm 250 μm 50 μm

**6.272** Vascular bundles within H KLUZL ÄILY [PZZ\L PU *Bassia prostrata*.

**6.273** Tangentially arranged vascular bundles in *Bosea cypria*, Amaranthaceae.

**6.274** Vascular bundles between rays in *Bosea cypria.*

#### *Several cambia periodically produce bands of xylem and phloem*

**6.275** *Welwitschia mirabilis*, Welwitschiaceae. Photo: P. Poschlod.

**6.276** Two tangential rows of vascular bundles in *Welwitschia mirabilis*.

**6.277** *Macrozamia moorei*, Cycadaceae.

ca ca ca

**6.278** A band of phloem between two bands of xylems in *Macrozamia moorei*.

*Several cambia periodically produce bands of xylem and phloem Several cambia produce irregular bands of xylem and phloem*

xylem xylem phloemca ca ca 500 μm 500 μm 500 μm

cambia

**6.279** Several concentric rows of cambia produce bands of xylem and phloem in the herb *Atriplex prostrata*, Amaranthaceae.


**6.281** Irregular bands of internal cambia in the herb *Polycarpaea divaricata*, Caryophyllaceae.

xylem phloem

xylem xylem parenchyma cambium 50 μm

**6.282** The zone between xylem belts in *Polycarpaea nivea* consists of a cambium, a phloem and an \USPNUPÄLKWHYLUJO`TH[PJILS[

**6.283** Alpine cushion plant *Silene acaulis*, Caryophyllaceae.

*Several cambia periodically produce irregular bands of xylem and phloem*

**6.284** Cushion of *Silene acaulis*  with taproot.

**6.285** Irregular bands of internal cambia in *Silene acaulis*.

**6.286** The zone between the two xylems in *Silene acaulis* consists of an anatomically undifferentiated cambium-phloem belt.

# 6.4 Intercalary meristems – Longitudinal growth far behind the tips in shoots and roots

Intercalary meristems are a special form of meristems. Intercalary meristems occur in grasses above nodes between leaf initials, in nodes on horsetails, and in root collars of mistletoes. Theses meristems originally are a product of apical meristems, which retain their meristematic activity far behind the inactivated apical meristems. Their activity is obvious in the elongation phase of the culm of grasses and horsetails. A long time HM[LY[OLMVYTH[PVUVM[OLÅV^LYZHUK[OLPUHJ[P]H[PVUVMHWPJHS meristems the culms are getting longer and longer due to the activity of intercalary meristems. In grass species with several nodes, multiple intercalary meristems are active until the culm YLHJOLZP[ZÄUHSSLUN[O

Mistletoes can only survive if the elongation of root collars follows the thickening of radial growth of the host. As soon as a haustorium touches the cambium of the host xylem, cells incorporate the foreign body. The mistletoe's strategy is to avoid isolation by forming new tissues in between its shoot and root. The original place of haustoria attachment remains, but the root elongates and enlarges in the cambial zone of the host.

#### *Horsetails*

**6.287** Initial phase of stem elongation in a fertile shoot of *Equisetum telmateia*.

#### *Macroscopic aspect of intercalary meristems Monocotyledons*

**6.288** Node of a Poaceae culm. The intercalary meristem is located above the node.

*Monocotyledons*

**6.289** Adult phase of stem elongation in shoots of a giant bamboo.

*Mistletoe*

**6.290** Mistletoe *Viscum album* on a branch of *Pinus sylvestris*.

**6.291** Internal structure of a node in *Equisetum arvense*.

**6.292** Elongation zone with unlig-UPÄLK TLYPZ[LTH[PJ JLSSZ PU SLHM sheaths of the grass *Milium effusum*.

#### *Microscopic aspect of intercalary meristems*

**6.293** Mistletoe haustoria of *Viscum album* in the xylem and phloem of an apple tree (*Malus domestica*).

# *Mistletoes*

xylem cambium phloem (host)

**6.294** Concentration of nuclei in the thin-walled haustorium of the parasite in the cambial zone of the host.

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# 7. Stem anatomical structures of major taxonomic units

This chapter describes the anatomy of major stem-forming taxa within the taxonomic hierarchic system. This system has seven WYPUJPWHSJH[LNVYPLZ^P[OZWLJPÄJLUKPUNZVU3H[PUUHTLZ!


Described in this chapter are species of the following groups:

	- Coniferopsida including Gnetales (some species of conifers, *Ephedra*, *Gnetum*, *Welwitschia*)
	- Angiosperms (many species of ÅV^LYPUNWSHU[Z¶ monocotyledons and dicotyledons (old term)

**7.1** Division: Basidiomycota (*Boletus edulis*)

*Fungi Brown algae Lichens Mosses*

**7.2** Class: Phaeophyceae **7.3** Lichenes **7.4** Class: Bryopsida

# *Domain* Eukarya *Kingdom* e.g. Plantae, Fungi

*Whisk ferns*

**7.5** Subdivision: Psilophytina

**7.6** Subdivision: Lycopodiophytina

**7.7** Subdivision: Equisetophytina

*Horsetails*

**7.8** Subdivision: Filicophytina

*Palm ferns*

**7.9** Class: Cycadopsida

*Seed plants*

**7.11** Class: Coniferopsida

**7.12** Order: Gnetales

**7.13** Angiosperms

**7.14** Angiosperms

**7.10** Class: Ginkgopsida

**7.15** Angiosperms

#### *Dicotyledons*

**7.16** Angiosperms

*Monocotyledons*

# 7.1 Stem-forming fungi and algae

# 7.1.1 Sporophytes of fungi

Described are microscopic structures of a few larger stems of fruiting bodies of Eubasidiomycetes. One cell type, the hyphae, form mushroom stems. The images demonstrate that the anatomical characteristics of hyphae create a great morphological diversity. Distribution, variation of density, orientation, diameter, wall thickness, chemical composition and different cell types determine the aspect and the construction of mushroom stems.

**7.17** A 5 cm-tall *Polyporus brumalis* mushroom on a branch.

*Macroscopic aspect of fungal fruiting bodies with stems*

**7.18** A 12 cm-tall *Ganoderma carnosum* mushroom, which grows on dead wood of *Abies alba*.

**7.19** Cross section of a stem of *Ganoderma carnosum* with a dark peripheral layer, lighter outer part and brown center with longitudinal tubes.

**7.20** Irregular distribution of hyphae around the holes in the central stem part of *Ganoderma carnosum*.

#### *Microscopic aspect overview Orientation of hyphae*

**7.21** Cross section of a stem of *Polyporus brumalis* with a dense peripheral layer, middle layer with few hyphae, and central strand of hyphae.

**7.22** Axially and parallel orientated hyphae in the peripheral zone of *Polyporus brumalis*.

**7.23** Radially oriented hyphae in the peripheral zone of *Ganoderma carnosum*.

**7.24** Irregularly oriented hyphae in the peripheral zone of *Boletus edulis*.

#### *Diameter and wall density of hyphae Composition of stems*

**7.25** Cross section of large and thick-walled hyphae with 2–3 μm diameter in the peripheral layer of *Polyporus brumalis*.

**7.26** Occurrence of hyphae with different diameters in the middle layer of a cross section of *Polyporus brumalis*.

**7.27** Red-stained peripheral layer and blue-stained central parts of the stem of *Boletus edulis* (Astrablue/ Safranin-stained).

**7.28** Brown-stained peripheral and blue-stained central layers of *Ganoderma carnosum* (Astrablue/ Safranin-stained).

# 7.1.2 Thalli and stems of brown algae

Described are a thallus and stems of brown algae. Large thallusforming brown algae primarily occupy rocky, permanently or temporally submerse coastal sites (benthos) of temperate and cold oceans.

Coastal brown algae are anchored with rhizoids, and form stems (cauloids) and leaves (phylloids) of various lengths. Small plants remain a few centimeter tall, very large ones, e.g. the giant kelp, can reach a length of up to 40 meters. The principal stem construction of all types is similar. The cortex consists VM JLSSZ ÄSSLK ^P[O JOSVYVWSHZ[Z HUK IYV^U Z\IZ[HUJLZ fucoxanthin) and a large center with less stained cells. The cellular structure is homogeneous in stems of small algae. Stems of *Laminaria* species in cold oceans are special, in that central, living cells form annual rings in a seasonal rhythm. It is a type of primitive secondary growth. Cell walls consist of a dense layer VMJLSS\SVZLÄIYPSZHUKHT\JPSHNPUV\ZSH`LYVMalginate (a polysaccharide). Stability is provided by the cellulosic layer, while ÅL\_PIPSP[`PZWYV]PKLKI`[OLHSNPUH[L7LYPWOLYHSJLSSZJVU[HPU photosynthetically active chloroplasts.

#### *Macroscopic aspect of large brown algae*

**7.29** *Fucus serratus* at a sea shore in Iceland.

*Annual rings*

**7.30** Washed-up, 6 m-long *Macrocystis pyrifera* (giant kelp) on the coast of South Africa.

longitudinally

phylloid phylloid cauloid rhizoid

**7.31** Phylloid (leaf), cauloid (stem) and rhizoids (roots) of *Laminaria* sp. on a rocky site in Iceland.

alginate cell wall

**7.32** Cross section of a *Laminaria* sp. stem with 6 annual rings.

**7.33** Cross section of a 5 cm-tall stem forming tree-like brown algae.

**7.34** Cross section of a 15 cm-tall tallus of *Fucus serratus*. Peripheral cells are axially, central cells longitudinally oriented.

*Microscopic aspect of stems and cells*

oriented cells proto-

**7.35** *Fucus serratus* cells with chloroplasts in the protoplast and double-layered walls.

# 7.2 Mosses – The oldest living plants

Described are stems of mosses (Bryopsida), a peat moss (Sphagnopsida) and a liverwort (Marchantiopsida). They include approximately 20,000 species and occupy all sites from the tropics to the arctic, and from dry to submersed sites.

The principal stem structure of mosses and peat mosses is simi-SHY(ULWPKLYTPZHUKHTHU[SLVMÄILYSPRLJLSSZ^P[O]HYPV\Z wall thickness surrounds a parenchymatic center. Parenchyma cells can be perforated by simple pits. This is the structure of the simplest types. Some species have developed a central strand consisting of leptoids, and others of leptoids and hydroids. Leptoids conduct photosynthetic products, the hydroids conduct water. The central parenchyma cells of most species are not perforated. Simple pits with large apertures could only be observed in a few walls. Leptoids are unlignifed, longitudinally enlarged cells with horizontal walls. Hydroids in *Polytrichum* sp. are lon-NP[\KPUHSS`LUSHYNLKSPNUPÄLKÄILYSPRLJLSSZ^P[OV\[WP[Z

The peat mosses belong to the simplest anatomical type, however, the mantle is divided into a very thin-walled peripheral and a thick-walled inner part.

The thallus of *Marchantia* sp. resembles a leaf rather than a stem. It consists of a layer of large parenchyma cells with large, simple pits and few oil cells. The top layer is composed of small cells with chloroplasts.

Mono- and multicellular extrusions of the epidermis cells, the rhizoids, occur on all observed species.

#### *Principal structure of moss stems*

#### *ơ*

50 μm

**7.36** *Neckera crispa*. Photo: A. Ber- **7.38** *Sphagnum compactum.*  gamini.

**7.37** Stem of *Neckera crispa* with a thick-walled mantle.

**7.39** Stem of *Sphagnum subnitens* with a mantle of thin- and thick-

leptoids 50 μm

**7.40** *Funaria hygrometrica.* **7.41** Stem of *Funaria hygrometrica* **7.42** *Polytrichum commune.* with a mantle of thick-walled cells and leptoids in the center.

walled cells.

*Types with leptoids in the center (sieve-tube-like cells) Types with leptoids and hydroids in the center*

**7.43** *Polytrichum commune* with a mantle of thick-walled cells and a center of hydroids and leptoids.

#### *Types with a thallus*

*Principal cell structure of moss stems*

**7.44** *Marchantia polymorpha*. **7.45** Thallus of *Marchantia polymorpha* with chloroplasts and a respiration cavern in the top layer. Starch grains in large parenchyma cells and rhizoids a the base of the thallus. Unstained slide.

25 μm

**7.46** *Thamnobryum alopecurum.*  Photo: A. Bergamini.

**7.47** Parenchyma cells with large simple pits in *Thamnobryum alopecurum*.

**7.48** <USPNUPÄLKSLW[VPKZPU*Thamnobryum alopecurum.*

**7.49** <USPNUPÄLKTSVUNSLWtoid with transverse walls in *Thamnobryum alopecurum.*

**7.52** Rhizoids are specialized epidermis cells. *Thuidium tamariscinum*.

*Principal cell structure of moss stems Rhizoids*

**7.50** <USPNUPÄLK SLW[VPKZ HUK YLK Z[HPULKSPNUPÄLK&O`KYVPKZHYLZ\Y-YV\UKLKI`\USPNUPÄLKWHYLUJO`TH cells in *Polytrichum commune.*

**7.51** 3PNUPÄLK ÄILYSPRL O`KYVPKZ \USPNUPÄLK SLW[VPKZ HUK WHYLUchyma cells with horizontal walls in *Polytrichum commune.*

# 7.3 Fern-like plants

# 7.3.1 Spikemosses, quillworts and clubmosses

Described are a few stems of the terrestrial genera *Selaginella* (spikemoss), *Lycopodium* (clubmoss) and the bulb of the swamp plant *Isoetes* (quillwort). They all have in common the absence of secondary growth, as well as the presence of concentric vascular bundles with a central xylem and a peripheral phloem. The xylem consists of tracheids with sclariform pits.

The families Selaginellaceae, Lycopodiaceae and Isoetaceae are distinguishable by the distribution of vascular bundles in the central strand (stele), and the individual species by the composition of the cortex and the form of vascular bundles.

by the presence of single, laterally extended vascular bundles

(polystele), which are surrounded by an endodermis.

#### *Selaginellaceae (spikemosses)*

This family contains approximately 40 species. All of them belong to the genus *Selaginella*. Their stems are characterized

**7.53** *Selaginella denticulata.*

**7.54** Stem of *Selaginella* sp. with isolated concentric vascular bundles.

scalariform pits

**7.55** Part of a vascular bundle in *Selaginella* sp.

**7.56** Tracheids with scalariform pits in a longitudinal section of *Selaginella* sp.

#### *Isoetaceae (quillworts)*

This family contains approximately 50 species. All of them belong to the genus *Isoetes*. They grow on wet sites. The bulb of *Isoetes lacustris* contains a central circle of tracheids where many laterally oblique emerging shoots are initiated. The vascular bundles are embedded in a very thin-walled parenchymatic tissue. The walls of tracheids are characterized by inten-ZP]LS`SPNUPÄLKHUU\SHYZ[Y\J[\YLZ(ULUKVKLYTPZPZHIZLU[

**7.57** *Isoetes lacustris.* Photo: M. Ctvrtlikova.

**7.58** A central strand and many leaf traces are embedded in a thinwalled parenchymatic tissue in *Isoetes lacustris*.

**7.59** The central ring consists of circular arranged tracheids in *Isoetes lacustris*.

**7.60** Tracheids with scalariform pits in the central strand in *Isoetes lacustris*.

87

#### *Lycopodiaceae (clubmosses)*

This family contains more than 10 genera and approximately 1,000 species. Stems of clubmosses are characterized by a large cortex and a central strand with vascular bundles. The strand is

**7.61** *Lycopodium alpinum*, an alpine cushion plant on dry sites.

**7.65** *Lycopodium annotinum*, a subalpine plant with long rhizomes.

**7.69** *Lycopodiella cernua*, an upright plant on subalpine bogs (Azores).

**7.62** Scalariform pits on the walls of tracheids in *Lycopodium alpinum.*

scalariform pits

**7.66** Scalariform pits on the walls of tracheids in *Lycopodium annotinum.*

outer cortex

surrounded by an endodermis. Different compositions of the cortex and distribution patterns of the central vascular bundles HSSV^ZWLJPLZPKLU[PÄJH[PVU

cortex

**7.63** The central strand (stele) with a large surrounding cortex in *Lycopodium alpinum.*

**7.64** Vascular bundles within the 50 μm

endodermis

central plectostele in *Lycopodium* 

*alpinum.*

**7.67** The central strand (stele) with a surrounding cortex in *Lycopodium annotinum.*

**7.68** Vascular bundles with a central plectostele in *Lycopodium annotinum.*

**7.71** Large, thin-walled cortex and slime ducts in *Lycopodiella innundatum*, a bog plant.

cortex tracheidphloem endodermis 50 μm

**7.72** Vascular bundles with a central plectostele in *Lycopodiella innundatum.*

**7.70** The central strand with a surrounding three-part cortex in *Lycopodiella cernua.*

# 7.3.2 Whisk ferns and moonworts

Described are two terrestrial species: *Psilotum nudum* of the family Psilotaceae (whisk ferns) and *Botrychium lunaria* of the family Ophioglassaceae (moonworts). The family Psilotaceae contains two genera with three species, and the family Ophioglossacea contains four genera with approximately 80 species.

9LWYLZLU[H[P]LZVM[OL[^VMHTPSPLZKPMMLYZPNUPÄJHU[S`MYVTLHJO other. *Psilotum nudum* is anatomically close to the lycopods, however, two features are different: the tracheids have scalariform and round pits, and the vessels are arranged star-like HYV\UK H ZJSLYPÄLK WP[Oactinostele). This is characteristic for

**7.73** *Psilotum nudum.*

**7.77** Groups of vessels at the tips of the star in *Psilotum nudum.*

**7.81** Tube-like arrangement of vessels (siphonostele) in *Botrychium lunaria.*

**7.74** Scalariform vessel pits (left) and bordered pits (right) in *Psilotum nudum.*

 100 μm

**7.78** Irregular distribution of tracheids and sieve cells in the root of *Psilotum nudum*.

**7.82** Cambium between xylem and phloem in *Botrychium lunaria.* nuclei

[OLMHTPS`(IZLU[HYLLUKVKLYTPZÄILYZHUKWHYLUJO`THJLSSZ within the xylem.

Stems of *Botrychium lunaria* are anatomically closer to dicotyledons rather than to lycopods. Characteristic is a central strand of xylem and phloem (siphonostele) with a cambium in the upper part of the plant. It centripetally produces a xylem, which consists of vessels with distinct perforation plates with large pits and rays. Fibers and axial parenchyma cells are absent. The central strand in the root is a closed concentric vascular bundle consisting of tracheids with bordered pits.

**7.75** Central cylinder surrounded by a large cortex and an epidermis in *Psilotum nudum.*

**7.79** *Botrychium lunaria*.

**7.83** 3PNUPÄLK ]LZZLSZ HUK \USPNUP-ÄLKYH`ZPU*Botrychium lunaria.*

actinostele

**7.80** Vessels with perforation plates in *Botrychium lunaria.*

**7.84** Root of *Botrychium lunaria* with a protostele.

# 7.3.3 Horsetails

The genus *Equisetum*, with approximately 20 species, is the only living genus within the Equisetophytina. Horsetails are anatomically chimeric: tracheids/vessels in vascular bundles within the internodes indicate a relationship to ferns, those in the nodes to dicotyledons (intercalary meristem) and the roots to the rhizomes of monocotyledons.

Vertical shoots with reduced leafs (sheaths) above nodes and long internodes are characteristic for all horsetails. Secondary growth is absent. Common for all species are intercalary meristems in the nodes, circular arranged vascular bundles (siphonostele) centripetal of the epidermis and a large cortex. The cortex consists of an epidermis, an outer part with thick-walled, VM[LUSPNUPÄLKÄILYZHUKPUULY[OPU^HSSLKWHY[^P[OSHYNLH\_Pally elongated schizogenous intercellulars (vallecular canals). The anatomy of the central strand is different in the internodes and nodes.

#### *Structure of the internodes*

In one group of species the central strand is bordered by an endodermis, e.g. in *Equisteum arvense* or *E. sylvaticum*. In the other group, the single vascular bundles are directly surrounded by the endodermis, e.g. in *E. limosum* or *E. hiemale*. The xylem of the vascular bundles of all species is reduced to a few isolated thick-walled tracheids or vessels. Their walls con-ZPZ[VMSPNUPÄLKYPUNZVYVM^PKLZWHJLKscalariform pits. Perforations plates and scalariform pits with very large apertures are hard to differentiate. The term tracheid/vessel is therefore used. Parenchyma cells surround the phloem. The sieve tubes have horizontal sieve plates. Lateral plates are absent. Within the area of the xylem is a large opening, the carinal canal, which occasionally contains tyloses.

#### *Macroscopic aspect*

**7.85** *Equisetum arvense* (left) and node of *Equisetum hiemale* (right).

**7.86** *Equisetum arvense* with dense outer cortex, small vallecular canals, circular arranged vascular bundles and a hollow center.

outer cortex

**7.87** *Equisetum palustre* with very large vallecular canals (aerenchyma) in the cortex and a small central strand.

**7.88** An endodermis separates the central strand and the cortex in *Equisetum sylvaticum*.

*Vascular bundles in internodes*

**7.89** An endodermis separates a single closed collateral vascular bundle from surrounding parenchyma in *Equisetum hiemale*.

**7.90** A single vascular bundle without distinct endodermis in *Equisetum limosum*.

#### *Structure of the nodes*

The origin of lateral shoots is in the nodes. Collateral, probably open vascular bundles are arranged in a compact circular belt of xylem and phloem. The xylem of the bundles consists of tracheids/vessels with bordered pits. A layer of small parenchyma cells in the pith and horizontally oriented vessels and an intercalary meristem divide the nodes axially. There is a continuum between bordered pits in the internodes and scalariform pits in the nodes.

#### *Structure of the root*

A single concentric vascular bundle with a single vessel is surrounded by a cortex (Carlquist 2011).

**7.91** Vessel walls with widely spaced pit apertures or perforation plates and annular thickenings in *Equisetum arvense.*

**7.92** Tyloses in a carinal canal of a vascular bundle in *Equisetum sylvaticum*.

*Structure of vascular bundles in nodes*

*Structure of nodes*

**7.93** Central strand and lateral shoots of *Equisetum arvense*.

**7.94** A single open? collateral vascular bundle with a xylem and phloem in *Equisetum arvense*.

node

**7.95** Node with a lateral branch in *Equisetum arvense*.

**7.96** Vessel transition from an internode to a node in *Equisetum arvense*. bordered pits

250 μm

**7.97** Bordered pits in vessels of a node of *Equisetum arvense*, longitudinal section.

**7.98** Bordered pits in vessels of a node of *Equisetum arvense*, cross section.

*Structure of the root*

**7.99** Concentric vascular bundle with a single vessel in *Equisetum arvense*.

# 7.3.4 Ferns

The Filicophytina include approximately 9,000 species. Presented here are common anatomical traits of ferns. Tree ferns, hemicryptophytes and water ferns occur from the tropics to the arctic zone and grow on very dry to very moist sites. Ferns form petioles, stems, rootstocks and rhizomes. Secondary growth is absent. Vascular bundles are arranged solitary, or in irregular groups lateral of petioles; they are circular arranged in root stocks and stems, or they form bands. The arrangement of vascular bundles varies within the plant; it is different in rhizomes and in petioles.

Most ferns have closed amphiversal vascular bundles with a SPNUPÄLK\_`SLTHUKZVSP[HY`WHYLUJO`THJLSSZPU[OLJLU[LY;OL \USPNUPÄLKZ\YYV\UKPUNILS[JVUZPZ[ZVMsieve tubes, companion cells, groups of parenchyma cells and an endodermis. The form and the arrangement of the cell types vary. Closed collateral vascular bundles occur in hydrophytes.

A very thin-walled endodermis, often with Casparian strips, surrounds vascular bundles. The parenchyma cells which separate the cortex from the vascular bundle are mostly centripetally ]LY`[OPJR^HSSLKHUKSPNUPÄLK;OLTHQVYP[`VMtracheids in all observed species have scalariform pits, however, transitions of scalariform pits to bordered pits occur frequently. Perforation plates on distal ends of tracheids occur in a few species. These types are real vessels. In a few species ZPL]LÄLSKZVJJ\YHZMHY as could be observed, only on lateral walls.

The structure of the cortex of petioles, rhizomes and stems greatly varies. In all cases one or more layers of thick-walled parenchyma cells surround a layer of more or less thin-walled parenchyma. The thick-walled cells can be either parenchyma JLSSZVYÄILYSPRLJLSSZ^P[OZSPNO[S`IVYKLYLKWP[Z;OPU^HSSLK parenchyma cells contain starch. Specialized cells or ducts of some species produce slime. See also White 1963.

**7.100** Tree fern *Cyathea cooperi.*

**7.101** Large hemicryptopyte *Woodwardia radicans*.

#### *Growth forms of ferns*

**7.102** Small hemicryptophyte

*Asplenium ruta-muraria.*

**7.103** Hydrophyte *Marsilea quadrifolia.*

**7.104** Stem of the tree fern *Cyathea cooperi.*

*Stems, root stocks and rhizomes*

**7.105** Rootstock with leaf bases of the hemicryptophyte *Dryopteris ÄSP\_THZ*

**7.106** Rootstock with leaf bases of the hemicryptophyte *Matteuccia struthiopteris.*

**7.107** Rhizomes of *Davallia canariensis.*

#### *Arrangement of vascular bundles*

parenchyma parenchyma vascular bundle

cortex 1 mm 1 mm

**7.108** Solitary in the center of the petiole of an annual stem of *Hymenophyllum tunbrigense.* Protostele.

vascular bundle

**7.109** Irregularly distributed in the annual stem of *Pteridium aquilinum*. Poly stele.

**7.110** Lateral in a petiole of *Athy-YP\TÄSP\_MLTPUH*Polystele.

**7.111** Circular in the rhizome of *Asplenium septentrionale.* Siphonostele.

vascular bundle

**7.112** Circular in the root stock of *Osmunda regalis.* Siphonostele.

**7.113** Arc-like in the rhizome of *Cryptogramma crispa*. Siphonostele.

**7.114** Arc-like in the petiole of *Culcita macrocarpa*. Siphonostele.

**7.115** Two bundles in the petiole of *Gymnocarpium robertianum.*  Polystele.

#### *Internal variation of vascular bundle arrangement*

**7.116** Round bundles, circular arranged in the root stock of *Gymnocarpium robertianum*. Siphonostele.

**7.117** A large and a small bundle in a petiole of *Polypodium vulgare.* 

**7.118** Different forms of bundles, circular arranged in the root stock of *Polypodium vulgare.* Siphonostele.

#### *Structure of closed amphiversal vascular bundles* pa pa ph xy en

**7.119** Laterally elongated xylem in a round vascular bundle of *Gymno-*

*carpium robertianum*.

**7.120** Eagle-shaped xylem in rhizome of *Polypodium vulgare*.

**7.121** Band-like xylem in a vascular bundle of the petiole of *Culcita* 

pericycle

*macrocarpa*.

*Endodermis of vascular bundles*

**7.122** Round xylem/phloem strand in the stem of the liana *Lygodium* sp.

**7.123** Endodermis with Casparian strips in *Cyathea cooperi*. ph xy pa en co

**7.124** Endodermis without distinct Casparian strips in *Marsilea quadrifolia*.

**7.125** Thin-walled pericycle and thick-walled endodermis separate central cylinder and cortex in *Polypodium vulgare*.

**7.126** Thin-walled endodermis IL[^LLU H SPNUPÄLK JVY[L\_ HUK HU \USPNUPÄLK WHYLUJO`TH [PZZ\L PU *Lygodium* sp.

#### *Wall structure of tracheids Wall structure of sieve elements*

**7.127** Scalariform pits in *Cyathea cooperi*.

v vv v 25 μm 25 μm

**7.128** Scalariform pits in *Blechnum* 

**7.129** Scalariform and bordered round pits in *Lygodium* sp.

50 μm

*spicant*.

#### *Structure of the cortex*

**7.131** Thin-walled parenchyma cells are surrounded by an epidermis in *Gymnocarpium robertianum*.

**7.135** Parenchyma cells with horizontal walls and slightly bordered pits in *Lygodium* sp.

cortex

cortex

> xy

**7.132** A layer of thick-walled parenchyma cells surrounds the vascular bundles in *Osmunda regalis*.

**7.136** Multilayered cortex in the rhizome of *Marsilea strigosa*.

**7.133** A layer of thick- and thinwalled cells occurs outside of a large, thin-walled parenchymatic zone in *Marattia fraxinea*.

cortex

**7.134** A dense belt of thick-walled cells surrounds the central xylem/ phloem strand in *Lygodium* sp.

**7.137** Multilayered cortex in the stem of *Cyathea cooperi*.

**7.138** Cortex with large aerenchymatic spaces of *Pilularia globulifera*.

**7.139** Starch in thin-walled parenchyma cells in *Culcita macrocarpa*, polarized light.

*Content of cortex cells*

**7.140** Dark-stained substances in parenchyma cells in *Marattia fraxinea*.

**7.141** Slime-conducting ducts in parenchyma cells of *Cyathea cooperi*.

ph xy

# 7.4 Seed plants

# 7.4.1 Palm ferns

Within the Cycadopsida, the familes Cycadaceae and Zamiaceae exist worldwide today, with ten genera and approximately 90 species. Most species occur in the tropics. The following presentation is primarily based on the collection of Greguss 1968.

Secondary growth is characteristic for all palm ferns. The presence of circular arranged tracheids is a common feature. Within the Cycadopsida, two radial growth types occur: one forms a simple siphonostele containing isolated vascular bundles or a closed xylem/phloem ring, the other has successive cambia, which form single vascular bundles or xylem/phloem rings. Collateral vascular bundles occur in petioles. The xylem is composed of radially arranged roundish tracheids. Annual rings have not been observed. Tracheids have scalariform or bordered pits with slit-like apertures. Pits are arranged in one or more axial rows. Rarely, tracheids with ephedroid perforation WSH[LZWLYKLÄUP[PVU]LZZLSZVJJ\YRays are homogenous and composed of thin-walled parenchyma cells, arranged in one to ZL]LYHS YV^Z;OL WOSVLT JVU[HPUZ ÄILYZ HUKsieve elements with lateral ZPL]LÄLSKZ\*VTWHUPVUJLSSZ^LYLUV[VIZLY]LK

Slime ducts occur in the pith and the cortex of some species. ;OL`HYLZ\YYV\UKLKI`ZTHSS\USPNUPÄLKexcretion cells or, in HML^ZWLJPLZHSZV^P[OSPNUPÄLK[YHJOLPKSPRLJLSSZ:OVY[SPNUP-ÄLKJLSSZVM[LU^P[OIVYKLYLKWP[ZVJJ\YPU[OLWP[OVMZVTL species (transfusion cells, terminus Greguss). Crystals in the form of druses, prisms or sand are frequent in parenchyma cells.

**7.142** *Cycas revoluta*

*Macroscopic aspect of palm ferns Pál Greguss' slide collection*

**7.143** *Cycas* sp.

**7.144** *Encephalartos* sp.

**7.145** Cycad slide collection at the Hungarian Natural History Museum Budapest, Department of Botany.

**7.146** One cambium forms circular arranged vascular bundles in *Ceratozamia mexicana*. Eustele.

#### *Types of radial growth*

**7.147** One cambium forms a closed belt of xylem and phloem in *Cycas* sp. Eustele. xylem pith cortex

**7.148** Successive cambia form single vascular bundles in *Cycas* sp.

**7.149** Successive cambia form several xylem/phloem rings in *Macrozamia moorei*.

#### *Collateral vascular bundles Xylem with tracheids*

**7.150** Stem of *Ceratozamia mexicana*.

**7.151** Petiole of *Cycas revoluta*.

**7.152** Petiole of *Zamia pygmaea*.

250 μm

ray tracheids

**7.153** Roundish tracheids, strictly radially arranged, in *Dioon spinulosum*.

**7.154** Roundish tracheids in *Zamia skinneri*.

**7.155** Scalariform pits in *Zamia furfuracea*.

**7.156** Bordered pits with slit-like apertures in *Encephalartos hilde-*

**7.160** One- to triseriate, homogeneous rays in *Dioon spinulosum*.

**7.157** Bordered pits with slit-like apertures in *Cycas revoluta*.

**7.158** Vessels with ephedroid perforation plates in *Zamia* sp.

**7.159** Uniseriate rays in *Cycas media*.

*brandtii*. *Width of rays*

**7.161** Multiseriate rays with thinwalled cells and a few transfusion cells in *Macrozamia pauli-guilielmi*.

**7.162** Ray with square, thin-walled cells, radial section of *Encephalartos septentrionalis*.

#### *Phloem*

**7.163** Collapsed sieve elements and square ÄILYZPU*Zamia skinneri*.

**7.164** +PZ[YPI\[PVU VM ÄILYZ `LSSV^ HUK [OPU walled sieve elements in *Zamia skinneri*.

**7.165** Sieve element with lateral sieve plates in *Encephalartos hildebrandtii*.

**7.166** Ducts in the pith of *Zamia skinneri*.

**7.167** +\J[Z ^P[O ZTHSS \USPNUPÄLK excretion cells in *Encephalartos hildebrandtii*.

**7.168** +\J[Z ^P[O ZTHSS \USPNUPÄLK L\_JYL[PVU JLSSZHUKSPNUPÄLKWP[[LKJLSSZPU*Dioon edule*.

*Ƥȋtransfusion cells) Crystals in parenchyma cells*

**7.169** Cells with unstructured walls in *Macrozamia moorei*.

**7.170** Cells with pitted walls in *Macrozamia pauliguilielmi*.

**7.171** Druses in *Zamia pygmaea*.

**7.172** Prismatic crystals in *Cycas revoluta*.

**7.173** Crystal sand in *Ence phalartos hildebrandtii*.

# 7.4.2 Ginkgoaceae

*Ginkgo biloba* is the only living species in the family of Ginkgoaceae. Fan-shaped leaves are characteristic for the deciduous tree. The species is native to southwestern China. This "living fossil" is frequently cultivated in temperate zones.

the latewood. Axial parenchyma cells do not exist. Ray height ]HYPLZIL[^LLU[^V HUKÄ]L JLSSZ9H` JLSSZ HYLUV[SPNUPÄLK Ray pits are intensively bordered and have slit-like apertures (taxodioid). Crystal druses occur in axial elongated chambers.

The stem/root anatomy of *Ginkgo biloba* has been described in detail by Greguss 1955.

The conifer-like xylem with annual rings is a product of secondary growth. Large earlywood and small latewood tracheids separate the square, radially arranged tracheids within the annual ring. Bordered tracheid pits are arranged in axial uni- to triseriate rows. Pit apertures are round in the earlywood and oval in The phloem is characterized by alternating layers of sieve cells HUKSPNUPÄLK[OPJR^HSSLKÄILYZ\*VTWHUPVUJLSSZHYLHIZLU[ :PL]L ÄLSKZ VU ZPL]L JLSSZ VJJ\Y VU YHKPHS ^HSSZ(ML^resin ducts occur in the pith and the phloem. Sclereids occur, but are rare. Large crystal druses and large quantities of crystal sand are characteristic. The rhytidome of older bark contains many layers of phellem.

**7.179** Alternating sieve cells HUKÄILYZ

cambium

 phloem

xylem

**7.180** Active and dead phloem.

uniseriate rays.

*Phloem*

collapsed sieve cells

sieve cells

cambium

with pits.

(phellogen and phellem).

resin duct

**7.181** :PL]LÄLSKZVUYHKPHS walls of sieve cells.

**7.182** Crystal druses and crystal sand.

phellem

cambium

ÄILY

# 7.4.3 Conifers

Today, seven families (Pinaceae, Araucariaceae, Podocarpaceae, Cephalotaxaceae, Cupressaceae, Taxodiaceae, Sciadopityaceae) are recognized within the conifers worldwide, together containing approximately 630 species. Conifers of the Pinaceae dominate the boreal zone in the Northern Hemisphere. Araucariaceae and Podocarpaceae are families of the Southern Hemisphere.

Plant growth forms and the forms of reproduction organs greatly vary. The presence or absence of heartwood is characteristic for many species. Stems on extreme sites reduce the xylem to radial strips.

Secondary growth is characteristic for all conifers. In common is the presence of square, radially arranged tracheids, often separated into earlywood and latewood. Only species growing in seasonal climates form more or less distinct annual rings. The presence or absence of axial parenchyma and axial and radial resin K\J[ZPU[OL \_`SLTPZ ZWLJPLZZWLJPÄJPits on axial tracheids occur in uniseriate (e.g. Pinaceae) or muiltiseriate rows (Araucariaceae). Pit apertures are mostly circular. The absence or presence of ray tracheids differentiates large groups. The form of the ray parenchyma cells is very variable.

The phloem is characterized by sieve cells with radial sieve ÄLSKZWHYLUJO`THJLSSZÄILYZHUKsclereids.

**7.184** *Picea abies*, Norway spruce, on a subalpine meadow.

*Conifers with one stem*

**7.185** *Pinus sylvestris*, Scots pine, on an Atlantic meadow.

*Stem cross sections*

*Reproduction organs*

**7.186** Fruits of *Juniperus nana*.

**7.187** Cone of *Pinus mugo*.

**7.188** *Picea abies* without heartwood.

**7.189** *Pinus sylvestris* with heartwood and sapwood.

**7.190** Radial stem strip with 840 annual rings of *Juniperus sibirica*.

**7.191** Twig of *Pinus sylvestris*.

#### *Variable ring distinctness and presence or absence of resin ducts*

**7.192** Distinct ring boundaries, without resin ducts, in *Fitzroya cupressoides*, South America.

**7.193** Weak ring boundaries, without resin ducts, in *Podocarpus lambertii*, subtropical South America.

**7.196** Left: Uniseriate pits in *Pinus banksiana*. Middle: Biseriate pits in *Araucaria angustifolia*. Right: Helical thickenings in earlywood tracheids in *Pseudotsuga menziesii*.

**7.197** Uniform pitting: tracheid and ray pits with round apertures in *Metasequoia glyptostroboides*.

**7.198** Uniform pitting: tracheid and ray pits with slitlike apertures in *Podocarpus falcatus*.

resin duct

**7.199** Heterogeneous pitting: fenestrate pits on ray-parenchyma cells and bordered pits with round apertures on ray tracheids and axial tracheids in *Pinus sylvestris.*

**7.200** Alternating rows of sieve cells and parenchyma cells with a few isolated sclereids in *Larix decidua*.

**7.201** Alternating rows of sieve cells HUKWHYLUJO`THJLSSZHUKÄILYZPU *Metasequoia glyptostroboides*.

**7.194** Distinct ring boundaries and resin ducts in *Pinus banksiana*,

resin duct

boreal North America.

**7.202** Resin ducts in the phloem of *Juniperus communis*.

**7.203** Ray dilatation in the phloem of *Podocarpus falcatus*.

# 101

# 7.4.4 Gnetales

#### *Ephedraceae*

*Ephedra* is the only genus within the family of Ephedraceae. All 30–45 leaf-less species grow on dry sites, mostly in arid regions. Their growth forms vary from dwarf shrubs and shrubs to lianas (*Ephedra campylopoda*).

Secondary growth is characteristic for all *Ephedra* species, and growth rings are generally distinct. The xylem is composed of vessels, tracheids, "ÄILY[YHJOLPKZ¹HUKrays. Foraminate perforation plates with distinct borders characterize vessels. Cell walls of vessels and tracheids frequently contain helical thickenings and bordered WP[Z^P[OYV\UKHWLY[\YLZHUKKPZ[PUJ[\USPNUPÄLK tori. "Fiber tracheids" are hybrids between parenchyma cells and tracheids: pits are simple (parenchyma-like), but horizontal walls are absent (tracheid-like). Classical parenchyma cells were not observed. \*Y`Z[HSZHUKPZMYLX\LU[S`WYLZLU[PUTVZ[\USPNUPÄLK parts of the xylem and phloem. Dark-stained substances occur mainly in the pith.

;OLWOSVLTPZJOHYHJ[LYPaLKI`WHYLUJO`THJLSSZÄILYZsclereids and sieve elements (sieve cells) with ZPL]LÄLSKZVU YHKPHS walls. Companion cells are absent. All large rays are dilated. Older stems contain a distinct, multilayered rhytidome.

See also Carlquist 1992.

**7.204** *Ephedra* sp. on a dry site in southwestern North America.

*Macroscopic aspect*

**7.205** *Ephedra* ZW^P[OÅV^LYZ7OV[V!(4VLOS

**7.206** *Ephedra distachya* ssp. *helvetica* with fruits. Photo: A. Moehl.

**7.207** Dwarf shrub *Ephedra nebrodensis* with distinct annual rings.

*Cross sections*

**7.208** Latewood of *Ephedra trifurcata* ^P[O [YHJOLPKZ ¸ÄILY [YHcheids" and vessels.

**7.209** Liana *Ephedra campylopoda* with irregular radial growth.

**7.210** Shrub *Ephedra gerardiana* on a dry site at high altitude, Ladakh, India, 4,400 m a.s.l.

#### *Structure of conducting elements*

ÄILY[YHJOLPK bordered pits

helical thickenings

helical thickenings

**7.211** Vessel with a foraminate per-

**7.212** 7P[Z ^P[O \USPNUPÄLK [VYP PU ¸ÄILY[YHJOLPKZ¹PU*Ephedra viridis*.

**7.213** Vessel and tracheids with IVYKLYLKWP[ZHUKH¸ÄILY[YHJOLPK¹ with simple pits in *Ephedra viridis*.

**7.214** Tracheids with bordered pits and helical thickenings in *Ephedra distachya*.

foration in *Ephedra viridis*.

**7.215** Longitudinal section of *Ephedra trifurcata*. Left: Tracheids with bordered pits. Right: "Fiber tracheids" with simple pits.

**7.216** Tri- to six-seriate rays with irregularly formed cells in *Ephedra viridis*.

**7.217** ,\_[YLTLS` SHYNL \USPNUPÄLK YH`ZIL[^LLUSPNUPÄLK[YHJOLPK]LZsel strips in *Ephedra gerardiana*.

**7.218** Prostrate and square ray cells in *Ephedra trifurcata*.

**7.219** Lateral walls with simple pits in *Ephedra viridis*.

**7.220** Axial and radial walls with simple pits in *Ephedra viridis*.

#### *Ray pitting Crystal sand Bark*

**7.221** Crystal sand in *Ephedra gerardiana*.

**7.222** Living phloem and dead phellem/phloem parts (rhyti dome) in *Ephedra nebro densis*.

**7.223** :PL]LÄLSKZVU YHKPHS walls of sieve cells in *Ephedra viridis*.

#### *Welwitschiaceae*

*Welwitschia mirabilis* is the only species within the family of Welwitschiaceae. The below-ground stem/root and the aboveground continuously growing two leaves are characteristic. The plant grows in the arid zone of Namibia and Angola. The stem/ root anatomy was described in detail by Carlquist & Gowans 1995.

#### **Anatomy of the leaf**

A layer of palisade cells is situated between anatomically undifferentiated epidermal surfaces. Open collateral vascular bundles are located in the central parenchymatic tissue.

#### **(UH[VT`VM[OLMLTHSLÅV^LYZ[HSR**

Collateral open vascular bundles are embedded in a parenchymatic tissue and a few ducts. The irregularly distributed bundles are arranged around a pith. The bundles consist of tracheids with annular thickenings and round, bordered pith-like structures. Phloem cells expand shortly after their formation and remain as collapsed structures.

#### **Stem/root**

The arrangement of the xylem and phloem is normally chaotic, however, in central parts of the stem successive cambia produce several layers of xylem/phloem zones. A circular, closed xylem/phloem is absent, a lateral vascular cambium seems to be absent, therefore the form of vascular bundles remains. ;OPU^HSSLK\USPNUPÄLKrays separate the bundles. The xylem consists of thick-walled vessels with simple perforation plates, [YHJOLPKZHUK[OPU^HSSLK\USPNUPÄLKWHYLUJO`THJLSSZ9HKPHS vessel and tracheid walls contain bordered pits without tori and with round to slit-like apertures. Pits are often arranged in alternate position. Parenchyma is pervasive. In parenchymatic aVULZPUJS\KPUN YH`Z[OLYLHYL]LY`[OPJR^HSSLKÄILYZJVUtaining a mantle of small prismatic crystals. Crystal sand occurs in most parenchyma cells.

The phloem consists primarily of very thick-walled gelatinous ÄILYZ(ML^[OPU^HSSLKsieve elements occur between them.

*ƪ ƪ*

**7.227** Distribution of vascular bundles.

**7.228** Randomly oriented vascular

bundles in a parenchymatic tissue containing ducts.

**7.229** Cambium between the xylem and the phloem.

bundle.

**7.230** Annular thickenings in tracheids. tracheid cambium

#### *Anatomy of the stem/root Anatomy of the stem*

**7.231** Chaotic orientation of the tissue in external parts of stems.

**7.232** Successive cambia between radially elongated vascular bundles in internal parts of stems.

*Anatomy of vascular bundles in the stem/root*

*Anatomy of vascular bundles*

**7.233** Cambial zone of an open collateral bundle.

**7.234** Phloem with thick-walled **7.236** Biseriate bordered pits. NLSH[PUV\ZÄILYZ

**7.235** Vessels with perforation **7.237** Uniseriate bordered pits. plates and tracheids.

ray

**7.238** Elongated sclereids with small prismatic crystals.

*Sclereids and crystals*

sclereid

**7.239** Layered sclereids surrounded by crystals.

**7.240** Crystal sand in thin-walled parenchyma cells.

 cortex phloem epidermis parenchyma 250 μm

**7.241** Thin-walled parenchyma and sclereids.

# *Anatomy of the cortex*

#### *Gnetaceae*

*Gnetum* is the only genus within the family of Gnetaceae. All 30 species, lianas and small trees with broad leaves, grow in the tropics. Described here is the small tree *Gnetum gnemon* from the Philippines. Its anatomy is described in detail by Carlquist 1996.

Secondary growth is characteristic. Growth rings are generally absent, however, density variations indicate intra-annual differences in climatic growing conditions. The xylem is composed of ]LZZLSZ [YHJOLPKZ ZLW[H[L ÄILYZ H\_PHS WHYLUJO`TH JLSSZ and rays. Vessels are solitary. Simple and foraminate perforation plates with distinct borders characterize vessels, with both types occurring within the same individual. Vessel walls contain small, vestured pits with oblique apertures. Radial walls of septate tracheids are perforated by large bordered pits. Round HWLY[\YLZ HUK KPZ[PUJ[ \USPNUPÄLK[VYP HYL JOHYHJ[LYPZ[PJHelical thickenings occur, but are rare. Horizontal walls of septate ÄILYZ HYL UV[SPNUPÄLK(\_PHS WHYLUJO`THPZ KVTPUHU[S` ]HZPcentric paratracheal and occasionally apotracheal. The width of the homocellular rays with prostrate cells varies between one HUKÄ]LYV^Z9H`WP[ZHYLZSPNO[S`[VKPZ[PUJ[S`IVYKLYLK4HU` small prismatic crystals are deposited in ray and pith cells.

The phloem is characterized by a few parenchyma and sieve elements (sieve cells) with ZPL]LÄLSKZVUYHKPHS^HSSZ\*VTWHUion cells are absent. Sieve cells collapse soon after formation. Rays are dilated. .LSH[PUV\ZÄILYZHUKZTHSSJY`Z[HSZVJJ\YPU the cortex and the phloem. Sclereids form a band between the cortex and the phloem. The phellem is interrupted by lenticels.

*Cross sections*

cortex

phloem

xylem

phellem tracheid ray

#### *Morphology of the plant*

**7.242** Broad, thin leaves of *Gnetum gnemon*.

**7.243** Breaking zone of a twig (apoptosis).

**7.244** Pith, xylem and bark.

pith

**7.245** Xylem with a growth zone.

parenchyma

vessel

growth zone

**7.246** Perforation plates.

**7.247** Vestured intervessel pits.

*Perforation plates and pits* septate tracheid

**7.248** Bordered pits on septate tracheids.

**7.249** Thin helical thickenings in tracheids.

pit crystals

**7.250** <UP[VÄ]LZLYPH[LOVTVJLSlular rays.

**7.251** Homocellular ray with prostrate and square cells.

**7.252** Ray cells with pits and small prismatic crystals.

**7.253** Ray cells with bordered pits.

**7.254** Conducting and collapsed sieve elements and gelatinous ÄILYZ

**7.255** Phloem, cortex with sclereids HUKNLSH[PUV\ZÄILYZHUKWLYPKLYT

**7.256** :PL]LÄLSKZPUZPL]LJLSSZ

phloem cortex phellem

**7.257** Lenticel.

# Gnetales: Conifers or Angiosperms?

Gnetales are seed plants (spermatophyta) that are taxonomically related to conifers. Bordered pits on tracheids in the xylem JVUÄYT [OH[ /V^L]LY THU` Z[LTHUH[VTPJHS MLH[\YLZ ZOV^ relations to angiosperms. A few features are common to all three families: the presence of vessels, tracheids with large bordered pits and small crystals in parenchyma cells and sclereids in the bark. Each genus has its own "specialty".

*Ephedra*THPUS`MVYTZSLHÅLZZZOY\IZHUKWYLMLYZHYPKJSPTH[LZ or dry sites in the Northern and Southern Hemisphere. The xylem has no axial parenchyma but "ÄILY[YHJOLPKZ¹^P[OZPTple pits. All perforation plates are foraminate.

*Welwitschia mirabilis* forms a subterranean stem and two continuously growing leaves. The plant is geographically isolated and occurs only in the desert of Namibia. The xylem has successive cambia and pervasive parenchyma. Collateral vascular bundles remain and do not grow together into a compact belt of xylem.

*Gnetum* grows as lianas and small trees with broad leaves. All species grow in moist tropical climates. The xylem consists of solitary vessels, tracheids and septate tracheids and paratracheal parenchyma. Perforations are simple and foraminate. A special feature are vestured vessel pits and bordered ray pits.

In common for *Welwitschia* and *Gnetum* is the presence of NLSH[PUV\ZÄILYZPU[OLWOSVLTHUK[OLJVY[L\_

**In conclusion:** Anatomical features in the xylem relate all the three families to the angiosperms. Solely the bordered pits indicate a relation to the conifers. It remains unclear to what extent site differences and geographical isolation drove stem evolution in such different directions.

# 7.4.5 Angiosperms: Monocotyledons and their growth forms

The monocotyledons are extremely manifold. There are approximately 60,000 species within 11 orders. Monocotyledons grow in all vegetation zones from tropical to arid and arctic zones, and in all habitats from extremely dry to submersed sites. Growth forms vary from little herbs to lianas and large trees (palms).

*Palms (Arecaceae)*

Within the Arecaceae family, there are approximately 2,600 species. They occur preferentially in tropical regions. A common characteristic for all species is the absence of secondary growth. Described here is the date palm, *Phoenix dactylifera*. Vascular bundles are irregularly distributed over the whole stem

**7.258** Date palm (*Phoenix dactylifera*) in an oasis of the Sahara desert.

# *Macroscopic aspect Microscopic aspect* ÄILY 500 μm

**7.259** Irregular distribution of vascular bundles (atactostele) in *Phoenix dactylifera.*

The more than 1,000 bamboo species occur primarily in southern Asia and South America. Most of the species form heavily SPNUPÄLKZ[YHPNO[Z[LTZ/LYL[OLZWLJPLZ*Phyllostachys bambusoides* is anatomically described. Vascular bundles are irregularly distributed over the whole stem cross-section (atactostele). Closed collateral vascular bundles are composed of two large Presented are exemplarily some few species and growth forms of different taxonomic groups in various habitats. This selection gives an impression of the anatomical variability within the monocots. So far, there are hardly any stem anatomical features observed that are common for the entire group.

cross section (atactostele). Closed collateral vascular bundles contain many small vessels with scalariform pits. The bundles are surrounded by thick-walled sheaths of ÄILYZ:LLHSZV;VTlinson *et al*. 2011.

**7.260** Closed collateral vascular bundles with dense sheaths in *Phoenix dactylifera.*

**7.261** Vessels with scalariform pits in *Phoenix dactylifera.*

#### vessels and an external group of phloem. Vessels are characterized by a large perforation plate in almost horizontal position, many small pits perforate lateral vessels. Sieve plates in sieve tubes occur as transversely perforated walls. A thick-walled ÄILY ZOLH[O Z\YYV\UKZ ]HZJ\SHYI\UKSLZ4VYLPUMVYTH[PVUPU Grosser & Liese 1971.

#### *Macroscopic aspect Microscopic aspect*

*Bamboo (Poaceae)*

**7.262** Bamboo (*Chusquea* sp.) in the Asian tropics, 20 m tall.

**7.263** Irregular distribution of vascular bundles (atactostele) in *Phyllostachys bambusoides*. 500 μm

**7.264** Closed collateral vascular bundles in *Phyllostachys bambusoides*.

ÄILY vessel parenchyma

**7.265** Vessel with small pits, ÄILYZ HUK WHYLUJO`TH JLSSZ in *Phyllostachys bambusoides*.

**7.266** Phloem with very large sieve tubes in *Phyllostachys bambusoides*.

#### *Grass-like terrestrial herbs (Cyperaceae)*

More than 5,000 species occur from the tropics to the arctic on extremely dry sites as well as on lake shores. The outlines of culms are triangular, but often roundish. Presented here are one species from a wet site (*Carex foetida*) and another from a dry, alpine site (*Kobresia simpliciuscula*).

Secondary growth is absent. The closed collateral vascular bundles are composed of a xylem with a few enlarged vessels and a round group of phloem. The bundles are often surrounded by a belt of thick-walled ÄILYZ:LLHSZV4L[JHSML

**7.267** *Carex pendula*.

**7.268** Triangular culm of the 10 cm-tall grass-like herbaceous *Carex foetida* on an alpine moist site.

**7.269** A belt of thick- ^HSSLK SPNUPÄLK ÄILYZ Z\Yrounds the closed vascular bundles in *Carex foetida*.

**7.270** Round culm of the 10 cm-tall grass-like herbaceous *Kobresia simpliciuscula* on an alpine, dry meadow.

**7.271** A belt of thick- ^HSSLK SPNUPÄLK ÄILYZ Z\Yrounds the closed vascular bundles in *Kobresia simpliciuscula.*

#### *Terrestrial grasses (Poaceae)*

More than 10,000 grass species occur from the tropics to the arctic on extremely dry sites as well as wet sites like lake shores. The outlines of culms are mostly roundish. Presented here are a 30 cm-tall grass species from a ruderal site (*Hordeum murinum*) and a 4 m-tall species from a moist Mediterranean site (*Arundo*  *donax*). Secondary growth is absent. The collateral closed vascular bundles are composed of a xylem with a few enlarged vessels and a round group of phloem. A belt of thick-walled ÄILYZVM[LUZ\YYV\UKZ[OLI\UKSLZ:LLHSZV:JO^LPUNY\ILY Berger 2017.

#### *Macroscopic aspect*

**7.272** *Hordeum vulgare*.

**7.273** Round culm of *Horde um murinum*, a 30 cmtall grass. Vascular bundles are circularly arranged (siphonostele).

**7.274** A belt of thin-walled, SPNUPÄLK ÄILYZ Z\YYV\UKZ the closed vascular bundles in *Hordeum murinum*.

*Cross sections of culms*

**7.275** *Arundo donax*, a 4 m-tall reed. Vascular bundles are arranged in a Fibonacci spiral pattern (atactostele).

**7.276** A belt of thick- ^HSSLK SPNUPÄLK ÄILYZ Z\Yrounds the closed vascular bundles in *Arundo donax*.

#### *Orchids (Orchidaceae)*

Approximately 20,000 autotroph and parasitic species occur from the tropics to the arctic on dry as well as on wet sites. The outlines of culms are mostly roundish. Presented here are a 10 cm-tall upright species from a dry site (*Spiranthes spiralis*) and a 40 cm-tall species from a moist site (*Epipactis atrorubens*). The closed collateral vascular bundles are composed of a xylem with a few small vessels and phloem. See also Stern 2014.

#### *Macroscopic aspect*

**7.277** *Spiranthes spiralis*, a 10 cm-tall species of a dry site in the temperate zone.

# *Cross sections of a culm*

**7.279** Xylem and phloem are anatomically not distinctly separated in the vascular bundle of *Spiranthes spiralis*.

**7.280** An endodermis and circularly arranged vascular bundles separate the pith and the cortex in *Spiranthes spiralis*.

*Sections of a culm*

en

**7.281** Rudimentary vascular bundles inside an endodermis in the bulb of *Spiranthes spiralis*.

#### *Macroscopic aspect*

**7.282** *Epipactis atrorubens*, a 40 cm-tall species of a wet site in the temperate zone. Photo: L.B. Tettenborn, Wikimedia Commons, CC BY-SA 3.0.

**7.278** Vascular bundles are irregularly distributed across the whole stem of *Spiranthes spiralis*.

**7.283** Vascular bundles are irregularly distributed across the whole stem of *Epipactis atrorubens*.

**7.284** The xylem surrounds the phloem in the vascular bundle of *Epipactis atrorubens*.

**7.285** Phloem and vessels of *Epipactis atrorubens*. Sieve tubes and companion cells are not differentiated in the phloem.

#### annular thickenings pit

**7.286** Annular thickenings in large vessels, and bordered pits in small vessels of *Epipactis atrorubens*.

#### *Lianas*

Monocotyledonous lianas occur in various families, e.g. in the Dioscoreaceae, Asparagaceae and Poaceae. Some species are perennial, e.g. the Mediterranean spiny *Smilax aspera*, or the tropical bamboo *Chusquea cumingii*. *Dioscorea communis* or *D. caucasica* have permanent subterranean bulbs and annual liana-like shoots. Characteristic for all liana-like species are collateral vascular bundles with large vessels (150–260 μm in radial diameter).

#### *Macroscopic aspect*

**7.287** *Dioscorea communis*, an annual liana.

**7.288** Annual shoot of *Dioscorea caucasica*, with circular arranged vascular bundles.

*Cross sections of culms/shoots*

**7.289** Closed collateral vascular bundle with large vessels and large sieve tubes in *Dioscorea caucasica*.

**7.290** Phloem with very large and small sieve tubes and small companion cells in *Dioscorea caucasica*.

*Perennial lianas*

**7.291** *Smilax azorica.* Photo:

JCapelo via Wikimedia Commons.

**7.292** Irregular distribution of vascular bundles in *Smilax azorica*.

**7.293** Xylem of *Smilax azorica* with very large vessels.

**7.294** Phloem of *Smilax azorica* with very large sieve tubes.

sieve plate

v

**7.298** Phloem of *Chusquea cumingii* with very large sieve tubes.

**7.295** *Chusquea cumingii*, a tropical bamboo species.

**7.296** Irregularly distributed vascular bundles inside of a dense belt of ÄILYZPU*Chusquea cumingii*.

**7.297** Xylem of *Chusquea cumingii*  with very large vessels.

#### *Hydrophytes*

There are species of various families in this life form. Most of them are distributed worldwide in fresh and marine aquatic environments. Presented here are two submerse species from [OL7V[HTVNL[VUHJLHLHUKAVZ[LYHJLHLHUK[^VÅVH[PUNZWLcies from the Hydrocharitaceae and Lemnaceae (today included in the Araceae family).

The submerse species have a central strand of conducting tissues inside of a more or less distinct endodermis. Phloem and xylem HYLKPMÄJ\S[[VKPMMLYLU[PH[L\ZLWVSHYPaLKSPNO[\*OHYHJ[LYPZ[PJ MVYHSSZWLJPLZPZ[OLWYLZLUJLVMWVVYS`SPNUPÄLKVY\USPNUPÄLK aerenchymatic tissues.

Potamogetonaceae are mainly submerse and ÅVH[PUNWSHU[ZPU fresh water. The family includes approximately 120 species. Zosteraceae are submerse marine plants. The family includes two species. Hydrocharitaceae are aquatic plants in both fresh water and marine habitats. The family includes approximately ZWLJPLZ 3LTUHJLHL HYL ÅVH[PUN O`KYVWO`[LZ;OL L\_HJ[ number of species is unknown.

**7.299** *Potamogeton pectinatus*, a 40 cm-long submerse aquatic plant.

co en 250 μm

**7.300** Stem of *Potamogeton pectinatus* with a large aerenchymatic cortex and a central strand which is surrounded by a thick-walled endodermis.

**7.301** Central strand of *Potamogeton pectinatus* consisting of a central air conducting canal, surrounding sieve tubes and parenchyma cells. ;OL \USPNUPÄLK ]LZZLSZ HYL KPMÄJ\S[ to recognize (use polarized light).

*Macroscopic aspect Longitudinal section*

**7.302** Vessels with helical thickenings in *Potamogeton natans*. v pa

#### *Macroscopic aspect*

**7.303** *Zostera marina*, a submerse marine plant. **7.304** Stem of *Zostera marina* with a large par-

*Cross sections of a shoot*

enchymatic cortex and a central strand which is surrounded by a thin-walled endodermis.

#### *Cross sections of a shoot*

**7.305** Central strand of *Zostera marina*, consisting of a central air-conducting duct with surrounding sieve tubes and parenchyma cells.

#### *Macroscopic aspect*

*Hydrocharitaceae*

**7.306** *Stratiotes aloides*HÅVH[PUNHX\H[PJWSHU[

*Macroscopic aspect*

**7.307** Culm of *Stratiotes aloides*. Vascular bundles are irregularly distributed within an aerenchymatic tissue.

**7.308** Vascular bundles of *Stratiotes aloides* consist of air-conducting tubes with surrounding sieve tubes and parenchyma cells.

**7.309** *Lemna minor*HÅVH[PUNHX\H[PJWSHU[

# aerenchyma ep pa 100 μm

**7.310** Plant body of *Lemna minor*, consisting of a thin-walled aerenchymatic tissue.

#### *Cross sections of the plant body* vab pa

**7.311** Heavily reduced vascular bundle in *Lemna minor*. The central cells probably represent sieve tubes.

#### *Trees and shrubs with secondary growth (***Dracaena***,* **Aloe***)*

Secondary growth is rare in monocots, but it occurs in a few families, e.g. in the Asparagaceae. Secondary growth is different than in dicots. The cambium is located outside of the conducting tissue for water and assimilates. Towards the inside it produces concentric vascular bundles with a phloem in the center, towards the outside it produces a uniform parenchymatic cortex with a periderm at the periphery. Vessels have round, simple pits. See also Chapter 5.2.

*Macroscopic aspect*

**7.312** *Aloe* sp*.*, a monocotyledonous plant with secondary growth.

**7.313** Xylem, phloem and periderm of *Aloe dhufarensis*.

#### *Cross sections of a stem*

**7.314** Formation of a vascular bundle within the cambial zone in *Aloe dhufarensis.*

# 7.4.6 Angiosperms: Dicotyledons and their growth forms

The dicotyledons include approximately 210,000 species in the basal orders Magnolids and Eudicotyledons (Rosids and Asterids; Christenhusz & Byng 2016). Growth forms and life forms cover a wide range (see Chapter 3), and vary from little herbs to lianas and large trees. Dicotyledons grow in all vegetation zones from tropical to arid and arctic zones and in all habitats from extremely dry to submerse sites.

#### *Annual herbs (therophytes)*

The height of annual herbs can vary from 3 cm to more than 4 m. They grow during one vegetation season. However, the growing time within this season differs. Some species grow in ]LY`LHYS`ZWYPUNHUKM\SÄSS[OLPYLU[PYLSPMLJ`JSL^P[OPUHML^ weeks, e.g. *Erophila verna*, others grow late in the season and last only for one or two months, and some have a longer life span within one year, e.g. *Helianthus annuus*. The spectrum of the xylem structure varies. It can be very light, with a density of Exemplarily presented here are some few species of different growth or life forms in various habitats. Excluded are species with successive cambia (for those see Chapter 6.3). The following short presentation will give an impression of the anatomical variability within the dicotyledons of the temperate zone. Taxonomic characteristics on the level of families are presented in Schweingruber *et al*. 2011 and 2013, and Crivellaro & Schweingruber 2015.

0.3 g cm-3[OPU^HSSLKHUKOHYKS`SPNUPÄLK*Erophila*) or heavy, with a density of 0.1 g cm-3, thick-walled and intensively ligni-ÄLK*Euphrasia*). The term "annual" can be misleading: not a single species has a life span of an entire year. Common for all annuals is the presence of just one growth ring, which is formed in the limited period of one astronomical year. Presented here are one very small and one very large annual plant.

*Macroscopic aspect*

**7.315** *Erophila verna*, a 3 cm-tall plant growing in early spring, with a life span of about four weeks.

**7.316** Root of *Erophila verna*^P[OWVVYS`SPNUPÄLKJLSSZPU[OLJLU[LYHUKSPNUPÄLKJLSSZH[[OLWLYPWOery of the xylem. Vessels are extremely small. Rays are absent.

*Very tall annual plant*

*Small annual plant*

**7.317** *Helianthus annuus*, a 200 cm-tall plant growing in late summer and fall, with a life span of about three months.

*Cross section of a shoot*

**7.318** Xylem of *Helianthus annuus*^P[O[OPU[V[OPJR^HSSLKÄILYZSHYNL]LZZLSZHUK]LY`KPZ[PUJ[ rays. xylem

#### *Perennial herbs (hemicryptophytes and geophytes)*

As for annual herbs, the height of perennial herbs can vary from 3 cm to 4 m. Perennial herbs grow over several vegetation WLYPVKZ;OL[LYT ¸OLYI¹PZ HUH[VTPJHSS` UV[ JSLHYS` KLÄULK -VSSV^PUN [OL JVTTVU ÅVYHZ LN(LZJOPTHUU *et al.* 2004), PUJS\KLKPU[OPZ[LYTHYLWSHU[Z^P[OZVM[WVVYS`SPNUPÄLKZ[LTZ VY^P[OZOVY[PU[LUZP]LS`SPNUPÄLKZ[LTZ;YHUZP[PVUZMYVTOLYIZ to dwarf shrubs are morphologically continuous. Most perennial OLYIZM\SÄSS[OLPYlife cycle during several vegetation periods. Their growth is interrupted during cold or dry seasons (dormant periods). The life span varies from two to approximately 50 years. Annual plants with two growth rings, which germinate PUMHSSZ[H`KVYTHU[K\YPUN^PU[LYHUKÅV^LYPUZWYPUN^PU[LY annuals), are an exception in temperate regions. The variability in stem structure is as large as in annual plants.

*Herb with long rhizomes*

**7.319** *Duchesnea indica*

**7.320** Rhizome of *Fragaria viridis* with a diffuse-porous xylem and four annual rings.

*Herb with a short rhizome*

**7.321** *Geranium sanguineum*

**7.322** Rhizome of *Geranium sanguineum* with a semi-ring-porous xylem with seven annual rings.

**7.323** *Paronychia argentea*

**7.324** Taproot of *Paronychia argentea* with a semi-ring-porous xylem with 15 annual rings.

500 μm

**7.325** *Antennaria dioica*, a small plant of colder climates.

**7.326** Taproot of *Antennaria alpina* with a dense, semi-ring-porous xylem with six annual rings.

### *Cushion plants with a tap root*

#### *Dwarf shrubs (chamaephytes and nanophanerophytes)*

Included are approximately 5–50 cm-tall, largely branched, perennial plants with hard, intensively SPNUPÄLK^VVK`Z[LTZ 9OPaVTLZHUKYVV[ZSP]LMVYHWWYV\_PTH[LS`Ä]L[V`LHYZ

*Small costal shrub with a taproot*

*Dwarf shrubs*

**7.328** Taproot of *Frankenia pulverulenta* with a very dense, diffuseporous xylem with six annual rings.

#### *Prostrate dwarf shrub*

**7.329** *Rhamnus pumila*.

**7.330** Stem of *Rhamnus pumila*  with a dense, semi-ring-porous xylem.

#### *Shrubs (nanophanerophytes)*

Included are intensively branched, perennial, 50–400 cm-tall WSHU[Z^P[OOHYKPU[LUZP]LS`SPNUPÄLK^VVK`Z[LTZ;OLVSKLZ[ known individuals reach ages of 800 years (*Juniperus sibirica*).

**7.331** *Paeonia lutea*.

500 μm

**7.332** Stem of *Paeonia suffruticosa* with a semi-ring-porous xylem.

**7.333** *Ribes rubrum*.

**7.334** Stem of *Ribes rubrum* with a semi-ring-porous xylem.

#### *Trees (phanerophytes)*

Included are perennial plants with one basal stem, more than 4 m OLPNO[^P[OOHYKPU[LUZP]LS`SPNUPÄLK^VVK`Z[LTZ;OL` can reach ages up to 5,000 years (*Pinus longaeva*).

**7.335** *Acer pseudoplatanus* **7.336** Diffuse-porous xylem with **7.337** *Quercus robur* small vessels in *Acer campestre*.

**7.338** Semi-ring-porous xylem in *Quercus robur*.

#### *Lianas*

Included here are annual and perennial plants which need support from other plants to grow upwards. Characteristic for all lianas is the presence of large vessels. However, the real demand for water conductance is related to the occurrence of transpiration stress.

*Perennial lianas*


**7.340** Partial ring with large vessels in *Calystegia arvensis*. It is formed in the same year as the closed ring.

**7.341** *Citrullus colocynthis*, a prostrate liana in the extreme desert.

**7.342** Heterogeneous tissue with large vessels (active only in the short rainy periods) in *Citrullus colocynthis*.

**7.343** *\*LSHZ[Y\ZÅHNLSSHYPZ*on a tree stem in the temperate zone.

ray vessel vessel

**7.344** Diffuse-porous xylem with large and small vessels in *Celastrus ÅHNLSSHYPZ*.

**7.345** *Clematis alpina*, one of the few lianas in subalpine and boreal/ arctic environments.

**7.346** Semi-ring-porous xylem in *Clematis alpina*, with a large earlywood containing many vessels.

#### *Succulents*

Included are annual and perennial terrestrial plants with waterstoring stems, growing mostly on dry sites. Common for all observed species is the extended water-storing tissue, be it in the pith, the xylem or the bark. Succulent plants occur in many taxonomic units. They are dominant in hot, arid regions, but are also frequent at dry sites in most other biomes.

**7.347** *Sedum annuum*, a 3 cm-tall herb with succulent leaves and stems, from the temperate zone at low to high altitudes.

*Annuals*

> **7.351** *Sempervivum montanum*, an alpine herb with a basic rosette and ZWVYHKPJ ÅV^LYZMYVT[OL[LTWLYate zone at high altitudes.

#### *Parasites*

Included are annual and perennial terrestrial plants. Growth forms are extremely different, but all of them maintain or complete their metabolism with nutrients from host plants.

**7.348** In *Sedum annuum*, a dense xylem is surrounded by a small phloem and a very large cortex, consisting of large, parenchymatic cells.

**7.352** Root collar of *Sempervivum montanum*^P[OHZTHSS\USPNUPÄLK xylem, a large phloem and a very large cortex.

**7.349** *Portulaca oleracea*, a pros- [YH[L OLYI^P[O ÅLZO`SLH]LZ HUK H succulent stem, from the temperate zone at low altitudes and dry sites.

**7.353** *Euphorbia canariensis*, a cactus-like plant, from the subtropical zone at dry sites.

**7.350** In *Portulaca oleracea*, a thinwalled, parenchyma-dominated xy SLTPZZ\YYV\UKLKI`HU\USPNUPÄLK phloem and cortex.

ca

xy

ph

**7.354** In *Euphorbia canariensis*, a \_`SLT ^P[O [OPU^HSSLK ÄILYZ HUK few vessels is surrounded by a very SHYNL\USPNUPÄLKWOSVLTHUKJVY[L\_

Common for all observed species is the large, water-storing cortex.

**7.355** The thin, annual shoots of *Cuscuta epithymum* are not self-supporting and attach to photosynthetically active host plants with haustoria.

**7.356** Circular arranged vascular bundles are surrounded by a large thin-walled parenchymatic cortex in *Cuscuta* sp.

*Annual parasite Semiparasite*

**7.357** The perennial *Viscum album* on apple trees. This semiparasite connects to the xylem of the host plant by haustoria (see p. 78).

**7.358** Annual shoot of *Viscum album*. Vascular bundles are sur-YV\UKLK I` \USPNUPÄLK WHYLUJO` matic cortex cells.

#### *Parasites*

**7.359** The chlorophyll-free *Lathraea squamaria* connects to the xylem of alder roots by haustoria.

**7.360** In *Lathraea squamaria*, a small YPUNVMSPNUPÄLK ]LZZLSZ Z\YYV\UKZ H large pith, and is itself surrounded by a small phloem and a large cortex. This is a typical succulent structure.

**7.361** The chlorophyll-free *Orobanche alba* is a parasite on several Lamiaceae, and connects to the phloem of the photosynthetically active host plant.

**7.362** In *Orobanche alba*, a ring of vascular bundles surrounds a large pith and is surrounded by a large cortex. This is a typical succulent structure.

*Hydrophytes and helophytes*

Included are annual shoots of plants which grow under water (hydrophytes) or are rooted in permanently wet soils (helophytes). Common for all hydrophytes and helophytes are unlig-UPÄLKaerenchymatic tissues. There are no principal anatomical differences between the two growth forms. Despite the homo-NLULV\ZZ[YLZZM\SHUHLYVIPJLU]PYVUTLU[ZWLJPÄJHUH[VTPJHS differences between species are related to taxonomy.

**7.363** Flower of *Myriophyllum spicatum* above the water table. The major part of the plant is submersed.

**7.364** The central cylinder in *Myriophyllum spicatum* is surrounded by a cortex with large, uniseriate aerenchymatic tubes.

**7.365** Submersed shoot of *Ceratophyllum demersum*.

**7.366** The central cylinder in *Cerato phyllum demersum* is surrounded by a cortex with small aerenchymatic ca nals between large parenchymatic cells.

**7.367** ;OLÅVH[PUNSLH]LZHUKÅV^ ers of *Nuphar lutea* root deep in the ground below the water table.

# *Hydrophyte* vascular bundle cortex 500 μm

**7.368** Single vascular bundles are surrounded by an aerenchymatic tissue in *Nuphar lutea*.

**7.369** Annual shoots of *Hippuris vulgaris*.

# central cylinder cortex 1 mm

**7.370** In *Hippuris vulgaris*, the small central cylinder is surrounded by a cortex with large aerenchymatic tubes.

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# 8. Evolution of stems

# 8.1 Paleobotanic evidence of stems

Writing a comprehensive story about Z[LTL]VS\[PVUPZKPMÄJ\S[ because evidence of only few early plants has been preserved over millions of years. Samples exist of plants which were fossilized under anaerobic conditions, enclosed in resin (amber) or carbonized. The majority of plants from sites with aerobic soil conditions are not preserved. Presented here are some evolutionary events of major importance, primarily based on WL[YPÄLKHUKJHYIVUPaLKWSHU[YLTHPUZ5V[KPZJ\ZZLKHYL[OL fossil ferns, the living seed ferns, and the ginkgos and angiosperms. For their structure refer to the description of their living representatives in Chapter 7.

#### *Early plant evolution*

A four-billion-years-lasting evolutionary process of water organisms lacking a nucleus (prokaryota) set the basis for life on land. First, marine microbial mats, the stromatolites, increased the amount of oxygen in the atmosphere through photosynthesis. Next, the marine algae developed polarity by forming long, NLV HUK OLSPVJLU[YPJHSS` VYPLU[LK H\_PHSS` YHTPÄLK JLSS\SHY strands.

The fact that ancient plant structures have survived for 400 million years is demonstrated by one fossil and few living green algae (chlorophyta). The modern green algae genus *Chara* is a typical living fossil.

Paleontologists assume that early fossil algae in tides zones had root-like structures. Shown here is a modern *Laminaria* species with rhizoids. 9OPaVPKZ HYL IHZHS ÄSHTLU[Z ^OPJO H[[HJO [OL whole plant to a stable ground. Water transport in aquatic algae occurs by diffusion, therefore vessels have not been developed.

#### *Fossil and recent green algae Recent brown algae*

**8.1** *Heterocladus waukahesha ensis*, a 5 cm-tall Silurian green alga with a central stem and many whorls. Reprinted from Taylor *et al*.

2009.

**8.2** Stem of *Palaeonitella cranii*, a Devonian green alga with a central stem and three whorls. Reprinted from Taylor *et al*. 2009.

**8.3** Stem of *Chara* sp., a comparable recent green alga showing four whorls.

**8.4** *Laminaria* sp., a modern brown alga with rhizoids.

**8.5** Cross section of a modern aquatic brown alga stem without liquid-conducting structures. The whole plant consists of parenchyma cells. Small, fairly thick-walled cells surround the periphery.

#### *The move to the land*

First land plants are known from the Upper Silurian and Upper Devonian period. Over the course of 60 million years, approximately from 420 to 360 million years ago, plants developed TLJOHUPZTZ[VZ\Y]P]LPUHUH[TVZWOLYLOVZ[PSL[VSPML;OLÄYZ[ terrestrial plants featured several new traits in comparison to WYLJLKPUNHX\H[PJWSHU[Z3PNUPÄLKLSLTLU[ZN\HYHU[LLKstability, tracheids and sieve cells allowed the transport of liquids, an epidermis prevented dehydration, stomata enabled gas exchange, and secondary growth provided longevity. All these features are so fundamental that they survived in all existing land plants until the present day.

Presented here are a few paleobotanical reconstructions. For further information the well-illustrated book from Taylor *et al.*  2009 is recommended.

#### *The earliest small land plants*

First land plants developed over the course of 20 million years in the late Silurian period. They reached a height of approximately 20 cm, were leaf-less, and had a simple stem structure. Characteristic is a central cylinder consisting of a round or starshaped xylem with tracheids and a phloem (protostele or actinostele). Tracheids have annular thickenings.

**8.6** Reconstruction of the Devonian *Rhynia gwynne-vaughanii*. Reprinted from Hirmer 1927.

**8.7** Cross section of a stem of *Rhynia gwynne-vaughanii* with a small xylem and a large cortex, surrounded by an epidermis. The cortex is divided into an inner and outer part.

**8.8** Cross section of a branch of *Rhynia gwynne-vaughanii* with an extremely small xylem and a large cortex, surrounded by an epidermis.

**8.9** Epidermis of *Rhynia gwynnevaughanii*.

**8.10** Cross-section of a stem of the Devonian *Asteroxyleon mackiei* with a star-shaped xylem (actinostele). Reprinted from Zimmermann 1959.

*Asteroxyleon*

*Rhynia*

**8.11** Tracheids of *Asteroxyleon mackiei* with annular thickenings. Reprinted from Henes 1959.

#### *The earliest trees*

First trees with a modern habit appeared in the Late Devonian period and lasted until the Early Carboniferous period. *Archeopteris* trees (syn. *Callixylon*) were widely distributed approximately 380 to 360 million years ago. With the presence of a coniferous wood structure, but still reproducing by spores, [OLZL[YLLZHYLJSHZZPÄLKHZprogymnosperms. Characteristic are tracheids with bordered pits, produced by a cambium (secondary growth).

#### *The Carboniferous clubmoss and horsetail forests (Lycophyta and Sphenophytes)*

A huge diversity of the spore-reproducing horsetail and clubmoss trees dominated the Late Devonian and the Carboniferous period approximately from 390 to 300 million years ago. Widely distributed were the genera *Lepidodendron* and *Sigillaria* (Lycophyta) and *Calamites* (Equisetales).

The structure of clubmoss trees is principally similar to modern trees. However, the proportions between xylem, phloem, cortex and periderm greatly differ. *Lepidodendron* trees are "bark trees"—a small xylem/phloem center is surrounded by an extremely large primary cortex and a very large periderm. The major part of the xylem is produced by a cambium (secondary growth). The arrangement of tracheids and rays is similar to conifers, however, cell walls are scalariform perforated. The absence of annual rings leads to the assumption that these trees grew in a tropical environment.

*Calamites* trees have a principally similar structure, but the bark is much smaller and the pith is enlarged. *Calamites* are "pith trees". The presence of carinal canals in some species indicates a relation to living herbaceous horsetails. Radial pitting on tracheids varies from scalariform to circular bordered.

1962.

**8.13** Reconstruction of clubmoss trees *Lepidodendron* sp. and *Sigillaria* sp., and the arborescent horsetail *Calamites* sp. Reprinted from Hirmer 1927.

**8.14** Stem reconstruction of the clubmoss tree *Lepidodendron*. One third of the stem radius consists of pith, xylem and phloem and two thirds of cortex and periderm (after Hirmer 1927).

#### *Sigillaria*

**8.15** Cross section of the xylem of *Sigillaria saulii*. Tracheids of the secondary xylem are radially arranged. Annual rings are absent. Reprinted from Henes 1959.

**8.16** Structure of tracheids of *Sigillaria saulii*. Characteristic for all Lepidphytales are scalariform wall structures with longitudinal thin strips. Reprinted from Hirmer 1927.

**8.17** Cross section of a root of *Stigmaria* sp., Lycopodiales, from the Lower Carboniferous. The secondary xylem consists of tracheae and rays. Reprinted from Hirmer 1927.

**8.18** Cross section of a root of *Calamites* sp. from the Lower Carboniferous, consisting of a secondary xylem, and a cortex with aerenchyma. Reprinted from Hirmer 1927.

#### *;OLÄYZ[gymnosperms (Cordaitales)*

Shown here is the anatomical structure of *Dadoxylon* sp. (syn. *Araucarioxylon*), one representative of an extinct group of conifer-like trees with secondary growth. The Cordaitales existed in the Carboniferous and were seed plants with a xylem similar in structure to conifers. The only difference to living conifers is the existence of bi- to triseriate rays. Multiseriate bordered pits are very similar to the modern Araucariaceae. The plants with large leaves have a similar aspect to some living Podocarpaceae. The absence of annual rings indicates that *Dadoxylon* trees grew in a tropical climate.

**8.19** Leaves of a species of the Cordaitales. Reprinted from Taylor *et al*. 2009.

**8.20** Cross section of *Dadoxylon* sp. without annual rings.

*Cordaitales* pits r

**8.21** Radial section of *Dadoxylon* sp. with multiseriate bordered pits on tracheid walls.

**8.22** Tangential section of *Dadoxylon* sp. with bi- to triseriate rays.

#### *The development of angiosperms*

Fossil stem remains of angiosperms are rare, but pollen records from the early Cretaceous period indicate a large taxonomic diversity 140 million years ago. Wood remains from the late Cretaceous already show an anatomical diversity comparable to that at present. Here, it is presented in Chapter 7 on modern material starting with palm ferns (Chapter 7.4.1) and ending with monocots (Chapter 7.4.5).

# 8.2 Evolution and homoplasy of wood anatomical traits

It makes sense to consult fossilized stems to analyze structural developments in stems. In Table 1 below, a few anatomical features are set in relation to fossil evidence and in a phylogeny-IHZLK [H\_VUVTPJ Z`Z[LT (SYLHK` H[ ÄYZ[ NSHUJL P[ ILJVTLZ obvious that there was no straightforward evolution of most features. Most of the features are homoplastic—they developed or disappeared in different taxa (convergent developments). Spicer & Groover 2010 and Carlquist 2012 have demonstrated this on the basis of anatomical traits of fossil and living trees. In contrast to previous studies, herbs are included in the analysis here, and angiosperms are grouped by growth form (trees, shrubs, lianas, large and small terrestrial herbs and hydrophytic herbs) instead of by phylogenetic units. Some results are therefore different to the aforementioned studies.

In Table 1, the appearance and occurrence of secondary growth and perforation plates of vessels are highlighted over a period of 400 million years.

*Secondary growth* is present in Devonian and Carboniferous Pteridophyta. It was lost in most living Pteridophytes. The presence of secondary growth in small herbs of modern Ophioglossaceae can be interpreted as a relict or a reinvention (Chapter 7.3.2). Radial growth, including radial growth with successive cambia, occurs in the majority of living seed plants (gymnosperms, dicotyledons). Secondary growth is principally absent in monocotyledons, however a reinvention began in Dracaenaceae and Agavaceae with a new mode of formation (Chapter 7.4.5). Secondary growth is absent in many dicotyledonous hydrophytes e.g. in Nymphaeceae, Ranunculaceae, Droseraceae and others. Since secondary growth exists in the majority of genetical taxa its absence can be interpreted as a retrogressive development.

*Vessels* are a characteristic of dicotyledons. They are absent in all Paleozoic taxa and fossil and modern gymnosperms. Vessels appeared parallel to the development of angiosperms in the Lower Cretaceous period. The occurrence of vessels in living Equisetaceae and in ferns of Ophioglossaceae seems to be a modern (Cretaceous?) development. A few taxa without vessels exist within the angiosperms. Phylogenetic studies lead to the assumption that at least *Amborella* is a relict of Pteridophytes, however Trochodendronaceae and Winteraceae are either relicts or retrogressive taxa. The following pictures clearly show that the presence of scalariform pits and pits with slit-like apertures are relictic, and large rays and ray dimorphism are modern features. Paleozoic and Cretaceous developments meet in a very few shrubs and trees.

**Table 8.1** Occurrence or absence of vessels and secondary growth in relation to major taxonomic units from Paleozoic times to the present.


#### *Amborellaceae*

**8.23** Radial arrangement of tracheids, no annual rings in *Amborella trichocarpa*.

**8.24** Tracheid pits of *Amborella trichocarpa* with slit-like apertures. Tori seem to be absent.

*Trochodendraceae*

**8.25** Uniseriate and bi- to four-seriate heterogeneous rays in *Amborella trichocarpa*.

**8.26** Radial arrangement of tracheids. Distinct early- and latewood in *Trochodendron aralioides*.

**8.27** Scalariform tracheid pits in *Trochodendron aralioides*. Tori seem to be absent.

**8.28** Uniseriate and bi- to multiseriate heterogeneous rays in *Trochodendron aralioides.*

**8.29** Radial arrangement of tracheids, no annual rings in *Drimys winteri.*

*Winteraceae*

**8.30** Tracheid pits with slit-like apertures in *Drimys winteri*. Tori seem to be absent.

**8.31** Uniseriate and bi- to four-seriate heterogeneous rays in *Drimys winteri*. Distinct sheath cells.

#### *Homoplasy and evolution*

,]VS\[PVUHY`[YLUKZPU \_`SLT L]VS\[PVU HYL KPMÄJ\S[[V YLJVNnize because functional adaptation to hydraulic and mechanic needs shaped anatomical structures. This is demonstrated here by relating a few anatomical characteristics to plant size. The following graphs express probabilities of occurrence, and are based on observations of 3,347 dicotyledons from seasonal climates in the Northern Hemisphere. The general trends shown PU[OLÄN\YLZILSV^^LYLJVUÄYTLK^P[OKL[HPSLKHUHS`ZLZVM the Apiaceae, Asteraceae, Brassicacea, Fabaceae, Rosacea and Lamiaceae (Schweingruber *et al*. 2013).

*Vessel diameters* <20 μm primarily occur in plants <10 cm in height. Such plants often grow in alpine zones. Vessel diameters >50 μm occur mainly in tall trees.

*Porosity* types correspond with plant size. Ring porosity occurs mainly in trees. It is absent in very small plants.

*Scalariform perforations* are rare. They are almost absent in ZTHSSWSHU[ZHUKVJJ\YVUS`PUHWWYV\_PTH[LS`VM[OLHUHlyzed specimens. They are absent in plants of the arid zone. *Simple perforations* dominate in most taxa of any size and in any climate type.

2-10 10-25 25-50 50-100 100-150 150-300 >300 Probability 0.0 0.5 Plant height (cm) Porosity ring-porous semi-ring-porous diffuse-porous 2-10 10-25 25-50 50-100 100-150 150-300 >300 Probability 0.0 0.6 Plant height (cm) fibers absent intra-annual thick-walled fibre bands fibers absent in the center parenchyma pervasive Fibers and parenchyma *Fibers and parenchyma*. Plant sizes shape the anatomy. Fibers dominate the xylem of all large plants. They are often absent in very small plants or in hydrophytes. In contrast, parenchyma dominates the anatomy of small plants. Pervasive parenchyma is absent in large plants, and very frequent in very small plants.

*Crystals* 7SHU[ ZPaL HSZV PUÅ\LUJLZ WO`ZPVSVNPJHS WYVJLZZLZ Prismatic crystals in the xylem and the bark are much more frequent in large than in small plants.

**8.32** Probability of occurrence of anatomical characteristics in relation to plant size, based on 3,347 dicotyledons from seasonal climates in the Northern Hemisphere. Reprinted from Schweingruber *et al.* 2013.

# 8.3 Parallel evolution of macroscopic and microscopic traits

The question is raised here of how local environmental fac- [VYZPUÅ\LUJL[OLTHJYVZJVWPJHUKTPJYVZJVWPJ[YHP[ZVMZ[LTZ Exemplarily presented and compared are the macroscopic aspect (plant formation), the SPML MVYT VM WSHU[Z [OL ÅVYPZ[PJ composition and the anatomical structure of stems of a subalpine forest, an oceanic shrub community, and an alpine herb community.

( Z\P[L VM ZWLJPÄJ LU]PYVUTLU[HS MHJ[VYZ LUHISLZ WSHU[Z ^P[O similar physiological qualities and surviving strategies to grow together.

*Plant formations* HYL KLÄULK I` [OLPY WO`ZPVNUVTPJ JOHYHJter, e.g. forests are plant formations with a dominance of trees. Heathlands are dominated by dwarf shrubs and meadows by herbs. *Plant communities*HYLKLÄULKI`[OLPYÅVYPZ[PJJVTWVZPtion, e.g. the tree storey of subalpine forests in the Alps consists of conifers, the shrub layer of heathlands consists of *Calluna vulgaris* and *Genista* sp., and alpine meadows are dominated by herbs. An ecophysilogical and taxonomic interplay determines formations and communities.

The term parallel evolution is related to taxonomic groups. If parallel evolved taxa are genetically far apart, e.g. conifers and dicotyledons, parallel developments are obvious, e.g. *Abies* and *Sorbus* are both trees. Parallel developments are less obvious for genetically closer related taxa, e.g. two genera within the conifers such as *Pinus* and *Abies* are both Pinaceae and always grow as trees, or *Vaccinium myrtillus* and *Vaccinium uliginosum* are both Ericaceae and always grow as dwarf shrubs.

**8.33** The tree layer in the ÄYMVYLZ[PU,TTLU[HS:^P[aLYSHUKSLM[PZKVTPnated by *Abies alba*, accompanied by *Sorbus aucuparia* (right).

#### *Atlantic heathland*

**8.34** Heathland at the North Atlantic coast, Cornwall, England (left). Dwarf shrub cushions are dominated by *Erica* sp., *Calluna vulgaris* (right), *Ulex* sp. and *Genista* sp.

**8.35** Alpine meadow at 2,900 m a.s.l. in the European Alps with *Carex curvula* and cushion plant *Minuartia sedoides*.

**8.36** *Minuartia sedoides*, a perennial alpine cushion plant with 85 annual rings in the taproot.

#### *4LZPJ,\YVWLHUÄYMVYLZ[(KLUVZ[`SV(IPL[L[\T*

Conifers, few bushes and tall herbs are characteristic for forests at the northern slopes of the Alps. Firs (*Abies alba*) prefer sites with high precipitation (>1,500 mm/year), long winters with longer frost periods and nutrient rich soils at altitudes between 1,300–1,700 m a.s.l. The tree layer is dominated by *Abies alba* and accompanied by *Sorbus aucuparia*. The shrub layer consists of *Lonicera nigra* and *Rosa pendulina*, and tall herbs such as *Adenostyles alliariae*, *Petasites albus*, *Prenanthes purpurea* and *([O`YP\TÄSP\_MLTPUH* dominate the herb layer.

Species within the tree, shrub and herb layers evolved parallel to adopt the same growth form. However, the anatomical stem structures of all occurring plants preserved their genetic heritage.

The xylem of the conifer *Abies alba* consists of tracheids; the anatomical structure expresses its coniferous heritage. Vessels of all dicotyledonous species in the tree, shrub and herb layers are part of parallel evolution. High soil water content and intensive water transport triggered the formation of large earlywood vessels with a KPHTL[LY VM ¶ T /V^L]LY [OL ]LZZLSÄILY WHYLUJO`TH WH[[LYUZ HYL Z[PSS ZWLJPLZZWLJPÄJ ,U]PYVUTLU[HS MHJ[VYZPUÅ\LUJLK[OLJHWHJP[`VM[OLJVUK\J[PVUZ`Z[LTI\[UV[ the arrangement of cell types. See also Keller *et al*. 1998.

#### *Isolated evolution of conifer Parallel evolution of vessel diameters in the tree and shrub layers*

**8.37** The xylem of *Abies alba*, Pinaceae, consists of tracheids.

# 250 μm

**8.38** Xylem of the 5 m-tall *Sorbus aucuparia*, Rosaceae, with large earlywood vessels.

**8.39** Xylem of the 1.5 m-tall *Lonicera nigra*, Caprifoliaceae, with large earlywood vessels.

**8.40** Xylem of the 1 m-tall *Rosa pendulina*, Rosaceae, with large earlywood vessels.

**8.41** Vessels in the rhizome of the 40 cm-tall *Adenostyles alliarae*, Asteraceae.

#### *Parallel evolution of vessel diameters in rhizomes of herbs*

**8.42** Vessels in the rhizome of the 50 cm-tall *Petasites albus*, Asteraceae.

**8.43** Vessels in the rhizome of the 80 cm-tall *Prenanthes purpurea*, Asteraceae.

**8.44** Vessels in the rhizome of the 80 cm-tall *([O`YP\T ÄSP\_MLTPUH*, Polypodiaceae.

#### *Heathlands along the European North Atlantic coast*

Dwarf shrubs and small bushes are characteristic for the vegetation along the northwestern European coasts. This coastal region is characterized by high precipitation (>1,000 mm/year), mild temperatures without frosts (e.g. annual mean temperature approx. 10°C in Cardiff, England), strong winds, granitic bedrock and thick, acidic, often wet organic soils.

The plant formation, the heath, is dominated by dwarf shrubs of different Ericaceae (genera *Calluna*, *Erica*) and Fabaceae (genera *Ulex*, *Genista*, *Cytisus*; Gorissen 2004).

,U]PYVUTLU[HS MHJ[VYZ OH]L PUÅ\LUJLK [OL TVYWOVSVNPJHS aspect of the plant formation (small shrubs) and the diameter of earlywood vessels (30–50 μm) in the xylem, but distribution WH[[LYUZVM]LZZLSZÄILYZWHYLUJO`THYLÅLJ[[OLNLUL[PJOLYP[HNL of the taxa. *Erica* species are diffuse-porous and Fabaceae species are semi-ring-porous, with vessels arranged in dendritic patterns.

The morphological aspect as well as the anatomical stem structure of the accompanying herbs vary greatly. All species preserved their external and internal taxonomical characteris- [PJZ;OLPY ZWLJPÄJ HUH[VTPJHS Z[Y\J[\YLZ LUHISL[OLT[V YLZPZ[ extreme local environmental conditions. Evolutionary processes therefore did not drive the basic anatomical structure of herbs in a similar direction.

*ȀƤ*

**8.45** Xylem of *Calluna vulgaris*, Ericaceae.

**8.46** Xylem of *Erica tetralix*, Ericaceae.

**8.47** Xylem of *<SL\_WHY]PÅVYH*, Faba-

**8.48** Xylem of *Genista cinerea*, Fabaceae.

**8.49** Cross section of a culm of the monocotyledonous *Agrostis stolonifera*, Poaceae.

#### *Divergent evolution of the anatomy of accompanying species*

ceae.

**8.50** Cross section of the stem of the dicotyledonous *Potentilla erecta*, Rosaceae.

**8.51** Cross section of the petiole of the fern *Pteridium aquilinum*, Polypodiaceae.

**8.52** Cross section of the stem of the moss *Sphagnum subnitens*, Sphagnaceae.

#### *Alpine meadows (Caricetum curvulae)*

Small, 3–10 cm-tall grasses and dicotyledonous herbs characterize meadows on gentle slopes on granitic bedrocks in alpine zones over 2,500 m a.s.l. in the Alps. The snow cover is reduced due to snow drift and vegetation periods are hardly longer than three months. *Carex curvula* and *Silene acaulis* and other species form dense cushions on dry, acidic soils, with an extended root system and minimal aboveground biomass. A few other grasses and dicots persist between those cushions (Klötzli *et al.* 2010).

Parallel evolution is expressed by small plant height and vessels with small diameters (15–30 μm). However, the general anatomical stem structure of all species present preserved their genetic heritage.

#### *Parallel evolution of vessel diameters of cushion plants*

**8.53** Cross section of the culm of the monocotyledonous *Carex curvula*, Cyperaceae.

**8.54** Cross section of the culm of the monocotyledonous *1\UJ\Z [YPÄK\Z*, Juncaceae.

**8.55** Cross section of the stem of the dicotyledonous *Silene acaulis*, Caryophyllaceae.

**8.56** Cross section of the stem of the dicotyledonous *Minuartia recurva*, Caryophyllaceae.

**8.57** Cross section of a culm of the monocotyledonous *Luzula alpinopilosa*, Juncaceae.

#### *Parallel evolution of non-cushion-forming herbs*

**8.58** Cross section of the stem of the dicotyledonous *Primula minima,*  Primulaceae, growing isolated or as a small cushion.

**8.59** Cross section of the stem of the dicotyledonous *Leontodon helveticus*, Asteraceae.

**8.60** Cross section of the stem of the dicotyledonous *Phyteuma hemisphaericum*, Campanulaceae.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 9. Anatomical adaptations to permanently changed environmental conditions

# 9.1 Anatomical and morphological plasticity of species

(SS^VVKHUH[VT`JSHZZPÄJH[PVUIVVRZKLZJYPIL[OLZ[Y\J[\YLVM so-called "normally grown" specimens. Nature, however, also produces a few giants, and many dwarfs. Here the question is raised to which degree local environmental factors modify the morphology and anatomy of stems of individual species. Exemplarily presented are the xylem structures of a few large (giants) and small individuals (dwarfs) of trees and herbs. Larger-thanaverage individuals have experienced mostly favorable growing conditions when they were young, and during their lifetime escaped the effects of extreme growth-limiting factors such as lack of nutrients, water or light, frost, or injury. In contrast, limiting and extreme factors negatively affected growth in small individuals. Giants are the winners, and dwarfs are the losers of competition.


Ń Trees, shrubs and herbs follow the same reaction mecha-UPZTZ[VLU]PYVUTLU[HSPUÅ\LUJLZ


**9.1** *Fraxinus excelsior.* Top: 20 m-tall as part of a riparian forest. Bottom: a 60 cm-tall browsed bush in a meadow.

**9.2** *Carpinus betulus*. Stem of a 10 m-tall dominant tree and a 50 cm-tall suppressed sapling.

**9.3** *Alliaria petiolata*. Bi-annual plants on a ruderal site, 12–65 cm in height.

**9.4** *Arabidopsis thaliana*. Annual plants on a ruderal site, 6–35 cm in height.

# *Anatomical characteristics of genetically large and small species*

#### *Fraxinus excelsior*

#### *Carpinus betulus*

Tall old trees of common ashes (*Fraxinus excelsior*) with large crowns and large stem diameters grow mostly on nutrient rich, moist soils. The xylem anatomy of dominant trees and suppressed individuals is extremely different. Characteristic for large trees are ring porosity, large dense latewood and large rays. Small individuals with small crowns and thin stems grow under the canopy and are periodically exposed to grazing. They survive periodic crown damages and can get old. Characteristic MVY [OLT HYL [PZZ\LZ JVUZPZ[PUN VM [OPU^HSSLK ÄILYZ ML^ ]LZsels and small rays. A large water-conducting earlywood in trees is in accordance with a large assimilating tissue and extensive ZHWÅV^0UJVU[YHZ[[OLML^]LZZLSZPUZ[LTZVMZ\WWYLZZLKVY JYPWWSLK[YLLZ YLÅLJ[[OLWYLZLUJLVMH YLK\JLKSLHMTHZZHUK PUKPJH[LH YLK\JLKZHWÅV^(THQVYM\UJ[PVUVMSHYNL YH`ZPU large trees is probably to enhance radial stem stability.

**9.5** Ring-porous xylem with a large earlywood zone and many large vessels in the cross section of a dominant tree of *Fraxinus excelsior*.

**9.6** -L^ ZTHSS ]LZZLSZ PU H ÄIYV\Z tissue in the cross section of the xylem of a browsed sapling of *Fraxinus excelsior*.

**9.9** ;YP[VÄ]LZLYPH[LOVTVNLULV\Z YH`Z IL[^LLU ÄILYZ WHYLUJO`TH cells and vessels in the tangential section of a dominant tree of *Fraxinus excelsior*.

**9.10** Uni- to biseriate homogeneous and heterogeneous rays between ÄILYZHUKH\_PHSWHYLUJO`THJLSSZPU the tangential section of a browsed sapling of *Fraxinus excelsior*.

**9.7** Diffuse-porous xylem with vessels in radial rows between thin- to [OPJR^HSSLKÄILYZPU[OLJYVZZZLJtion of a dominant tree of *Carpinus betulus*.

**9.11** Slender uni- and biseriate homogeneous rays in the tangential section of a dominant tree of *Carpinus betulus*.

Hornbeam (*Carpinus betulus*) grows mostly on shallow soils. Trees form the 10 m-tall canopy and suppressed individuals (saplings) are in the herb layer in the shadow. The xylem anatomy of dominant trees and suppressed individuals is fairly similar.

Common are vessels in radial rows and radial vessel-free zones. Vessels in trees are slightly larger than those in suppressed trees. Rays in trees are distinctly distinguished from the ground tissue and those in suppressed trees are similar to the axial parenchyma cells. A functional explanation of the xylem structure between [YLLZHUKZHWSPUNZPZKPMÄJ\S[

250 μm

**9.12** Wide uni- and biseriate rays in the tangential section of a suppressed sapling of *Carpinus betulus*.

Garlic mustard (*Alliaria petiolata*) is a biannual herb growing on deep, rich soils. Dense stands are composed of 15–100 cm-tall individuals. The xylem anatomy of dominant and suppressed individuals is very different. The second ring in large individuals contains 60–80 μm wide, radially arranged vessels within a thin- ^HSSLK ÄILY[PZZ\L9H`Z HYL ZPTPSHY[V H\_PHS WHYLUJO`TH JLSSZ ZVTLHYLPUJVUÅ\LU[NYV\WZ;OLZLJVUKYPUNPUZTHSSPUKP]PK\HSZ contains a few vessels with a diameter of 20–30 μm within a thin- [V[PJR^HSSLKÄILY[PZZ\L9H`ZHYLZPTPSHY[V[OLVULZPUSHYNL PUKP]PK\HSZI\[[OLÄILYZHYLZTHSSLY(SHYNL^H[LYJVUK\J[PUN earlywood with vessels in large individuals is in accordance with HSHYNLHZZPTPSH[PUN[PZZ\LHUKL\_[LUZP]LZHWÅV^0UJVU[YHZ[[OL ML^ ]LZZLSZPU Z[LTZ VM[OL ZTHSSPUKP]PK\HSZ YLÅLJ[ H YLK\JLK SLHMTHZZHUKPUKPJH[LHYLK\JLKZHWÅV^;OLKLUZLÄILY[PZZ\L guarantees stability of the very thin stem. Dense stands suggest that root competition is the major growth-limiting factor.

f

v

#### *Alliaria petiolata Arabidopsis thaliana*

Thale cress (*Arabidopsis thaliana*) is an annual herb and grows on dry to medium moist sites with rich soils. Stands are composed of 5–50 cm-tall individuals. The center of large and small PUKP]PK\HSZOHZUVÄILYZ"P[JVUZPZ[ZTHPUS`VM\USPNUPÄLKWHYLUchyma cells. Rays are absent in large and in small individuals. Very small vessels with a diameter of 15–25 μm are arranged in long radial rows. Vessels are almost absent in small individu-HSZ:THSS]LZZLSZN\HYHU[LLZHWÅV^PUSHYNLPUKP]PK\HSZ>H[LY conductance in small individuals occurs probably through ÄILYZ+PMMLYLU[YVV[ZWHJLZHYLNYV^[OSPTP[PUN

**9.13** Large radially arranged vessels ^P[OPU H[OPU^HSSLK ÄILY[PZZ\LPU the cross section of a large individual of *Alliaria petiolata*.

**9.17** 9H`Z H\_PHS ÄILYZ HUK WHYLU-JO`THJLSSZHYLUV[VYKPMÄJ\S[[VKPZtinguish in the tangential section of a large individual of *Alliaria petiolata*.

**9.14** Few small vessels within a [OPU[V[OPJR^HSSLK ÄILY[PZZ\LPU the cross section of a small individual of *Alliaria petiolata*.

**9.18** 9H`Z H\_PHS ÄILYZ HUK WHYLU-JO`THJLSSZHYLUV[VYKPMÄJ\S[[VKPZtinguish in the tangential section of a small individual of *Alliaria petiolata*.

**9.15** Small, radially arranged ves-ZLSZ^P[OPUH[OPU^HSSLKÄILY[PZZ\L in the cross section of a large individual of *Arabidopsis thaliana*.

**9.19** 9H`Z H\_PHS ÄILYZ HUK WHYLUchyma cells are indistinguishable in the tangential section of a large individual of *Arabidopsis thaliana*.

250 μm

v

f

**9.16** Very few small vessels within H [OPU [V [OPJR^HSSLK ÄILY [PZZ\L in the cross section of a small individual of *Arabidopsis thaliana*.

**9.20** 9H`Z H\_PHS ÄILYZ HUK WHYLUchyma cells are indistinguishable in the tangential section of a small individual of *Arabidopsis thaliana*.

# ͡Ǥ͚ơ

=LNL[H[PVU aVULZ HYL [OL YLZ\S[ VM JSPTH[PJ PUÅ\LUJLZ AVULZ HYLKLÄULKI`JSPTH[PJJVUKP[PVUZHUK[OLWO`ZPVNUVT`VM[OL vegetation, e.g. the constantly wet tropical zone is dominated by evergreen trees, epiphytes and lianas, or the dry cold-temperate (boreal) zone is dominated by conifers and evergreen or summer-green dwarf shrubs (Pfandenhauer & Klötzli 2014).

Here the question is raised to which degree zonal climatic conditions affect the physiognomy and the anatomy of stems. Wheeler *et al*. 2007 analyzed the xylem of almost 6,000 trees from the tropics to the boreal zones around the globe. They found that the xylem in trees of different vegetation zones is expressed in a modest way by ring distinctness and the water conductivity system (vessels). Based on their results, presented here are frequently occurring anatomical structures of trees and shrubs from different latitudinal and altitudinal zones.


# 9.2.1 Trees in the tropics, the temperate and the boreal zone

#### *Tropical rain forest*

Trees with adaptations to a persistently warm, aseasonal climate with more than 3,000 mm of annual precipitation. Water transport through the tall stems with heights up to 50 m to the large, evergreen crowns occurs through a few large vessels. Most trees in lower altitudes of tropical rain forests do not form annual ring boundaries, making cross-dating impossible.

**9.21** *Microberlinia brazzavillensis*, Fabaceae, a 40 m-tall evergreen tree. Diffuse-porous wood with 5–10 vessels/mm2 , >200 μm vessel diameter and paratracheal parenchyma.

**9.22** *Pseudobombax munguba*, Malvaceae, a 30 m-tall deciduous tree. Diffuse-porous wood with approx. 5–10 vessels/mm2 , >200 μm vessel diameter, very [OPJR^HSSLKÄILYZML^]LZZLSZHUK a lot of parenchyma.

**9.23** *Diospyros ebenum*, Ebenaceae, a 25 cm-tall evergreen tree. Diffuse-porous wood with black heartwood, approx. 5 vessels/mm2 , >200 μm vessel diameter, and parenchyma around vessels and in bands.

**9.24** *Sarcotheca* sp., Oxalidaceae, an evergreen tree. Diffuse-porous wood with approx. 5 vessels/mm2 , T]LZZLSKPHTL[LYTHU`ÄILYZ and few axial parenchyma cells.

#### *Temperate forest*

Trees with adaptations to a temperate seasonal climate with 1,000 mm of annual precipitation. Water transport through the tall stems with heights up to 35 m to the large, deciduous crowns occurs mostly through many small vessels (diffuseporous wood), or in a few species through large vessels in the earlywood and small vessels in the latewood (ring-porous ^VVK;OL HUH[VTPJHS JOHYHJ[LY VM [OL LHYS`^VVK YLÅLJ[Z H tropical and that one in the latewood a temperate climate. Principally all trees and shrubs in temperate climates form annual YPUNZ\*YVZZKH[PUNPZ[OLYLMVYLWVZZPISL(IV\[VMWLYLUnial dicotyledonous herbs also produce annual rings.

**9.25** *Fagus sylvatica*, Fagaceae, a 25 m-tall deciduous tree. Diffuseporous wood with approx. 300 vessels/mm2 , 100 μm earlywood vessel diameter.

**9.26** *Acer pseudoplatanus*, Sapindaceae, a 25 m-tall deciduous tree. Diffuse-porous wood with approx. 150 vessels/mm2 , 100 μm vessel diameter.

**9.27** *Pyrus communis*, Rosa ceae, a 15 m-tall deciduous tree. Diffuseporous wood with approx. 300 vessels/mm2 and 100 μm vessel diameter.

**9.28** *Ulmus glabra*, Ulmaceae, a 25 m-tall deciduous tree. Ringporous wood, latewood vessels in tangential bands.

#### *Boreal forest*

Trees with adaptations to a seasonal climate with warm short summers and cold winters in the conifer belt of the boreal zone, dominated by spruces (*Picea* sp.), larches (*Larix* sp.) and pines (*Pinus* sp.). Birch trees (*Betula pendula*) and mountain-ash (*Sorbus aucuparia*) are common accompanying tree species. Water transport through the tall stems with heights up to 25 m to the evergreen crowns occurs primarily through the earlywood tracheids. The anatomically diffuse-porous structure of the accompanying deciduous species is like in the temperate zone. All trees, shrubs and perennial herbs form annual rings. Cross-dating is possible in individuals with more than 20 rings.

**9.29** *Larix sibirica*, Pinaceae, a 15 m-tall deciduous tree.

**9.30** *Pinus sibirica*, Pinaceae, a 25 m-tall evergreen tree.

500 μm

**9.31** *Betula pendula*, Betulaceae, a 10 m-tall deciduous tree. Diffuseporous wood with approx. 150 vessels/mm2 and 100 μm vessel diameter.

**9.32** *Sorbus aucuparia*, Rosaceae, 10 m-tall deciduous tree. Diffuseporous wood with approx. 300 vessels/mm2 and 50 μm vessel diameter.

# 9.2.2 Shrubs in the tropics, the Mediterranean and arctic zone

#### *Subtropical African dry climate, Sahara*

Shrubs with adaptations to persistent drought in a tropical desert (Sahara) with less than 5 mm of annual precipitation. Very few shrubs and dwarf shrubs survive the extremely long drought periods and often also impact by grazing. Very intensive growth

**9.33** *Calligonum azel*, Polygonaceae, a 1 m-tall shrub. Diffuse-porous wood with approx. 40 vessels/mm2 and 100 μm vessel diameter.

**9.34** *Euphorbia calyptrata*, Euphorbiaceae, a 30 cm-tall dwarf shrub. Growth zones absent. With approx. 200 vessels/mm2 and 40 μm vessel diameter.

#### *European thermo-Mediterranean zone*

Shrubs and dwarf shrubs with adaptations to winter rain and summer drought, with less than 200 mm of annual precipitation. 4HU`ZOY\IZVMU\TLYV\ZMHTPSPLZWYVÄ[MYVT[OL^PU[LYYHPUZ and survive the extreme summer droughts. Most of them form

Dwarf shrubs with adaptations to persistently cold temperatures

**9.37** *Teucrium chamaepitys*, Lamiaceae, a 20 cm-tall dwarf shrub. Semi-ring-porous wood with approx. 400 vessels/mm2 and 25 μm vessel diameter.

*Arctic zone*

mm2

**9.38** *Fumana ericoides*, Cistaceae, a 10 cm-tall dwarf shrub. Diffuseporous wood with approx. 400 vessels/mm2 and 30 μm earlywood vessel diameter.

250 μm 250 μm

ing is therefore impossible.

**9.35** *Zilla spinosa*, Brassicaceae, a 40 cm-tall dwarf shrub. Growth zones absent. With approx. 150 ves-

and 50 μm vessel diameter.

sels/mm2

**9.39** *Daphne gnidium*, Thymelaeaceae, a 60 cm-tall dwarf shrub. Diffuse-porous wood with approx. 250 vessels/mm2 and 40 μm vessel diameter.

growth zone

**9.36** *Farsetia aegyptia*, Brassicaceae, a 40 cm-tall dwarf shrub. Distinct growth zones. With approx. 200 vessels/mm2 and 400 μm vessel diam-

eter.

true annual rings. Radial growth greatly varies. Plants on shallow soils form small rings (<1 mm width), those on deep soils produce larger rings (2–3 mm). Cross-dating is principally pos-ZPISLI\[KPZ[\YIHUJLZÄYLNYHaPUNKVTPUH[LJSPTH[PJLMMLJ[Z

occurs for very short periods after rainfall events. Most of the species have no visible growth zones, and if there are any they correlate to rain events rather than regular seasons. Cross-dat-

**9.40** *Launaea lanifera*, Asteraceae, a 30 cm-tall dwarf shrub. Without growth rings. With approx. 200 vessels/mm2 and 50 μm vessel diameter.

the one or two summer months. They form mostly distinct, but extremely small annual rings (0.05–0.5 mm). Cross-dating is KPMÄJ\S[I\[WVZZPISL

**9.43** *Betula nana*, Betulaceae, a 50 cm prostrate dwarf shrub. Diffuseporous wood with approx. 200 vessels/mm2 and 40 μm vessel diameter.

**9.44** *Salix arctica*, Salicaceae, a 40 cm-tall dwarf shrub. Diffuseporous wood with approx. 300 vessels/mm2 and 50 μm vessel diameter.

and short vegetation periods. Dwarf shrubs from a few families (Ericaceae, Salicacea and Betulaceae) grow only during

**9.41** *Cassiope tetragona*, Ericaceae, a 20 cm-tall dwarf shrub. Diffuseporous wood with >500 vessels/

and 20 μm vessel diameter.

250 μm 250 μm 250 μm 250 μm

**9.42** *Empetrum nigrum*, Ericaceae, a 30 cm prostrate dwarf shrub. Diffuseporous wood with approx. 500 vessels/mm2 and 35 μm vessel diameter.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 10. Anatomical adaptations to temporarily changed environmental conditions

Two principal capacities characterize seed plants. Primarily, cell formation pathways determine the basic structure of plant bodies, and the formation of different cell types. This is shown PU[OLMVSSV^PUNÄN\YL

*Node* c determines the formation of fusiform mother cells, vessels and axial parenchyma mother cells.

*Node* dKL[LYTPULZ[OLMVYTH[PVUVMÄILYZ[YHJOLPKZHUK]LZZLSZ *Node* e determines the formation of parenchyma cells or vessels. *Node* f guides living parenchyma cells in various directions.

Secondarily, the cambium and cells are able to react to shortterm environmental changes with a reduction or an enhancement of their activity. This is described in detail in Larson 1994, Timell 1986, Schweingruber 2007 and Fromm 2013. In the following chapter the anatomical reactions to various environ-TLU[HSPUÅ\LUJLZHYLKLTVUZ[YH[LKPUHJVUKLUZLKMVYT

*Two basic processes* determine temporal anatomical reactions in all growth forms—from herbs over shrubs to trees—in all climatic zones and sites:


secondary-cell-wall formation and SPNUPÄJH[PVU HUK [V intensive stress with the formation of callus tissue or ducts, and the excretion of various substances.

Short-term environmental conditions determine the anatomy of [OL\_`SLT/V^L]LY[OLZ[Y\J[\YLZNYLH[S`]HY`ILJH\ZLTVKPÄ-JH[PVUZVJJ\YIHZLKVUNLUL[PJ[H\_VUZWLJPÄJPUMVYTH[PVUHUK based on the location of the environmental impact (tropism). Cambial and cellular reactions are mostly combined. Reactions to the impact occur immediately or with a time lag. Anatomi-JHS YLHJ[PVUZHYLTVZ[S`UV[\UPX\L[VZWLJPÄJLU]PYVUTLU[HS PUÅ\LUJLZ

In the following pages, the full range of variability has to be limited to showing a few examples for conifers and deciduous trees.

# ͙͘Ǥ͙ơǦ during the vegetation period

# 10.1.1 Individual small and large annual rings and missing rings

"Large" and "small" are relative terms, and are always related to the mean values of width within a ring sequence. Extremely small or large rings express cambial reactions to short-term events in the leaf or root area of plants. A few extremely small rings can be the consequence of extreme droughts, cold days, short vegetation periods, or crown damage by insects. These are *negative pointer years*.

If environmental conditions are too extreme, annual rings can be missing completely; cambial and cellular activities are interrupted for at least one vegetation period. Missing rings can only be detected by cross-dating with other samples.

In contrast, above-average precipitation or warm days can lead to the formation of a few extremely large rings. These are *positive pointer years*.

**10.2** Extremely small ring with very few earlywood tracheids in the conifer *Metasequoia glyptostroboides*. A severe summer drought reduced the cambial activity mainly at the beginning of the growing season.

*Negative pointer years*

**10.3** Extremely small ring with one earlywood tracheid in the conifer *Juniperus communis*. A very short and cold vegetation period in the subarctic environment reduced the cambial activity at the beginning of the growing season.

**10.4** Extremely small ring in the deciduous tree *Pyrus pyraster*. The reason for this growth reduction is unknown.

**10.5** Extremely large ring in the conifer *Larix dahurica*. An unusually rainy and warm period probably induced accelerated growth at the northern timberline in eastern Siberia.

*Positive pointer years*

**10.6** Two rings with large latewood zones in a sequence of smaller rings in a subfossil *Quercus* sp. from a bog with usually high water table in northern Germany. Larger rings may be the result of a larger aerobic root zone in drier years.

**10.7** Two rings with large earlywood zones in the root collar of the herb *Silene vulgaris* in a meadow of the temperate zone, polarized light.

# 10.1.2 Discontinuous growth – Wedging rings

The presence of wedging rings is an indication of a principal plant physiological law. Each cambial cell and each living differentiated cell reacts autonomously to external mechanical stresses. This capacity permits an optimal reaction to internal HUK L\_[LYUHSPUÅ\LUJPUNMHJ[VYZ HUK JVUZLX\LU[S`[OL WO`ZPological optimization of plants (Schweingruber 2007).

Wedging rings in increment cores or discs only provide a limited insight into the actual presence of rings within the whole plant. Nogler 1981 demonstrated that wedging or missing rings are only locally missing in stems. Ring wedging occurs in all growth forms and taxonomic units with secondary growth. Individual or several rings wedge out in short or long periods of suffering from stress, after position changes, or just in relation [V[OLMVYTH[PVUVMÅ\[LKZ[LTZ

**10.8** Schematic diagram of a locally absent, wedging or missing annual ring. Model: Nogler 1981.

**10.10** 4HU`^LKNPUNYPUNZPUHaVULVMÅ\[PUNVM the stem of the conifer *Juniperus nana*.

**10.11** Wedging areas in a branch of *Pinus mugo* which changed its position after rock fall events.

**10.12** >LKNPUN YPUNZPUHaVULVMÅ\[PUNVM[OL dwarf shrub *Empetrum nigrum*.

**10.13** Multiple wedging rings in the ring-porous wood of *Ulmus glabra* after intensive browsing.

# 10.1.3 Individual small and large latewood zones and latewood zones with thin- or thick-walled tracheids

Noticeable latewood zones in conifers are related to reduced or enhanced cambial activity or cell-wall formation activity of living tracheids. If the cell walls remain small they are called light years (low density; Kaennel & Schweingruber 1995). Reduced JLSS ^HSS NYV^[O PZ H YLZ\S[ VM SV^ [LTWLYH[\YLZ PUZ\MÄJPLU[ water supply in the late summer or a lack of photosynthetic products after crown or root damages. Reduced cambial and cell activity often occurs together.

**10.14** Individual ring with small latewood consisting of thin-walled tracheids in *Larix decidua*.

**10.15** Individual ring with large latewood consisting of thick-walled tracheids in *Pinus sylvestris*.

*Blue rings in conifers*

**10.16** Sequence of small rings with small latewood zones consisting of thin-walled tracheids in *Larix decidua*.

# ͙͘Ǥ͙Ǥ͜ǡƤȋDzblue rings")

3PNUPÄJH[PVU PZ [OL SHZ[ Z[LW PU [OL WYVJLZZ VM JLSS MVYTH[PVU If this process is interrupted, and the cells are not fully ligni-ÄLK[OL`HWWLHYIS\LPUZSPKLZZ[HPULK^P[O(Z[YHIS\L:HMYHUPU These so-called "blue rings" were found exclusively in conifers of the northern high latitudes. It is believed that low temperatures in fall induce this feature. See Piermattei *et al*. 2015.

**10.17** Latewood in a ring of *Larix decidua* growing at the treeline. The last section of the latewood tracheids is still in formation and the \USPNUPÄLK JLSSZ HWWLHY \UZ[HPULK or blue.

250 μm 250 μm 250 μm

**10.18** <USPNUPÄLK SH[L^VVK aVUL in *Pinus sylvestris* growing at the northern tree line.

**10.19** Completely formed latewood zone in *Pinus sylvestris* growing at the northern tree line. Primary walls HYLSPNUPÄLKYLK ZLJVUKHY`^HSSZ HYL\USPNUPÄLKIS\L

**10.20** Frost ring and "blue ring" in the latewood of *Larix decidua* growing at the tree line. Extreme frost in early fall damaged the cambium cells and interrupted cell-wall formation. Callus tissue is formed and the ZLJVUKHY`^HSSZYLTHPU\USPNUPÄLK

### 10.1.5 False rings and density variations

False rings are normally formed rings with distinct latewood borders and cannot be anatomically distinguished from true HUU\HS YPUNZ;OLPYPU[YHHUU\HS VJJ\YYLUJL JHU ILPKLU[PÄLK only by cross-dating. The term density variation is based on x-ray analysis of conifers and indicates intra-annual structural variations without distinct tangential borders. The difference between the two types is gradual.

*False rings* look like normal rings, containing a latewood zone with an abrupt transition from late- to earlywood. They occur mainly in Mediterranean and arid regions and are primarily triggered by short, intensive summer droughts.

*Density variations* of conifers are characterized by intra-annual tangential zones of thicker-walled and often smaller tracheids in relation to previous and following zones. They occur from the tropics to the temperate zone and are almost absent in boreal and arctic zones. Intra-annual density variations are triggered by short-term climatic events such as cold or dry periods during the growing season or slight damage to crown or roots. The susceptibility varies within the conifers. Cupressaceae are more, Pinaceae are less susceptible.

*Intra-annual tangential structural changes* of some deciduous plants are based on rhythmic formation of different cell types Z\JO HZ ÄILYZWHYLUJO`TH ÄILYZ]LZZLSZ VY WHYLUJO`TH]LZsels. Theses changes generally occur in fast-growing stems of trees, shrubs and herbs from the tropics to the temperate zone. Such structural changes are triggered by genetic characteristics as well as short-term climatic events.

#### *Few density variations in conifers*

**10.23** Large rings with two density variations in the latewood of *Pinus sylvestris* growing on sand dunes in Germany. Zones of smaller and larger, and thin- and thick-walled tracheids indicate variable water availability.

**10.24** Two density variations in *Cupressus sempervirens*. The more intensive variation is characterized by a few rows of small, thick-walled tracheids. The weaker variation is expressed just by smaller tracheids.

**10.21** False rings in *Pseudotsuga men ziesii* growing in Arizona, USA, an area with monsoon rains. There are two rings within a growing sea-ZVU ^P[O [OL ÄYZ[ YPUN PU[LYY\W[LK from the second by a severe summer drought.

*Multiple density variations*

#### drought triggered irregular latewood growth.

**10.25** Multiple density variations in *Pinus elliottii* in tropical Tahiti.

**10.22** Large rings with two density variations in the latewood of *Pinus sylvestris* growing on shallow soil in Valais, Switzerland. Late summer

**10.26** ;HUNLU[PHS IHUKZ VM ÄILYZ alternate with bands of parenchyma in *Ficus sycomorus*, Egypt. This is characteristic for many tropical species.

*Intra-annual structural changes*

**10.27** Tangential bands of thick- and [OPU^HSSLK ÄILYZ HS[LYUH[L PYYLN\ larly in a shoot of a 40 cm-tall annual herb *Lepidium campestre* in the temperate zone in Davos, Switzerland.

**10.28** Tangentially arranged parenchyma cells in the latewood of the ring-porous *Quercus robur* of the temperate zone in Switzerland.

# ͙͘Ǥ͙Ǥ͞Ƥcracks

All cracks are the result of short-term events.

*Radial cracks* in living conifer trees occur just in the earlywood of individual years, or along rays over many rings. Due to insuf-ÄJPLU[^H[LYÅV^PUZ\TTLY[OL[OPU^HSSLKearlywood cells contract laterally and split along the weakest zone—the rays. Tangential rows of radial splits can be dendrochronologically dated. Radial cracks in dry logs are a result of anisotropic tissue contractions. Extreme shrinkage of degraded, subfossil wood produces cogwheel-like stem sections. If splits in the xylem reach the cambial zone they induce traumatic reactions. Therefore cracks can be overgrown by accelerated cell production. Radial cracks in bark are the result of enlarged stem circumferences.

*Tangential cracks* (ring shake) are a result of mechanical stress and occur along anatomically weak zones in various species. Genetic predisposition or extreme wind events are the most frequent causes of ring shake and resin pockets. Stems of the ring-porous chestnuts split along the earlywood vessels, those of spruces along the thin-walled earlywood tracheids. Resin pockets occur mainly in stems of conifers in windy mountainous regions. Extreme tangential splitting is characteristic for a few alpine herbs, e.g. *Saxifraga* sp. and *Androsace* sp.

*Micro-cracks* mainly occur in cell walls of conifer tracheids. Extremely intense, very short events of tension or pressure due to rock fall, wind storms or avalanches can break tracheids irreversibly. The stability of wood with broken tracheid walls is probably not much reduced. The abnormalities can be detected by microscopy with polarized light.

**10.29** Radial cracks in a disc of *Fraxinus excelsior* are a result of intensive tangential shrinkage.

**10.30** Large radial cracks in the anaerobically degraded peripheral zone of a Neolithic, waterlogged

**10.31** Extremely large radial cracks in the degraded, light-colored sapwood and the dark heartwood of a Neolithic, waterlogged post of *Quercus* sp.

**10.32** Many radial frost cracks in the heartwood of *Larix laricina* in the boreal zone. Material: Y. Begin.

**10.33** Intra-annual radial cracks along rays in the earlywood of *Larix dahurica*.

post of *Abies alba*.

*Microscopic aspect of radial cracks*

**10.34** Frost split in *Larix sibirica*. The split opened and triggered accelerated radial growth and induced the formation of traumatic resin ducts.

*Radial splits in the bark*

**10.35** Radial splits in the bark of *Populus nigra* are a result of stem expansion.

**10.36** Radial splits in the bark of *Aristolochia macrophylla* are a result of stem expansion.

#### *Tangential cracks along weak, thin-walled zones*

**10.37** Tangential split in *Picea glauca*, indicating a thin-walled latewood after a volcanic event. Material: G. Jacoby.

250 μm 250 μm

**10.38** Tangential split along a thin-walled earlywood zone in *Larix dahurica*.

*Wind-induced tangential cracks*

**10.39** Tangential splits in a ray-less stem of the perennial herb *Saxifraga muscoides*.

**10.40** *Picea engelmannii* with onesided branches in a windy area.

**10.41** Longitudinal section trough resin pockets of *Picea abies* in an alpine region.

**10.42** Macroscopic aspect of a resin pocket in *Pinus sylvestris* in an alpine region.

**10.43** Microscopic aspect of a resin pocket in *Picea abies*. The crack borders are surrounded by callus tissues.

**10.44** A stone hit a the stem of *Pinus sylvestris* in a rock fall.

*Shock-induced cracks in axial tracheid walls*

**10.45** Cracks in axial tracheid walls of *Picea abies* under polarized light.

**10.46** Zone of crushed tracheid walls in *Picea abies* under polarized light.

#### 147

# ͙͘Ǥ͚ơǦ

# 10.2.1 Abrupt growth changes

Abrupt growth changes are the result of long-lasting environmental changes. A long-term reduction in growth can be the consequence of crown damage by repeated insect defoliation, environmental pollution, pruning, a change in water availability or increased competition. Sudden increased growth can be the consequence of improved light conditions, e.g. after thinning, or improved hydrological conditions. Abrupt growth changes are rarely permanent. They can in extreme cases lead to the death of individuals, however, in most cases they are reversible. Their causes can only be evaluated in relation to observations or measurements of environmental conditions.

# reduced growth 500 μm 500 μm

*Abrupt negative growth changes*

**10.49** Growth reduction of the deciduous *Castanea sativa* after crown damage by the chestnut blight fungus (*Cryphonectria parasitica*).

**10.47** Growth reduction in the conifer *Pinus mugo* after crown damage by a rock slide.

**10.50** Enhanced growth in the boreal conifer *Larix dahurica*, its cause is unknown.

*Abrupt positive growth changes*

**10.51** Enhanced growth in the prostrate mountain dwarf shrub *Arctostaphylos rubra*, its cause is unknown.

**10.52** Enhanced growth in the prostrate arctic shrub *Betula nana*, its cause is unknown.

### 10.2.2 Structural changes

Discussed here is the anatomical structure of changes in cell type, cell size, cell-wall density and cell orientation after root exposure in conifers and deciduous trees. Structural changes are the result of positional changes within the plant body, of extensive plant destruction, or long-term environmental changes, e.g. by root exposure, stem cutting and extreme suppression of individuals. This can only be ecologically explained in the context of external observation.

*Root exposures* are expressed by changes of tracheid diameters, ÄILY KPTLUZPVU ÄILY^HSS [OPJRULZZ ]LZZLS KPZ[YPI\[PVU WH[ terns, tyloses, ray width and ring distinctness.

*Resprouting stumps after they have been cut* indicate a reorganization of tissues. After removal of the crown, conifers and KLJPK\V\ZWSHU[ZYLK\JLYHKPHSNYV^[OJOHUNLÄILYKPYLJ[PVUZ and reduce ray height. Conifers reduce latewood zones and tracheid walls, deciduous trees reduce vessel frequency and diameters. These reactions are an expression of dramatic physiological changes. Instead of using their own photosynthetic resources plants after crown removal use resources of neighboring individuals through anastomosing roots.

Dying stems in beech cohorts lose their foliage. Due to reduced ZHWÅV^[OLWYVWVY[PVUVMwater conduction to nutrient storage changes. The conducting area of vessels continuously reduces its capacity, while the amount of parenchymatous tissue (rays) increases. See also Gärtner 2003.

**10.53** Exposed roots of *Fagus sylvatica* on a rock.

*Macroscopic view of exposed roots*

**10.54** Exposed roots of *Fraxinus excelsior* at a river bank.

*Anatomical changes after root exposure in conifers*

**10.55** Exposed roots of *Picea abies* after an extremely intensive rainfall in the Swiss Alps.

**10.56** Root of *Pinus sylvestris* in an avalanche track. Only the diameters of tracheids are changing.

**10.57** Root of *Juniperus sabina*VUHZJYLLÄLSK Only the diameters of tracheids are changing. The position of the root did not change.

**10.58** Root of *Pinus sylvestris* on a path. Exposure triggered changes in tracheid diameters and the formation of resin ducts. Scars in the exposed part of the root are a result of sustained injuries.

#### *Anatomical changes after root exposure in ring-porous deciduous trees*

**10.59** Root of *Fraxinus excelsior* on a river bank. \*LSS ^HSSZ VM ÄILYZ HYL [OPJRLY ]LZZLSZ HYL SLZZ frequent, and rays are smaller after exposure.

**10.60** Root of *Prunus amygdalus* on a path. Cell ^HSSZVMÄILYZHYL[OPJRLY]LZZLSZHYLZTHSSLYHUK less frequent, and rays are smaller after exposure.

r

**10.61** Root of *Quercus petraea* on a river bank. New large rays are initiated, and rings are more distinct after root exposure.

*ơǦ*

**10.62** Root of *Amelanchier ovalis* on an eroded slope. Vessel diameters are JVU[PU\V\ZS` YLK\JLK HUKÄILY^HSS[OPJRULZZPUJYLHZLZ HM[LY YVV[ L\_WVsure.

**10.64** Root of *Eucalyptus* sp. on a river bank. Vessel frequency and diameter PZYLK\JLKHUKÄILY^HSS[OPJRULZZPUJYLHZLZHM[LYYVV[L\_WVZ\YL

**10.63** Root of *Lycium chanar* in a river bed. Ring width increases, vessel distribution patterns distinctly change, and vessel diameter is reduced after root exposure.

**10.65** Root of *Fagus sylvatica* on an enlarged path. Tyloses are formed in the root part after exposure.

#### *Macroscopic aspect of decapitated trees*

**10.66** Decaying stump of *Picea abies* with a living external part.

**10.67** Longitudinal section of a completely overgrown stump of *Abies alba*. The tracheids are axially oriented.

**10.68** Decapitated *Fagus sylvatica* with adventitious sprouts.

**10.69** Intensively pollarded shrub of *Krascheninnikovia ceratoides*. The twigs are used for goat fodder in Ladakh, India.

#### *Structural changes after decapitation in conifers*

**10.71** >H]`ÄILYKPYLJ[PVUHM[LYKLJHWP[H[PVUPU*Thuja occidentalis*.

**10.70** Reactions to decapitation in *Thuja occidentalis* are the reduction of radial growth, latewood formation and cell-wall thickness, and local changes in the direction of tracheids.

*Structural changes after decapitation in conifers* traumatic resin ducts

**10.72** Reactions to decapitation in *Picea abies* are a reduction in radial growth, the formation of traumatic resin ducts, and a change in the direction of tracheids.

**10.73** Structure in *Picea abies* after decapitation: Irregularly oriented tracheids and partially biseriate short rays with one to four cells.

**10.74** Structure in *Picea abies* before decapitation: Straight, axially oriented tracheids and exclusively uniseriate rays with four to ten cells.

after decapitation 250 μm 250 μm

**10.75** Reactions to decapitation in *Fagus sylvatica* are a reduction in radial growth, vessel frequency and diameter, an enlargement of rays, HUKHJOHUNLPUÄILYKPYLJ[PVU

**10.76** Structure in *Fagus sylvatica* after decapitation: Wavy axial tissues containing many very large, but short rays.

**10.77** Structure in *Fagus sylvatica* before decapitation: Straight, axi-HSS`VYPLU[LKÄILYZHUK]LZZLSZHUK slender small and large rays.

*Structural changes in deciduous trees under extreme competition stress*

**10.78** Dying suppressed individuals of *Fagus sylvatica*.

**10.79** A continuous reduction in vessel frequency and diameter, and an enlargement in rays occurs after loss of foliage in *Fagus sylvatica*.

**10.80** Twelve-year-old, 3 cm-tall seedling of *Fraxinus excelsior* in a meadow. Grasses compete with it for light and resources, resulting in few, small vessels, uni- to biseriate rays and thin- to [OPJR^HSSLKÄILYZ

# 10.3 Eccentricity and irregular stem forms

Changes in vertical position, irregularly applied tension or pressure, unfavorable growing conditions, or a genetic constitution can be expressed by eccentric growth in roots, stems and branches. Conifers generally form additional cells at the side of pressure, while deciduous plants form additional cells at the side of tension. The cambium is sensitive to short- or long-term JOHUNLZ ,HJO[VWVSVNPJHS YLHJ[PVUPZ[OLYLMVYL YLÅLJ[LKPU H cross section.

In a cross section, circular annual rings indicate an upright, while oval rings indicate an oblique position. Growth reactions are indicators for extremely local changes within the plant. If a stem changes position the reaction is only expressed by eccentricity in this stem and not in any other plant parts.

Intra-annual dating of pressure or tension changes during the growing period is possible with some restrictions. If the cam-IP\TPZUV[M\SS`HJ[P]LVYPMOVYTVUHSPUÅ\LUJLZHYLKLSH`LK the anatomical reaction does not indicate the exact moment VM[OLL\_[LYUHSTLJOHUPJHSPUÅ\LUJL(U`JOHUNLZVMIHSHUJL K\YPUN [OL KVYTHU[ WLYPVK HYL YLÅLJ[LK PU [OL UL\_[ NYV^PUN period, immediately in the early or in the late earlywood. Intraannual trophic changes are most reliable in fast-growing plants because many cambial cells are at disposition for reactions.

Eccentricity only indicates that an event took place that triggered a change in inclination. The exact cause can only be reconstructed through observation of the environmental conditions.

**10.81** Snow creeping causes stems of *Fagus sylvatica* to grow with a curved stem basis.

*Macroscopic aspect of trees with eccentric growth*

**10.82** An avalanche bent these stems of *Larix decidua*. They regenerated in a vertical position.

**10.83** Leaning trees of *Platanus occidentalis* with LJJLU[YPJZ[LTZHM[LYZL]LYHSÅVVKL]LU[Z

**10.84** Leaning stem of the tree *Rhus typhina*, ^P[OHU\WYPNO[WVZP[PVUVM[OLZOVV[PU[OLÄYZ[ year, leaning from the second year onwards. Accelerated growth occurs at the tension side.

#### *Reaction to a single event*

**10.85** Leaning rhizome of the perennial herb *Lythrum salicaria*, with an upright position of the ZOVV[PU[OLÄYZ[[^V`LHYZSLHUPUNMYVT[OL[OPYK year onwards.

**10.86** Changing inclination of a root of the shrub *Corylus avellana*. The root grew vertically in the ÄYZ[ `LHY HUK H[ HU HUNSLMYVT[OL ZLJVUK `LHY onwards.

#### *Reaction to a multiple events*

**10.87** Changing inclinations of a stem of *Picea abies* in an avalanche track. ;OLZLLKSPUNNYL^\WYPNO[PU[OLÄYZ[ML^`LHYZ[OLUNV[OP[HUKPUJSPULK twice.

**10.88** Changing inclination in a stem of *Pinus mugo* on an instable ground VMHIVN;OLZLLKSPUNNYL^\WYPNO[PU[OLÄYZ[[^V`LHYZHM[LY[OH[TV]LK twice.

**10.89** The Å\[LKZ[LTVMH[YLLVM*Pyrus communis*.

**10.90** ,JJLU[YPJÅ\[LKYOPaVTLVM[OLOLYI*Hippocrepis comosa*. The rhizome grew toward one ZPKL MVY Ä]L `LHYZ [OLU ]HYPV\Z SVJHS JHTIPHS activities formed three lobes.

**10.91** ,JJLU[YPJ Å\[LK Z[LT VM [OL K^HYM ZOY\I *Eriogonum jamesii*. The shoot grew upright in the ÄYZ[Ä]L`LHYZ[OLU]HYPV\ZSVJHSJHTIPHSHJ[P]Pties periodically formed lobes.

**10.92** Prostrate conifer *Juniperus sibirica* on a rocky dry slope in the Polar Ural. Photo: S. Shyiatov.

#### *Reaction to unfavorable local growing conditions* year 1150

**10.93** Very eccentric, horizontally growing stem of *Juniperus sibirica*. The cambium is only active towards the ground-facing side. Older stem sections are eroded. The ring sequence

**10.94** Prostrate dwarf shrub *Dryas octopetala* on a wind-swept, rocky slope in the high arctic of Greenland.

**10.95** Extremely eccentric, horizontally growing, 20 cm-long stem of *Dryas octopetala*. The cambium is only active towards the ground-facing side. The stem hasn't changed its position for 120 years.

year 1989

154 *Ch 10. Anatomical adaptations to temporarily changed environmental conditions* YLÅLJ[ZJHTIPHSHJ[P]P[`VM `LHYZ

# 10.4 Reaction wood – Reaction to mechanical stress

The formation of reaction wood is a principal survival strategy of terrestrial plants. It is responsible for the formation of directed growth, and for regeneration after gravity-related deformations.

9LHJ[PVU ^VVK PZ KLÄULK HM[LY ,]LY[ HZ ¸>VVK ^P[O more or less distinctive anatomical characters, formed in parts of leaning or crooked stems and branches. Compression wood

#### **Compression wood Tension wood**


occurs in conifers at the compressed parts (e.g. lower side of branches) and tension wood in angiosperms at the tension side (e.g. upper side of branches)."

;OLWYLZLUJLVMPU[LUZP]LS`SPNUPÄLKprimary walls is common for compression wood and tension wood. Each living cell reacts independently from another to mechanical stress.

	- Ń Occurs at the tension side of leaning parts of trees, shrubs and herbs.
	- Ń :LJVUKHY`^HSSZJVU[HPUTVZ[S`NLSH[PUV\ZÄILYZ (G-layer), which are low in lignin and high in cellulose.
	- Ń 4HJYVÄIYPSZHYLVYPLU[LKWHYHSSLS[V[OLH\_PZVM[OLÄILY
	- Ń Fibers are angular in circumference. Intercellulars are absent.
	- Ń )YHUJOLZHYLÅL\_PISL^P[OHOPNOYLZPZ[HUJL[V[LUZPVU HUKOPNOÅL\_PIPSP[`

See also Gardiner *et al*. 2014, Ghislain & Clair 2017 and Onaka 1949.

**10.96** Branch and twigs of the conifer *Picea abies*. Compression wood occurs at the underside of branches.

**10.97** Wind-exposed trees of *Picea engelmannii*. Compression wood occurs at the lee side of the stems.

**10.98** Small tree of the dicotyledonous *Euphorbia balsamifera*. Tension wood occurs at the upper side of the branches.

**10.99** Dicotyledonous herb *Lupinus* sp. Tension wood occurs at the upper side of the branches.

# *Occurrence of compression wood and tension wood*

# 10.4.1 Compression wood in conifers

Compression wood appears on polished discs as dark brown zones, and in stained slides as intensively stained layers. +PYLJ[LKYLHJ[PVUZ[VNYH]P[`PUÅ\LUJLZHYLVI]PV\Z

#### *Macroscopic aspect Microscopic aspect*

**10.100** Polished disc of *Picea abies*. The dark zones represent compression wood. Its position PUKPJH[LZ[OLPUÅ\LUJLVMNYH]P[`HYYV^Z

**10.101** Compression wood in a twig of *Pinus sylvestris*.

**10.102** Compression wood in a twig of *Taxus baccata*. Only tracheids form compression wood. Axial parenchyma cells and rays are not responsive.

**10.103** Thick-walled round latewood tracheids with intercellulars in a leaning small *Picea abies* tree.

**10.105** Longitudinal spiral-like cracks in latewood tracheids walls indicate THJYVÄIYPSZPU[OLJVTWYLZZPVU^VVKVM*Picea abies*.

**10.104** Round, thick-walled latewood tracheids with large, distinct tertiary

**10.106** Changing direction in the angle of helical thickenings in *Taxus baccata*PUKPJH[LZ]HYPV\ZNYH]P[H[PVUHSPUÅ\LUJLZ:VTLOLSPJHS[OPJRLUPUNZPU tracheids are left-turning, and some are right-turning.

### 10.4.2 Tension wood in angiosperms

;LUZPVU^VVKPZKPMÄJ\S[[VYLJVNUPaLVUWVSPZOLK^VVKKPZJZ Macroscopically it appears on planed longitudinal sections as a felt-textured surface. The chemical composition and the position of NLSH[PUV\ZÄILYZ^P[OPUWSHU[ZKPMMLY;LUZPVU^VVK primarily occurs on the tension side in the xylem of angiosperm trees, shrubs and herbs, but also in the cortex and the phloem of angiosperm herbs (e.g. in *Euphorbia* sp.), shrubs (e.g. *Daphne* sp.) and trees (e.g. *Broussonetia papyrifera*) and in gymnosperms (e.g. *Gnetum gnemon*;OLWYLZLUJLVMSPNUPÄLK WYPTHY` ^HSSZ PZ JVTTVU MVY NLSH[PUV\Z ÄILYZ PU [OL \_`SLT The anatomical and chemical composition of tension wood varies, which is shown by differences in acceptance of Astra-IS\L:HMYHUPUZ[HPU;OLM\UJ[PVUVMNLSH[PUV\ZÄILYZWYVIHIS` varies between different types. Their occurrence in species with extremely different positions within the phylogenetic tree leads [V[OL HZZ\TW[PVU[OH[ NLSH[PUV\Z ÄILYZ H[SLHZ[[OVZLPU[OL cortex, are of polyphyletic origin.

**10.107** Planed board of a *Populus* sp. The feltlike surface is characteristic for tension wood. Photo: A. Crivellaro.

*Macroscopic aspect Microscopic aspect – Location within the xylem and bark*

**10.108** Twig with tension wood (blue) in the latewood of the second and third ring in a cross section of *Quercus robur*.

**10.109** Stem of *Fagus sylvatica* with tension wood (violet) in the earlywood.

**10.110** Stem of *Ficus carica* with tangential bands of different intensities of tension wood (violet) in the earlywood.

**10.111** Shoot of *Linum bienne* with tension wood (blue) in the whole ring of the annual plant.

**10.112** Tension wood in the bark of the Gnetaceae *Gnetum gnemon*, polarized light.

#### *Ƥ*

**10.113** Dark-blue- to red-stained secondary walls of the tree *Fagus sylvatica*, Fagaceae.

**10.114** Light-blue-stained secondary walls of the tree *Betula pendula*, Betulaceae.

**10.115** Light-blue-stained secondary walls of the annual herb *Linum bienne*, Linaceae.

**10.116** Pink-stained secondary walls and bluestained tertiary walls in the alpine prostrate dwarf shrub *Salix retusa*, Salicaceae.

**10.117** Red- to dark-blue-stained secondary and tertiary walls in *Sorbus aucuparia*, Rosaceae. Tension wood is hardly expressed.

**10.118** +V\I[M\S[LUZPVU^VVK;OL ÄILYZ^P[O thin blue secondary or tertiary walls are located at the tension side of the annual herb *Cannabis sativa*, Cannabaceae.

**10.119** +HYRIS\LZ[HPULK NLSH[PUV\Z ÄILYZ PU *Urtica dioica*<Y[PJHJLHL;OLZLÄILYZ^LYL\ZLK for textiles.

#### *Ƥ*

**10.120** )S\LZ[HPULK NLSH[PUV\Z ÄILYZ PU *Linum usitatissimum*3PUHJLHL;OLZLÄILYZHYL\ZLKMVY textiles (linen).

**10.121** 7PURZ[HPULKNLSH[PUV\ZÄILYZPU*Gnetum gnemon* .UL[HJLHL ;OLZL ÄILYZ HYL \ZLK MVY strings of musical instruments.

# 10.5 Cell collapse and lateral ray compression

Cell collapse occurs in the xylem, the phloem, the cortex and the phellem. As soon as the turgor in thin-walled cells exceptionally decreases, the negative pressure makes cells collapse.

\*LSS JVSSHWZLPU[OL \_`SLTPZ[OLÄYZ[ Z[LWVM H JOHPU YLHJ[PVU Direct mechanical damage, e.g. by hail stones or woodpeckers, primarily causes cell damage which induces callus formation. Physiological imbalances between transpiration and available water resources can cause negative pressure in water conduc- [P]LHYLHZ;OLYLMVYL[OPU^HSSLK\USPNUPÄLK]LZZLSZ[YHJOLPKZVY ÄILYZJHUJVSSHWZL;OPZVJJ\YZLNHM[LYIYV^ZPUNpollarding, heavy frost damage in the leaf area, or extreme summer droughts.

Cell collapse in the phloem is a normal, ontogenetic phenomenon for many species. As soon as sieve elements are no longer turgescent, they collapse. The occurrence of collapsed sieve elements is an indication for a non-conductive phloem. Periodic collapse of tangential bands of sieve tubes leads to a radial contraction and, in consequence, to bent rays. Cork cells in the phellem mostly collapse soon after their formation.

;OLJV\YZLVMHYH`PU[OL\_`SLTPZPUÅ\LUJLKI`[OLWYLZZ\YL in vessels. Vessel development normally has the highest priority within the xylem formation. Due to high pressure in vessels, adjoining rays may change their course. They get compressed, but do not collapse.

Subfossil and fossil wood is often deformed, e.g. by heavy ice or sediment loads. The weakest components of the xylem—mostly earlywood tracheids—collapse. Anaerobically degraded wet wood from archaeological sites shrinks dramatically, which is microscopically expressed by deformed, compressed cells.

**10.122** Woodpecker marks on a *Salix* sp.

*Macroscopic phenomena causing cell collapse*

**10.123** Browsed *Fraxinus* sp.

**10.124** Frost damage on the dwarf

**10.125** Wounded bark of an old *Salix* sp.

**10.126** Collapsed vessels in the alpine *Salix glaucosericea* after a heavy late frost.

shrub *Arctostaphylos uva-ursi*.

**10.127** Collapsed tracheids and bent rays in *Larix sibirica* after a heavy late frost.

*Cell collapse in the xylem*

r

#### *Cell collapse and cell deformation in the phloem*

**10.128** Collapsed and compressed sieve elements at the external part of a vascular bundle in the petiole of *Cycas revoluta*.

# si si

**10.129** Collapsed sieve elements between parenchyma cells in the phloem of the dicotyledonous tree *Schinus molle*.

**10.130** Compressed and deformed tangentially arranged sieve elements and bent rays in the conifer *Larix decidua*.

**10.132** Living rays, bent in the area of large vessel in *Rosa elliptica*.

#### *Cork cell collapse in the phellem Lateral compression of rays Macroscopic aspect of subfossil wood*

**10.133** Subfossil stems in Quaternary river sediments of the Alps.

**10.131** Collapsed dead cork cells in the phellem of *Fagus sylvatica*.

*Microscopic aspect of subfossil and fossil wood*

**10.135** Compressed tracheids in the Carboniferous WL[YPÄLK^VVKVM*Dadoxylon* sp.

**10.136** Laterally compressed xylem in the Neolithic, anaerobically decomposed and dehydrated wood of *Fraxinus excelsior*.

# compressed

500 μm

**10.134** Compressed xylem of a stem of *Larix decidua* in a glacier moraine.

uncompressed

# 10.6 Cambial wounding – Callus formation, overgrowing of wounds

All plants with secondary growth have been confronted with cambial wounding by mechanical, environmental, biological or pathogenic causes for more than 300 million years. Repair mechanisms have therefore been developed to isolate the living tissues from destruents and pathogens. Wounds can be caused mechanically by environmental factors, such as hail, lighten-PUNZ[VYTZÄYLYVJRMHSSÅVVKZH]HSHUJOLZPJLWSH[LZWY\Uing or decapitation, and—human-induced—by grafting. Many wounds are biologically induced by animals, such as insects, birds, herbivores, rodents, and many others. Pathogenic infestations can also cause cambial reactions, e.g. mistletoes, witches' brooms, fungi, or cancerous agents.

If meristematic cells are affected, a reaction chain starts. The ÄYZ[ HUH[VTPJHSS` ]PZPISL Z[LW PZ [OL MVYTH[PVU VM H JOLTPJHS and mechanical protective zone, called barrier zone. Chemical boundaries compartmentalize parts of stems adapted to their anatomical structure (Shigo 1989; see also Chapter 12.4). All cell elements can be plugged with phenols, and vessels with tyloses. Protective reactions are weak along the stem axis, but intense in radial and tangential directions. In a second step, callus tissue formed. Living meristematic and parenchymatic cells produce undifferentiated cells. The further wound occlusion process is characterized by accelerated growth, the transformation of undifferentiated callus cells to original xylem and phloem cells, and the reorganization of the cell structure corresponding to the stem axis.

Monocotyledonous plants are not able to close wounds. The excretion of phenolic slime, and the formation of tyloses and suberin layers around wounds protect the culms of bamboo against decay (Liese & Köhl 2015).

**10.137** Fire scars on *Castanea sativa*.

#### *Macroscopic aspect of scars*

**10.138** Open scar on a stem of *Eucalyptus capillosa*.

**10.139** :\Y]P]PUN JHSS\Z HM[LY ÄYL VU H Z[LT VM *Robinia pseudoacacia*.

**10.140** Barrier zone at the base of a dead branch of *Acer pseudoplatanus*.

*Barrier zones*

**10.141** Compartmentalized zone of a mechanical wound in the dwarf shrub *Arctostaphylos uva-ursi*.

**10.142** Compartmentalized and overgrown zone of a mechanical wound in the tree *Fraxinus excelsior*.

#### *Callus formation and open scars*

callus

**10.143** Frost rings with callus cells in *Larix decidua*.

**10.144** Frost crack with lateral callus cells in *Larix decidua*.

**10.145** Scar from a rodent on a shoot of *Fraxinus excelsior*.

**10.146** Scar on an annual stem of the 2 cm-tall herb *Knorringia pamirica*.

**10.147** Completely healed wound with a hidden scar on a 10 cm-tall seedling of *Fraxinus excelsior*.

**10.148** Completely healed hail wound with a hidden scar on a small branch of *Pinus mugo*.

**10.149** (\_PHSWHYHSSLSÄILY HUK YH` structure on the inside of the scar.

**10.150** Scar on a broken stem of *Picea abies*.

**10.151** \*OHV[PJÄILYHUKYH`Z[Y\Jture on the outside of the scar.

#### *Reorientation of the axial Ƥ*

# 10.7 Prevention of wounds

Ontogentic processes in plants have the potential to create wounds. Obvious are scars created by dropping buds, leaves, twigs, fruits and rhizoids. Plants prevent wounds by the formation of periderms, especially phellem (cork), before the potential wound is exposed to destruents (see also Chapter 6.1.3).

Stem expansions created by radial growth cause tangential tension in the bark. Before bark cracks, living parenchymatic cells react with accelerated cell-wall growth and cell division, the phloem and cortex therefore dilate. Parallel to the expansion, phellem layers seal the endangered zone with cork (see also Chapter 6.2.1). Many small plants never form complete stems, they keep the form of vascular bundles for many years, however, they are linked by inter-fascicular cambia. Stem or root segregation is a widespread phenomenon in many taxonomic units, and in many biomes. A few examples may illustrate this here. Plant stems reorganize themselves by forming new phloem or periderm belts around a part of the xylem. Stems of many alpine cushion plants segregate near the soil surface. In consequence, living parenchyma cells change their physiological mode and form phloem, and partially phellem, all around the partial stems. The cactus *Carnegia gigantea* in American deserts has a full stem at the base, but a circle of single stems inside the cortex. A few species within the family of Primulaceae form little stems within a cortex without cambium.

Pathogenic organisms are able to penetrate meristems. Hosts and parasites live in a symbiosis. Hosts react to parasites, e.g. mistletoes, with accelerated growth without obvious structural disturbances.

**10.152** Leaf scars on a stem of the succulent *Aeonium arboretum*. Brown spots represent cork layers.

*Leaf scars*

**10.153** Macroscopic aspect of leaf scars on a long shoot of a twig of *Acer pseudoplatanus*.

**10.154** Microscopic aspect of leaf scars on a shoot of *Castanea sativa*.

**10.155** Macroscopic aspect of the scar of a shed twig in *Quercus robur*.

*Twig abscission*

**10.156** Longitudinal section trough a breaking zone of a twig of *Quercus robur*.

**10.157** Microscopic detail of the breaking zone of a twig of *Quercus robur*, ploarized light. Crystal druses are characteristic.

#### *Fruit abscission Rhizoid abscission*

**10.158** Apple (*Malus sylvestris*) with petiole on a twig before shedding.

**10.159** Microscopic aspect of the abscission zone between petiole and twig in *Malus sylvestris*.

**10.160** Dead rhizoids on the stem of the climber *Hedera helix*.

**10.161** Interrupted connection between the stem and a rhizoid in *Hedera helix*.

sclerenchyma

*Stem thickening and prevention of cracks in living stem parts*

 ph small cells enlarged cells 50 μm

**10.162** Longitudinal crack in the bark of *Betula pendula*.

**10.164** Cortex enlargement by cell wall growth, and reinforcement by sclerenchymatic tissues in the alpine herb *Androsace villosa*.

**10.165** Alpine herb *Antennaria dioica*.

# *Initial stages of stem segregation in herbs*

**10.166** Formation of lobes is the ÄYZ[ Z[HNL VM Z[LT ZLNYLNH[PVU *Antennaria dioica*.

**10.167** Alpine herb *Arnica angustifolia*.

**10.168** Vascular bundles in the rhizome of *Arnica angustifolia* represent incompletely segregated stems.

#### *Segregated stems of herbs*

**10.169** Alpine herb *Potentilla nitida*.

**10.170** Microscopic aspect of a segregated rhizome of *Potentilla nitida*, completely surrounded by a periderm.

**10.171** Top: Alpine cushion plant *Saussurea hypsipeta*. Bottom: Segregated stems of *Saussurea glanduligera*. Material: J. Dolezal.

**10.172** Microscopic aspect of a segregated stem of *Saussurea andryaloides*. Material: J. Dolezal.

**10.173** *Carnegia gigantea* in a desert in northern Mexico.

**10.174** Macroscopic aspect of a segregated stem of *Carnegia gigantea*. The cortex is rotten.

**10.175** Macroscopic aspect of a cross section of a segregated stem of *Carnegia gigantea.*

haustorium

**10.176** Semiparasite *Phoradendron* sp. on the conifer *Juniperus* sp.

**10.177** Macroscopic aspect of mistletoe haustoria (*Viscum album*) in a stem of *Abies alba*.

**10.178** Microscopic aspect of a haustorium of *Phoradendron* sp. in the stem of *Juniperus* sp. Callus tissues are absent.

#### *Suppression of a rejection reaction between host and parasite*

# 10.8 Resin and gum ducts

Resin ducts are part of the defense system of some plants (see also Chapter 4.12), and secrete resin and gums in the living parts of roots, stems, twigs and needles. Resin canals are axially and radially linked and form a network. The occurrence and distribution of resin ducts in conifers is primarily related to taxonomy. In some genera, e.g. *Pinus* and *Picea*, resin ducts occur in the bark and the xylem. In other species, resin ducts occur only in the bark, and in some they are completely absent. The frequency of resin ducts in conifers is related to stress, and increases with higher stress levels. If stress occurs mechanically in the cambial region, e.g. after woodpecker attacks, or through intensive fungal infections or insect infestations, tangential rows of ducts in the xylem and phloem make intra-annual dating of the event possible. If stresses occur in the crown, e.g. by defoliators, the frequency of resin ducts is increased, but the diffuse distribution of ducts make intra-annual dating impossible. Tangential rows of resin ducts after wounding indicate sporadic long-term reactions.

**10.179** *Pinus canariensis* with many resin ducts.

*Conifers with resin ducts*

**10.180** Sapwood of *Pinus sylvestris* with living resin ducts.

*Ƥ*

**10.181** Longitudinal section of the intraxylary network of resin ducts in *Pinus sylvestris*.

**10.182** *Pinus ponderosa* with resin ducts in the xylem and the cortex. resin duct

xylemphloem cortex cambiumperiderm 500 μm 500 μm 500 μm

> **10.183** *Microbiota decussata* with resin ducts in the phloem and the cortex, but no ducts in the xylem.

**10.184** *Pilgerodendron uviferum* without resin ducts.

#### *Long-term reaction to crown and stem damage*

**10.185** Spruce bud worm (*Choris-* **10.187** Windbreak on *Picea abies*. *toneura fumiferana*) infestation on *Picea engelmannii*.

**10.186** The frequency of resin ducts is extremely high for *Pseudotsuga menziesii*.

**10.188** Scar on *Picea abies* with periodic formation of traumatic resin ducts.

**10.189** Woodpecker scars on a stem of *Pinus mugo.*

*Short-term reaction to woodpecker attacks*

**10.190** Traumatic resin ducts in the xylem of *Abies alba*. The damage occurred in the dormant period or just at the beginning of the growing season. Firs do not normally form resin ducts in the xylem.

**10.191** Traumatic resin ducts in the phloem of *Juniperus oxycedrus*, caused by a bird attack.

**10.192** Scar caused by the fungus *Monilia* sp. on a twig of *Prunus armeniaca*. The fungus attacks the cambium, initiating scars and dieback of twigs.

*Reactions to fungal attacks*

**10.193** Tangential, traumatic gum ducts in the xylem of *Prunus cerasus*. The fungal attack occurred in early summer, just after earlywood formation.

*Unknown cause*

**10.194** An unknown attack triggered traumatic ducts in the xylem of *Elaeagnus pungens*.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 11. Coexistence of algae, fungi and vascular plants

# 11.1 Mycorrhizae – Coexistence of vascular plants and fungi

The symbiotic relationship between fungi and the roots of vascular plants is called mycorrhiza. Hyphae transport inorganic nutrients—mainly carbohydrates—from the soil to the plants, while [OLM\UNPWYVÄ[MYVTVYNHUPJZ\IZ[HUJLZWYV]PKLKI`[OL]HZJ\SHY plants. Mycorrhizae occur in most of the terrestrial plants.

#### *Ectomycorrhizae*

Fine roots are coated by a mycelium of numerous fungus species. The hyphae penetrate the intercellulars between the endodermis (*syn*. rhizodermis) cells and form the Hartig net. The O`WOHLM\UJ[PVUHSS` YLWSHJL[OL ÄUL YVV[Z ,J[VT`JVYYOPaHPZ most abundant in conifers (e.g. in *Picea*, *Abies* and *Pinus)*, and in deciduous trees (e.g. *Fagus* and *Quercus)* of the temperate zones in the Northern Hemisphere.

#### *Endomycorrhizae*

Hyphae within the cortex cells of plants characterize endomycorrhiza. The most abundant endomycorrhiza is arbuscular mycorrhiza (vesicular-arbuscular mycorrhiza, or VAM). It is found in the majority of terrestrial plants. Anatomically characteristic are globular (vesicle) or irregular, tree-like (arbuscular) terminal ends of hyphae within the living cortex cells of roots.

Mycorrhizal symbiosis is essential for orchids. Since orchid seeds don't have their own nutrient reserves, they are relying on symbiotic fungi (basidiomycetes) for successful germination. Fungi also provide organic and inorganic nutrients to orchids without photosynthetic capacity, e.g. *Neottia nidus-avis*. In this case the vascular plant acts as a parasite of the fungus (holoparasite).

**11.1** Ectomycorrhiza on *Picea* ÄUL roots. Slide: S. Egli.

**11.2** ,J[VT`JVYYOPaH VU H ÄUL YVV[ of *Picea abies*. Section stained with Lugol's iodine, hyphae appear purple. Slide: S. Egli.

*Endomycorrhiza*

**11.3** ,J[VT`JVYYOPaHVUHÄULYVV[ of *Picea abies*. Hyphae surround the root and penetrate the rhizodermis. Slide: S. Egli.

**11.4** Ectomycorrhiza in intercellu-SHYZVM[OLYOPaVKLYTPZVMHÄULYVV[ of *Picea abies*. Slide: S. Egli.

**11.5** Endomycrorrhiza with globular vesicles in the roots of *Allium porrum*. Slide: S. Egli.

**11.6** *Neottia nidus-avis*, a holoparasite without chlorophyll. Photo: A. Moehl.

**11.7** Cross section of *Neottia nidusavis* with vascular bundles in the center and a parenchymatous cortex.

**11.8** Endomycorrhiza in living cortex cells of *Neottia nidus-avis.* Groups of hyphae surround cell nuclei.

# 11.2 Lichens – Coexistence of algae and fungi

Hyphae of fungi and algae live together in a symbiotic association and form a morphological and physiological unit. Partners are principally ascomycetes and cyanobacteria. More than 20,000 species occur from the tropics to the arctic, and from extremely dry to aquatic sites. Some thousand species form leaflike, not self-supporting thalli, some form small, self-supporting and upright, horizontal or hanging stems. Fungi build the corpus and are responsible for water uptake, while the photosynthetic algae provide organic nutritive substances. The periphery is light-permeable and extremely hydrophilic. This allows shortterm photosynthetic reactions of the algae by moistening the Z\YMHJL3PJOLUZWYVK\JLZWLJPLZZWLJPÄJHJPKZ^OPJOWHY[PHSS` crystallize. The arrangement of hyphae is related to growth forms. Upright types form a dense external tube, hanging types form a dense central cable-like strand, and leaf-like forms do not have stabilizing elements. The following images describe the principal lichen structure and some anatomical variations of different growth forms.

*Macroscopic aspect of lichen growth forms*

**11.9** Upright *Cladonia* sp. with 4 cm-tall stems with reproductive organs.

**11.10** Hanging *Usnea barbata* with up to 20 cm-long strands.

**11.11** Leaf-like, not selfsupporting *Hypogymnia physodes* with 3 cm-long, ÅH[[OHSSP

*Principal structure of lichens*

**11.12** Structure of *Roccella* sp. Characteristic are the translucent periphery, the green hydrophilic layer of hyphae with algae, and the central hydrophobic layer of hyphae. Material: C. Scheidegger.

**11.16** Leaf-like *Hypogymnia physodes* without dense

hyphae zones.

**11.17** Top: Large crystals in *Roccella* sp. Bottom: Small crystals around the central strand of *Usnea barbata,*  polarized light.

like stem.

**11.13** Upright *Alectoria nigricans* with a dens tube-

**11.14** Hanging *Usnea hirta* with a dense central strand.

**11.15** Thick-walled hyphae in the central strand of *Usnea hirta*.

170 *Ch 11. Coexistence of algae, fungi and vascular plants*

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 12. Wood decay

The "system Earth" is based on a closed carbon cycle. In one half organisms store the carbon in organic substances. This is partially described for the "system plant" in the previous chapters. In the other half, different organisms disaggregate the organic Z\IZ[HUJLZPU[VPUVYNHUPJZ\IZ[HUJLZ+LZJYPILKOLYLIYPLÅ`HYL traces of processes and organisms which fragment wood into smaller particles. Wood can be decayed by photochemical processes, bacteria, archaea, fungi, insects and vertebrates.

# 12.1 Abiotic decomposition

Ultraviolet light and acid rain determine the aspect of old wooden houses on sunny slopes. The originally light-colored lignin turns into a dark brown through photodegradation. Cell walls in dense latewood are broken and stain dark red with Safranin. Since latewood cells decay slower than earlywood cells, mechanical erosion by wind and rain creates a wavy density WYVÄSL VU[OL Z\YMHJL\*LSSZ VU[OL Z\YMHJL VM ZPS]LY` ZOPUPUN ^VVKLUIVHYKZHYLKLSPNUPÄLK:LLHSZV,]HUZ*et al*. 1996 and George *et al.* 2005.

#### *Macroscopic and microscopic aspect of abiotic wood decomposition*

**12.1** Five-hundred-year-old wooden alpine house. The aged wood appears brown.

**12.2** Logs of *Larix decidua* in an alpine house. Ultraviolet light stained the lignin brown and mechanical weather conditions modeled the surfaces.

**12.3** (UH[\YHSKLUZP[`WYVÄSLPU[OL^VVKVM*Larix decidua* due to ultraviolet insolation and weathering.

**12.4** Broken secondary walls in the latewood of a beam of *Larix decidua* due to ultraviolet radiation. Slide stained with Safranin.

**12.5** Wooden shingles of *Picea abies* on a roof appear gray due to the impact of acid rain.

**12.6** +LSPNUPÄLKZ\YMHJLVMHYHPUL\_WVZLKILHT of *Picea abies*. The exposed secondary walls of tracheids appear blue when stained with Astrablue/Safranin.

# 12.2 Anaerobic decay – Absence of oxygen

:\IMVZZPSUVUWL[YPÄLK^VVKPZ^LSSRUV^UPUKLUKYVJOYVUVSogy from riverbeds, bogs and clay pits. Subfossil wood is preserved because shortly after death it was covered by loamy sediments. Due to a lack of oxygen, only some bacteria and archaea are able to decompose the wood. Different stages of decomposition are expressed by discoloration, the loss of weight and shrinkage.

The alterations by microscopic anaerobic degradation are demonstrated exemplarily on Astrablue/Safranin-stained slides taken from subfossil late glacial pine stumps (*Pinus* cf. *sylvestris*) of a clay pit in Zurich, Switzerland, oak stems from riverbeds of large European rivers, and posts from a prehistoric lake dwelling settlement in Switzerland.

0U[OLÄYZ[ Z[HNLKLJH`PUconifers occurs mosaic-like, cell by cell. In the next stage, the cellulose structure of secondary walls is broken up. In the last stage, the secondary walls contract. During all stages, the primary walls remain largely chemically untouched and keep their original form. Therefore the general wood structure is preserved, and subfossil and fossil wood can IL[H\_VUVTPJHSS`PKLU[PÄLK

*Macroscopic and microscopic aspect of pine stumps decayed under anaerobic conditions*

**12.7** Late glacial stump of *Pinus sylvestris* in-situ deposited in grey loam, Zurich, Switzerland, 13,000 years BP. Photo: U. Büngen.

**12.8** Cross section of a late glacial stump of *Pinus sylvestris*. The dark center is preserved by resin, and the light, contracted periphery is heavily anaerobically decayed.

**12.9** Cross section overview of a late glacial stump of *Pinus sylvestris*. Transition between areas preserved by resin (red) and areas in decay (blue).

phe phe 500 μm 50 μm

**12.11** Cross section of the transition zone between the well-preserved and the decayed zone in a stump of *Pinus sylvestris*. A few latewood [YHJOLPKZ OH]L KLSPNUPÄLK ZLJVUKary walls (blue).

secondary wall secondary wall secondary wall secondary wall primary wall primary wall primary wall bordered pit

**12.12** Cross section of the latewood in a slightly decayed zone of a stump of *Pinus sylvestris*. All second-HY`^HSSZHYLKLSPNUPÄLKIS\L;OL primary walls of tracheids and rays HYLUV[KLSPNUPÄLKYLK

**12.13** Cross section of the latewood in a heavily decayed zone of a stump of *Pinus sylvestris*. The sec-VUKHY` ^HSSZ HYL KLSPNUPÄLK HUK contracted (dark blue). The primary ^HSSZHYLZ[PSSSPNUPÄLKYLK

ery of a heavily decomposed stump of *Pinus sylvestris*. The anatomy of the bark is perfectly preserved.

**12.10** Bark structures at the periph-

**12.14** Radial section of the latewood in a slightly decayed zone of a stump of *Pinus sylvestris*. Pits in the secondary walls are largely decomposed.

#### *Macroscopic aspect of stems decayed under anaerobic conditions*

**12.15** Holocene stems of *Quercus* sp. in sediments of a Central European river bed. Photo: W. Tegel.

**12.16** Holocene stem of *Pinus sylvestris* in a lake in the boreal zone of northern Scandinavia. Photo: T. Bartholin.

**12.17** Conifer posts in a Neolithic lake dwelling settlement in Northern Italy (Fiave). Photo: W. Schoch.

*Macroscopic and microscopic aspects of deciduous tree stems decayed under anaerobic conditions*

**12.18** Wet cross section of a Holocene *Quercus* stem. The sapwood is light-colored and the heartwood appears black. Different stages of degradation are expressed by different degrees of shrinkage (see Chapter 13.5). Photo: W. Tegel.

**12.21** Microscopic Astrablue/Safranin-stained cross section of a Neolithic post of *Alnus* sp. +LZWP[L PU[LUZP]L KLNYHKH[PVU ZWLJPLZZWLJPÄJ features are well preserved.

**12.19** Microscopic unstained cross section of the black heartwood zone of a subfossil *Quercus* sp. All anatomical features including tyloses are preserved.

primary wall secondary wall

**12.22** Cell-wall degradation in a very soft, decayed *Alnus* stem. Primary walls determine the ZWLJPLZZWLJPÄJ Z[Y\J[\YL :LJVUKHY` ^HSSZ HYL almost completely degraded. Tertiary walls are preserved, but separated from the primary walls.

**12.20** Microscopic tangential section of a black heartwood zone of *Quercus* sp. All parenchyma JLSSZ HYL ÄSSLK ^P[O KHYRsubstances (phenols), giving the heartwood its macroscopic black appearance.

**12.23** Different levels of cell-wall degradation in a very soft, decayed *Alnus* stem. Rays consist of parenchyma cells and are well preserved. Cell- ^HSSZ[Y\J[\YLZVMH\_PHSLSLTLU[ZÄILYZHUK]LZsels) are decomposed.

# 12.3 Aerobic decay – Wood-decaying fungi

Hundreds of fungus species decay wood. Fungi can be differentiated by their fruiting bodies, while differentiation by hyphae is very limited. Some fungi grow as parasites on living trees, but most decompose dead wood. Some fungi attack carbohydrates, while others attack lignin or suberin (lipid polymers). Fungi grow from cell to cell through pit apertures or enzymatically dissolve cell walls. They cause discoloration as blue stain or randomly green wood, or decay patterns as brown rot, soft rot and white rot. Some decay patterns are shown here exemplarily. Their natural variability is much more diverse.

*Blue stain fungi* produce radial blue stripes in the sapwood of freshly felled logs, mainly in *Pinus* and *Larix*. The blue stripes are just an optical effect due to light refraction. Fungi, mainly ascomycetes, do not degrade cell walls nor do they reduce the stability of the wood. Hyphae absorb sugars, starch proteins and lipids in parenchyma cells. Thick, brown hyphae primarily follow rays and grow from cell to cell through pit apertures. Cell composition as well as cell-wall structures are perfectly preserved.

*Brown rot fungi*, mainly of the family of Polyporaceae, decay living and dead wood. Characteristic is the cubiform brown decay pattern and the dramatic loss of weight and bending strength. Enzymes decay cellulose and hemicellulose. The cellular composition of decayed wood does not change. Anatomical details of rays remain, however, those in tracheids disappear or are indistinct. Since the cellulosic structure is decomposed, wood structure disappears in polarized light. Decomposition artifacts limit an anatomical species determination.

*Soft rot fungi* mainly decay construction wood of conifers and KLJPK\V\Z[YLLZ\*OHYHJ[LYPZ[PJHYLJ\IPMVYTHUKÄIYV\ZKLJH` patterns on logs and boards in humid conditions. It is mainly ascomycetes that primarily decay cellulose and hemicellulose and—to a small amount—also lignin.

*White rot fungi* produce simultaneous rot and successive white rot. Both types occur mainly on deciduous trees but also on conifers in the forest. Decayed wood is spongy and often appears whitish. Characteristic for *simultaneous white rot* are irregular dark lines which consist of concentrations of darkcolored hyphae. Affected wood loses a lot of weight. Basidio-T`JL[LZHUKHZJVT`JL[LZÄYZ[H[[HJRJLSS\SVZLHUKSPNUPUHUK later also hemicellulose. In *successive white rot*, basidiomyce- [LZHUKHZJVT`JL[LZÄYZ[H[[HJRSPNUPUHUKOLTPJLSS\SVZLHUK later also cellulose. Characteristic for damages from the fungus *Ganoderma lipsiense* are enlarged decay zones.

#### *Macroscopic aspect of major discolorations and rottenness caused by fungi* mycelium

**12.24** Blue stain in *Pinus sylvestris*. Characteristic are radial dark stripes in the sapwood of pines and larches.

**12.25** Green rot in *Fagus sylvatica*.

**12.26** Concentrated hyphae (mycelium) of *Armillaria* sp. in the cam-

bial zone of a tree.

**12.27** Brown rot in a stem of *Picea abies*. Characteristic are cubiform,

**12.28** Red rot fungus (*Fomes annosus*) in a root and a stem of *Picea abies*.

**12.29** Soft rot caused by a species of *Polyporus*. Characteristic is soft, ÄIYV\Z^VVKPUTVPZ[JVUKP[PVUZ

**12.30** Simultaneous white rot with bleached parts and dark demarcation lines in *Betula pendula*.

dark brown areas in conifers. barrier zone

**12.31** Successive white rot and a dark demarcation zone in *Quercus* sp. The little hollows indicate the decay.

#### *Microscopic aspect of fungal hyphae in wood*

**12.32** Radial section of a bluestained *Pinus sylvestris* with thick brown hyphae in rays and tracheids. The hyphae do not stain with Astrablue/Safranin.

**12.33** Radial section of a brown rotten *Pinus sylvestris* with thick, red-stained, non-septate hyphae in tracheids.

**12.34** Cross section of white rotten *Fagus sylvatica* with thin, bluestained hyphae.

**12.35** Radial section of a green rotten *Betula pendula* with thin, bluestained hyphae in a vessel.

*Hyphae growing through cell walls Brown rot decay*

**12.36** Septate hyphae growing through pit openings in *Pinus sylvestris*.

**12.37** Hyphae enzymatically dissolve cell walls in *Pinus sylvestris*.

**12.38** Preserved cell wall structure in *Pinus mugo* (unstained slide).

**12.39** Largely decomposed secondary cell-wall structures in tracheids and preserved cell-wall structures in rays of *Pinus mugo*.

**12.40** Hyphae within the walls of latewood tracheids in *Pinus sylvestris*.

# *Soft rot decay* secondary wall secondary wall25 μm 25 μm 25 μm 25 μm

**12.41** Advanced decay in *Pinus sylvestris*. Hyphae decomposed and KLSPNUPÄLK ZLJVUKHY` ^HSSZ 7YP-THY`^HSSZHYLZ[PSSSPNUPÄLK;LY[PHY` walls are structurally preserved but ZSPNO[S`KLSPNUPÄLK

#### *Simultaneous white rot decay*

**12.42** White rot decay in *Betula pendula*. Secondary walls of most ÄILYZHYLIYVRLUHUKKPZZVS]LK

**12.43** Advanced decay in *Betula pendula*. Some secondary walls HYL IYVRLU ZVTL HYL KLSPNUÄLK and some are completely gone. Primary walls of decomposed cells are KLSPNUPÄLK

### *Successive white rot decay*

**12.44** Holes in a stem of the dwarf shrub *Calluna vulgaris*.

**12.45** Selective degradation of cells around a hole in a stem of *Calluna vulgaris*.

**12.46** Holes in a stem of the conifer *Pinus mugo*, polarized light. Tracheids around the holes are delig-UPÄLK

**12.47** Various degrees of decay of pits around a hole in a stem of *Pinus mugo*.

# 12.4 Compartmentalization – The natural limit to fungal growth

*Macroscopic and microscopic aspect of compartmentalization*

wall 3

wall 2

Shigo 1989 described the CODIT concept (COmpartmentalization of Decay In Trees). It states that hyphae of fungi cannot grow unlimited because stems form radial tangential and axial fungicide barrier zones. Hyphae initiate phenolic excretion of living parenchyma cells (rays and axial parenchyma). Four walls limit the expansion of hyphae.

The CODIT concept is not a means of healing stems, but it can compartmentalize damages.


**12.49** Longitudinal section of a compartmentalized dead branch of *Acer* sp.

**12.50** Cross section of a compartmentalized wound of a rhizome of the herb *Mentha* sp.

**12.51** Cross section of a barrier zone in *Betula pen-*

*dula*.

wall 1 250 μm

**12.52** Radial section of a barrier zone in *Pinus mugo*.

**12.48** Cross section of a compartmentalized wound

in *Acer* sp.

# 12.5 Decay by xylobiontic insects

Thousands of insect species are part of the recycling process of wood and bark. Some specialize in feeding on living plants, many prefer dead logs and timber, and a large group is responsible for the decomposition of rotten wood. Xylobionts evolved in many taxonomic units, e.g. in beetles, termites, wasps, bees, TPULYÅPLZants and woodlice. Each insect species prefers spe-JPÄJ ^VVK JVUKP[PVUZ (SS KLJVTWVZLYZ SLH]L feeding traces in the wood or in the bark. Beetles destroy the cambium, the peripheral parts of the living xylem, and the most active part of the phloem. Affected trees can therefore die. Larvae of miner ÅPLZ (NYVT`aPKHL MLLK \W HUK KV^U PU [OL JHTIPHS aVUL where they consume nutrients. Since they do not destroy all meristematic cells, these galleries can be closed by callus cells. These scars are called WP[O ÅLJRZ (NYVT`aPKHMMLJ[LK Z[LTZ don't die. Longhorn beetles (Cerambycidae), wood borers (Anobiidae) and carpenter ants (*Camponotus* sp.) feed on dense, dry, dead wood, where they form galleries of various forms. The YLTHPUPUN I\YYV^Z HYL TVZ[S` ÄSSLK ^P[O coprolites. Termites have the most powerful mandibles, and destruct extremely dry wood. Various insect larvae, e.g. of goat moth (Cossidae) and the Asian longhorn beetle, form large scale galleries, and can often kill the trees. Wasps peel externally eroded plants, and \ZL[OL^VVK ÄILYZMVY[OL JVUZ[Y\J[PVU VM[OLPY ULZ[Z>HZW KLSPNUPM` [OL ÄILYZ JOLTPJHSS` ZV [OL ÄILYZ ILJVTL ÅL\_PISL Woodlice (Oniscidea) are the last members in the wood decay chain, they live in moist mull.

For more information see e.g. Wermelinger 2017.

#### *Galleries of bark beetles—cambium miners which can kill the host trees*

**12.53** The bark beetle *Ips typographus*. The beetle lives in the cambial zone of *Picea abies* in Europe. Photo: B. Wermelinger.

**12.54** Galleries of the European spruce bark beetle *Ips typographus* in the bark of *Picea abies*.

**12.55** Beetle galleries in the wood of a post.

**12.56** Gallery of a bark beetle in the bark of *Picea abies*. Affected is the whole living phloem as well as the dead rhytidome.

**12.57** Bark beetle gallery in the wood of *Picea abies*. Affected are the two last rings.

**12.58** Bark beetle gallery in the wood of *Fagus sylvatica*. Affected is just the last ring.

#### *ƪȋȌȄ*

**12.59** Peripheral xylem zone of *Salix* sp. with galleries. Larvae of TPULY ÅPLZ MLLK PU [OL ZVM[ JHTbial zone before they leave the tree trough the bark.

**12.60** 7P[O ÅLJRZ PU HSS YPUNZ VM H young *Salix* tree.

**12.61** 7P[OÅLJRZPU[OLSH[L^VVKVM *Betula pendula*;OLÅLJRZHYLÄSSLK with callus tissue.

**12.62** 7P[O ÅLJRZPU[OL LHYS`^VVK wood of *Tasmannia xerophila*. The ÅLJRZHYLZ\YYV\UKLKI`JVSSHWZLK JLSSZHUKÄSSLK^P[OJHSS\Z[PZZ\L

**12.63** Heavily decayed sapwood of a beam of *Quercus* sp.

**12.64** Traces of a beetle in a dry beam of *Fagus sylvatica*.

**12.65** The irregularly formed galler-PLZ HYL ÄSSLK ^P[O frass, composed VM JVWYVSP[LZ HUK ÄUL ^VVK WHY- [PJSLZ ;OL PUZLJ[ LH[Z YH`Z ÄILYZ and vessels.

**12.66** ;OLYV\UKNHSSLYPLZHYLÄSSLK ^P[OMYHZZJVWYVSP[LZHUKÄUL^VVK particles).

**12.67** Longhorn beetle (*Hylotrupes bajulus*, Cerambycidae). Photo: B. Wermelinger.

*Sapwood and heartwood destruents*

**12.68** Galleries of larvae of longhorn beetles occur mainly in the earlywood.

**12.69** The Asian longhorn beetle (*Anoplophora glabripenni*) lives in the wood of various deciduous trees. Photo: D. Hölling.

**12.70** The larvae of the goat moth *Cossus cossus* feed in tree trunks, here in a stem of *Salix* sp.

#### *Sapwood and heartwood destruents—Anobiidae wood borers*

**12.71** Exit holes of wood borer larvae in a wooden tool made of *Fagus sylvatica* wood.

**12.72** Irregular internal galleries of wood borer larvae in wood of *Fagus sylvatica*.

**12.73** Round wood borer galleries, polarized light. The insect larvae prefer to feed on the soft earlywood.

**12.74** Frass with wood dust in coprolites, polarized light. Particles have a length of 10–20 μm.

*Sapwood and heartwood destruents—carpenter ants*

*Mull consumers—woodlice*

**12.75** Carpenter ant *Camponotus* sp. Photo: B. Wermelinger.

**12.76** Galleries of carpenter ants. **12.77** Woodlouse in a rotten stem of *Fagus sylvatica*.

**12.78** Common wasp *Vespula vulgaris*. Photo: B. Wermelinger.

**12.79** Nest of *Vespula vulgaris*. Photo: B. Wermelinger.

**12.80** 4VZ[S` KLSPNUPÄLK SHYNL ÄILYZVMTHU`KPMMLYLU[WSHU[ZPU H nest of *Vespula vulgaris*. Ray cells and vessels are absent.

**12.81** 4VZ[S` KLSPNUPÄLK ZTHSS ÄILYZVMHU\UJV]LYLKULZ[VM*Vespula vulgaris* in a wet meadow.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 13. Fossilization, permineralization, Ƥǡ carbonization and wet wood conservation

# 13.1 Fossilization

When remains or traces of organisms are preserved in sediments they are called fossils. The process of fossilization (syn. WL[YPÄJHtion or permineralization) begins when organisms, e.g. stems, are buried and remain in anaerobic conditions. Most WL[YPÄLK stems are therefore in a horizontal position. Many fossil stems ^LYLWYVIHIS`KYPM[^VVK[OH[^HZI\YPLKI`Å\]PHSZLKPTLU[Z Stems in upright position were more likely embedded in volcanic ashes. When supersaturated groundwater penetrates the wood, the minerals precipitate within cellular spaces and crystalize. Most frequent are calcium carbonate (CaCO3) or silicate minerals (SiO2). Fossilization occurs mainly in association with marine or volcanic hydrothermal water. Characteristic for fossils is the conservation of microscopic structures that can be observed on polished disks or on micro-sections. Observations VUMVZZPSPaLK^VVKSLHKZ[VMV\YWYPUJPWHSÄUKPUNZ!


See also Taylor *et al*. 2009 and Selmeier 2015.

**13.1** Stem disc of a Triassic *Araucarioxylon*Z[LTMYVTHWL[YPÄLKMVYLZ[ in Arizona, USA.

#### *Ƥ*

**13.2** ,YVKLK WL[YPÄLK Z[LTZ MYVT Å\]PH[PSL ZLKPTLU[Z VM [OL <WWLY Triassic, approx. 200 million years HNV7L[YPÄLK-VYLZ[(YPaVUH<:( Photo: V. Markgraf.

**13.3** Broken and dislocated petri-ÄLKZ[LTZMYVTÅ\]PH[PSLZLKPTLU[Z Upper Triassic, approx. 200 million `LHYZHNV7L[YPÄLK-VYLZ[(YPaVUH USA. Photo: V. Markgraf.

**13.4** Reconstruction of the *Araucaria* forest at the Chinle Formation (Trias). Display at the Visitor Center VM[OL7L[YPÄLK-VYLZ[

**13.5** Modern *Araucaria araucana* forest in Patagonia, Chile.

**13.6** *Quercoxylon* sp. with distinct annual rings. Miocene, approx. 20 million years ago, Great Basin, Idaho, USA.

**13.7** Conifer with indistinct annual rings. Miocene, approx. 20 million years ago, Great Basin, Idaho, USA.

**13.8** *Palmoxylon* sp., of unknown age and locality, with distinct vascular bundles.

**13.9** 7L[YPÄLKliana of unknown age and locality.

**13.10** *Rhynia gwynne-vaughanii*, a herb with a central stele. Lower Devonian, approx. 400 million years ago, Scotland.

#### *ƤƤ*

*Macroscopic aspect of polished disks*

**13.11** Cross section of *Paradoxylon leuthardii*, one of the Cordaitales without annual rings. Upper Triassic, approx. 220 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.12** Tangential section of *Paradoxylon leuthardii* with biseriate rays. Upper Triassic, approx. 220 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.13** Radial section of *Paradoxylon leut hardii* with multiseriate bordered pits on tracheids. Upper Triassic, approx. 220 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

#### *ƤƤ*

v

*Ƥ*

**13.14** *Sequoia* sp. with distinct and indistinct rings. Early Oligocene, approx. 34 million years ago, Florissant, Colorado, USA.

**13.15** Mimosoideae, Fabaceae, without annual rings, with paratracheal parenchyma. Oligocene, approx. 28 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.16** *Celtixylon* cf., Ulmaceae, with distinct annual rings. Oligocene, approx. 28 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.17** *Palmoxylon*, Arecaceae, with KPZ[PUJ[ SPNUPÄLK WHY[Z VM ]HZJ\SHY bundles. Oligocene, approx. 28 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.18** Cross section of *Paradoxylon leuthardii*, without growth rings, indicating tropical rain forest as habitat? Upper Triassic, approx. 220 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.19** Cross section of *Paradoxylon leuthardii*, with growth rings, indicating seasonal climate. Upper Triassic, approx. 220 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.20** Lauraceae cf. with growth rings, seasonal climate. Oligocene, approx. 28 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.21** .YV^[O YPUNZ PU H WL[YPÄLK ring-porous oak stem in Miocene deposits, approx. 15 million years ago, Oregon, USA.

**13.22** )HYYPLY aVULZ PU H WL[YPÄLK stem of a eucalypt tree. Australia, age unknown.

**13.23** Barrier zones in a stem of a Lauraceae cf. tree. Oligocene, approx. 28 million years ago, Vosges Mountains, France. Slide: R. Buxtorf.

**13.24** Hyphae in a vessel of a Meliaceae tree. Reprinted from Selmeier 2015.

**13.25** Coprolites of a beetle in a Lauraceae tree. Reprinted from Selmeier 2015.

#### *Indicators of Ƥ*

#### *Indicators of Ƥ*

**13.26** Decayed cell walls in *Paradoxylon leuthardii*. Preserved are the thick primary walls and thin tertiary walls. Slide: R. Buxtorf.

**13.27** Radially compressed dicotyledonous wood. Slide: R. Buxtorf.

**13.28** Heavily tangentially compressed wood of *Paradoxylon leuthardii*. Slide: R. Buxtorf.

**13.29** The root of a herb squeezed the soft, anaerobically decayed xylem tissue. Reprinted from Selmeier 2015.

**13.30** Single crystals in individual tracheids in *Paradoxylon leuthardii*. Slide: R. Buxtorf.

#### *Permineralization under polarized light*

**13.31** Many crystals in large earlywood vessels in Ulmaceae wood. Slide: R. Buxtorf.

**13.32** Irregular crystallization of silicates without any wood anatomical context in Ulmaceae wood. Slide: R. Buxtorf.

**13.33** Crystallization of different minerals relating to groups of tracheids in *Paradoxylon leuthardii*. Slide: R. Buxtorf.

# 13.2 Permineralization of archaeological artifacts

Permineralization also takes place on metallic archaeological artifacts. The scabbards and handles of iron swords, bronze daggers and axes often contain small mineralized remains of wood. Transverse and longitudinal breaks of particles clearly show the HUH[VTPJHSZ[Y\J[\YL:WLJPLZPKLU[PÄJH[PVUPZ[OLYLMVYLWVZZPISL If highly concentrated liquids of iron, copper or sulfur soak the wood, minerals precipitate on cell walls and preserve their structure.

**13.34** Wooden scabbard permineralized by dissolved iron. Photo: W. Tegel.

*Permineralization of archaeological artifacts*

**13.35** Wooden scabbard permineralized by dissolved copper. Photo: W. Tegel.

**13.36** Microscopic structure of archaeological *Quercus* sp. wood, permineralized by iron. Photo: W. Tegel.

**13.37** Microscopic structure of archaeological *Alnus* sp. wood, permineralized by sulfur. Photo: W. Tegel.

# ͙͛Ǥ͛Ƥ

\*VHSPÄJH[PVU PZ H ]LY` ZSV^ WYVJLZZ PU ^OPJO ^VVK PZ [YHUZformed into coal at geological time scales. The anaerobic decay VMVYNHUPJZ\IZ[HUJLZPZ[OLÄYZ[Z[HNLVMJVHSPÄJH[PVUZLL\*OHWter 12.2). The ratio of carbon increases during anaerobic decay processes, and the wood loses its stability. The process starts with soft wood, which under high pressure and high temperatures turns into brown coal, IP[\TPUV\Z JVHS HUK ÄUHSS` anthracite. (UPUJYLHZLPUJHYIVUJVU[LU[KLÄULZ[OLZLNYHKLZ·^VVKJVU- [HPUZIYV^UJVHSHUKHU[OYHJP[L JHYIVU>OLU softened logs come under mechanical pressure by sediments or ice, the thin-walled earlywood cells collapse, and the stems get KLMVYTLK,HYS`Z[HNLZVMJVHSPÄJH[PVUHYLRUV^UMYVTZ[LTZPU medieval glacier deposits and late glacial clays (subfossil wood). Remnants of wood (xylite, lignite) with well-preserved structures are frequently found in brown coal layers (Eocene to Oligocene). .LULYHVYL]LUZWLJPLZJHUILPKLU[PÄLKHZSVUNHZ[OLLHYS` wood zones are not too much distorted (Dolezych 2005).

Tertiary wood is often so well preserved that even stages of cell wall decay can be recognized. However, in most cases the earlywood zones are compressed, and the corresponding ray features HYLKPMÄJ\S[[VVIZLY]L+VSLa`JO

#### *Fossil tree stump*

**13.38** Stumps from a brown coal bed. Photo: Elbwestfale 2003, via Wikimedia Commons, CC BY-SA 3.0.

*Taxonomically and climatologically relevant features in cross sections of conifers*

**13.39** Conifer *Taxodium taxodii* cf. without growth zones. Oligocene, approx. 34–23 million years ago. Material: M. Dolezych.

**13.40** Conifer *Sciadopityoxylon wettsteinii* without resin ducts and distinct earlywood and latewood zones. Miocene, approx. 25–5 million years ago. Material: M. Dolezych.

**13.41** Conifer *Doliostroboxylon priscum* with resin ducts. Eocene, approx. 56–38 million years ago. Material: M. Dolezych.

*Taxonomically relevant features in radial and tangential sections of conifers*

**13.42** Conifer *Larix decidua* with biseriate tracheid pits. Interglacial period, Italian Alps, >50,000 years ago.

**13.43** Fenestrate vessel-ray pits in the earlywood of *Sciado pity oxylon wettsteinii*. Mio cene, approx. 25–5 million years ago. Material: M. Dolezych.

**13.44** Small round pits in the latewood of *Taxodium gypsacum*. Miocene, approx. 25–5 million years ago. Material: M. Dolezych.

**13.45** Large round pits in

the early- and latewood of *Do lio stro boxylon priscum*. Eocene, approx. 56–38 million years ago. Material: M.

Dolezych.

**13.46** Uniseriate rays with three to ten cells in *Taxodium gypsacum*. Miocene, approx. 25–5 million years ago. Material: M. Dolezych.

#### *Cell wall decay in latewood*

**13.47** Perfectly preserved cell walls in latewood tracheids of *Glyptosto-IV\_`SVU Y\KVSÄP*. Middle Miocene, approx. 15 million years ago. Material: M. Dolezych.

**13.48** Anaerobically decayed cell walls in latewood tracheids of *Larix decidua* from the moraine of an alpine glacier. Middle Ages, approx. 1400 AD. Material: M. Dolezych.

**13.49** Various stages of anaerobic decay in the latewood zone of *Sciadopityoxylon wettsteinii*. Miocene, approx. 15 million years ago. Material: M. Dolezych.

ew

ew

ew

lw

ew

**13.50** Primary and tertiary cell walls are preserved, secondary walls are com pletely decayed in latewood trache ids of *Taxodium taxodii* cf. Oligocene, approx. 34–23 million years ago. Material: M. Dolezych.

**13.51** Wood of *Larix decidua*, heavily radially compressed by a glacier. Moraine of an alpine glacier, Middle Ages, approx. 1400 AD.

**13.52** Radial compression is indicated by bent rays in wood of *Larix decidua*. Interglacial period, Italian Alps, >50,000 years ago. Slide stained with Astrablue/Safranin.

**13.53** Compression in a branch of *Abies alba* cf., polarized light. Interglacial period, Switzerland, approx. 45,000 years ago.

**13.54** Compression of two weak zones in ringless wood of the conifer *Doliostroboxylon priscum*. Eocene, approx. 56–37 million years ago. Material: M. Dolezych.

**13.55** Intensively compressed earlywood zones between thick-walled latewood zones in *Taxodium gypsacum*. Miocene, approx. 15 million years ago. Material: M. Dolezych.

**13.56** Intensively compressed earlywood and latewood zones in *Sequoiadendroxylon* sp. Czech Republic, middle Miocene, approx. 15 million years ago.

# 13.4 Carbonization

Carbonization is the process of transforming wood into charcoal by pyrolysis. Charcoal used to be one of the most important energy resources in pre-industrial times because it was easy to transport. Most human civilizations produced charcoal PURPSUZ^P[O[OL^VVKSVZPUN¶VMP[Z^LPNO[\*OHYJVHS burning and grazing were the main reasons for the depletion of ancient forests all over the world. With the switch to fossil fuels like coal and coal oil, charcoal lost some of its importance. It also used to be important for the production of gun powder (black powder). Today, charcoal is still widely used, e.g. for SVJHSTL[HSS\YNPJWYVJLZZLZJOLTPJHSÄS[LYPUNIHYILJ\LZHUK charcoal crayons.

;OLJHYIVUPaH[PVUWYVJLZZKVLZUV[KLZ[YV`[OLZWLJPLZZWLJPÄJ anatomical structures of the material. Also, charcoal is very stable against biodegradation in both dry and wet environments. Charcoal is therefore of relevance to historical studies,

**13.57** Establishment of a charcoal kiln in the Black Forest. Photo: T. Ludemann.

HZ HUJPLU[ WYLOPZ[VYPJ ÄYLZ·JOHUNPUN O\THU ILOH]PVY HUK vegetation patterns from the tropics to the arctic—can be docu-TLU[LK^P[O[OLPKLU[PÄJH[PVUVMWHY[PJSLZHZZTHSSHZHJ\IPJ millimeter.

During the carbonization process, cell walls shrink by approx. HUKHSZVSVZL[OLPYÄIYPSSVZLZ[Y\J[\YL/V^L]LY[OLZ[Y\Jture of pits and perforations, and of artifacts caused during decay before the carbonization process, remain. Longitudinal cracks and tangential cell collapses are a sign of vapor pressure during the heating phase.

Binocular stereomicroscopes and episcopic microscopes facilitate the observations. Thin sections, embedded in two component epoxy resin, are the basis for photographic presentations (see Chapter 2).

*Carbonization artifacts*

**13.58** Radial cracks in this conifer are caused by vapor pressure during the heating phase.

**13.59** Thin carbonized cell walls in *Alnus* sp.

**13.60** Conifer with an insect gal-SLY`ÄSSLK^P[Ocoprolites.

**13.61** Charcoal cross section of *Fraxinus excelsior*.

**13.62** Charcoal cross section of

*Alnus* sp.

**13.63** Charcoal cross section of *Fagus sylvatica*.

# 13.5 Wet wood conservation

Woods in anoxic sediments can keep their form over millennia. However, decomposition takes place on a cellular level. Alterations become obvious when logs or archaeological artifacts dry out. Radial shrinking and weight loss are macroscopic signs of intensive decomposition, cell collapses are visible on a microscopic level. Despite many deformation artifacts, parts such as YVV[ZMYVTWSHU[ZHSVUNSHRLZOVYLZJHUILPKLU[PÄLK

Natural preservation occurs in the heartwood of logs of pines soaked in resin, and of oaks with high levels of phenols. The central parts of stems resist decomposition at the air, but the sapwood decays. The situation is much worse in wood of lake dwellings. Most wooden artifacts are of small dimensions, and made mostly of wood of deciduous trees. Five-thousandyear-old Neolithic stems of deciduous trees with a diameter of JT SVZL HWWYV\_PTH[LS`  VM [OLPY VYPNPUHS ^LPNO[ HUK ZOYPURYHKPHSS`^OLUKLO`KYH[LK\W[V;OLYLMVYLHSHYNL WHY[VMO\THUJ\S[\YHSOLYP[HNL^V\SKILSVZ[^P[OV\[HY[PÄJPHS preservation. Archaeologists preserve the form of large artifacts mainly with polyethylene glycol (PEG), e.g. the Swedish warship Vasa. Smaller wooden instruments are preserved by freeze KY`PUN VY KPTL[O`S L[OLY [YLH[TLU[;OL MVYT HUK Z\WLYÄJPHS traces of human treatment of ancient wet wood can therefore be kept in museums. The cell wall structures of wet wood do survive preservation techniques.

**13.64** Late glacial stems of *Pinus sylvestris* that have been exposed to air. The stump surfaces decay and ZJPLU[PÄJPUMVYTH[PVUHIV\[[OLSPML of those trees is lost.

#### *Macroscopic changes on wet wood after dehydration*

**13.65** Disc of an air-exposed *Pinus sylvestris* stem. The large central part is preserved due to natural impregnation with resin. The peripheral zone without resin decays.

**13.66** Disc of a water-saturated archaeological *Quercus* sp. Its form is perfectly preserved.

**13.67** Disc of the same *Quercus* stem, but dehydrated. Intensively decayed parts shrink dramatically, as indicated by the original outline.

**13.68** Not dehydrated, heavily degraded wood of late glacial *Pinus sylvestris*. The cell form and primary cell walls (red) remain. Secondary walls lost their original structure HUKHYLKLSPNUPÄLKIS\L

*Microscopic changes on wet wood after dehydration*

**13.69** Dehydrated, heavily degraded wood of late glacial *Pinus sylvestris*. Cells lost their original form due to contraction, and degraded secondary cell walls appear as unstructured bodies.

**13.70** Dehydrated, heavily degraded wood of late glacial *Pinus sylvestris*. The cell forms remain, but the secondary walls are gone.

#### *Microscopic changes on wet wood after dehydration* roots

**13.72** Modern wood of *Fraxinus excelsior*, stained with Safranin. All microscopic structures can be recognized.

**13.76** Modern wood of *Quercus petraea*, stained with Safranin. All microscopic structures can be recognized.

**13.73** Dehydrated, heavily degraded wood of Neolithic *Fraxinus excelsior*. Lateral contraction deformed the original structure, but major anatomical characteristics for the species remain. collapsed vessels parenchyma cell ÄILY

**13.77** Dehydrated, heavily degraded wood of Neolithic *Quercus* sp. Lateral contraction deformed the original structure, but major anatomical characteristics for *Quercus* remain e.g. ring porosity, large rays, dark heartwood.

**13.74** Dehydrated, heavily degraded wood of Neolithic *Fraxinus excelsior*. It maintained its anatomical structure despite some lateral deformation by roots from plants of the lake shore.

**13.78** Dehydrated, heavily degraded wood of Neolithic *Quercus* sp. Alterations due to dehydration are spe-JPÄJ[VJLSS[`WLZ-PILYZHYLOLH]PS` compressed, parenchyma cells with cell contents less so.

Material: Swiss National Museum.

**13.75** Dehydrated, heavily degraded wooden artifacts, made from *Fraxinus excelsior*, maintained their original form after conservation treatment by freeze drying.

**13.79** Well preserved anatomical structure of wood of Neolithic *Quercus* sp. after conservation treatment by freeze drying and polyethylene. Cell walls are decomposed.

**13.80** Neolithic wooden bowl conserved with diethyl ether resin.

*ơ* primary and secondary walls

**13.81** Cell walls without structure after freeze drying. Electron-optical photograph. Reprinted from Bräker *et al*. 1979.

**13.82** Partially preserved cell wall structure after diethyl ether treatment. Electron-optical photograph. Reprinted from Bräker *et al.* 1979.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# 14. Technically altered wood products

This chapter gives an extremely brief overview over a few frequently used products made from technically altered wood plywood, particleboards, granulated cork stoppers, hardboards, pencils and paper.

Naturally grown wood used to be one of the most valuable removable resources in pre-industrial times. Today, large industries modify wood from its natural form and structure, and change its physical and chemical properties to create products that suit modern human needs. In construction, shrinking and swelling of materials is an undesirable feature. This can be avoided with the production of plywood, particleboards and hardboards in ^OPJO[OLÄILYKPYLJ[PVUVM[OL^VVKPZHY[PÄJPHSS`HS[LYLK-PILYZ in plywood run horizontally and vertically, particleboards and JVYR Z[VWWLYZ OH]L ÄILYZ Y\UUPUN PU HSS KPYLJ[PVUZ HUK OHYKboards feature compressed wooden structures. These processed products do not react in an anisotropic way anymore. For other products, the formal design has priority, e.g. in wooden pencils. Paper industries chemically disaggregate wood species with long ÄILYZ

#### *Ƥ*

glued joint

**14.3** Macroscopic aspect of various chipboards, consisting of small wood particles and synthetic products.

**14.1** Macroscopic aspect of glued plywood. **14.2** Microscopic aspect of a multi-layered plywood. Longitudinally and vertically oriented layers stabilize the board.

*Ƥ*

**14.4** Chipboard with large particles and large ZWHJLZ^P[OZ`U[OL[PJÄSSLYZ^OP[L

**14.5** Particleboard with small particles in polar-PaLK SPNO[;OL Z`U[OL[PJ ÄSSLYZ HYL UV[ IPYLMYPUgent.

#### *Hardboards*

**14.7** Compressed wooden structure of *Populus* ZW;OLPU[LUZP[`VM[OLJVTWYLZZPVUPZYLÅLJ[LK in the bending of the rays.

*Wooden pencils*

#### *Granulated cork stoppers*

**14.8** Wine cork stoppers. **14.9** Chips in a cork stopper. Cells of small par- [PJSLZHYLÄSSLK^P[OHPYKHYR

**14.10** Wooden pencils with paint coating. The lead is embedded in a holder of *Thuja*, *Juniperus* and many other species.

**14.11** Cross section of a lead pencil with a holder of *Larix* wood.

**14.12** The ground-up wood pulp is visible in this old book page.

**14.13** Macerated conifer tracheids with a length of up to 4 mm.

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (*http://creativecommons.org/licenses/by/4.0/*), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# Acknowledgements

During the last 40 years many friends, colleagues, students and institutions provided material or supported the book with critical remarks or inspiring thoughts. Here I just mention and thank institutions and persons who essentially supported the book.

The Swiss Federal Research Institute WSL, Birmensdorf ,provided [OL[LJOUPJHSPUMYHZ[Y\J[\YL HUK ÄUHUJPHS Z\WWVY[ \*VSSLHN\LZ from the phytopathological and entomological department provided material for macro-photographs or micro-sections.

;OL>:3)PYTLUZKVYMÄUHUJPHSS`Z\WWVY[LKL\_J\YZPVUZ[V.LVYgia and Greenland, the WSL shop was always ready to support TL^P[OVMÄJLTH[LYPHSHUK)LY[/VL^LJRLUL]LYNV[[PYLK[V solve my IT problems.

The heads of the Botanical Gardens in Bern, Birmensdorf, Basel, Zurich, Orsières, Munich, Tübingen, Padova, Helsinki, Funchal, Tbilissi, Ekaterinburg, Krasnoyarsk and Magadan allowed me to sample rare plants.

Sandro Luchinetti from the Daples Schenkung Zurich technically developed microtomes which were essential for producing perfect micro-sections.

Discussion during the International Dendroanatomical Weeks with participants from all over the world were very inspiring.

Special thanks go to Ulf Buentgen in Cambridge (UK), Alan Crivellaro in Padova (Italy), Jiri Dolezal in Trebon (Czech Republic) and Holger Gärtner in Birmensdorf (Switzerland) for intensive ZJPLU[PÄJKPZJ\ZZPVUZ

I especially would like to thank my wife Elisabeth for her generosity. Her patience was almost unlimited during hundreds of JVTTVUÄLSK[YPWZHUKSVULS`SVUNZ\TTLYZ:OLOHKTLVU the long leash.

Fritz Hans Schweingruber

# References and recommended reading


© The Author(s) 2018

F. H. Schweingruber, A. Börner, *The Plant Stem*, https://doi.org/10.1007/978-3-319-73524-5


# Index of keywords

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calcium oxalate ................ 39, 51, 69


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F. H. Schweingruber, A. Börner, *The Plant Stem*, https://doi.org/10.1007/978-3-319-73524-5 © The Author(s) 2018



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*Funaria hygrometrica* . . . . . . . . . . . . . . . . . . 85

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