Xylem Structure and Introductory article Function · PDF fileXylem Structure and ... evolution and adaptation of plants to the terrestrial environment. ... tissue: phloem and xylem

Xylem Structure andFunctionAlexander A Myburg, North Carolina State University, Raleigh, North Carolina, USA

Ronald R Sederoff, North Carolina State University, Raleigh, North Carolina, USA

Vascular plants have evolved a highly specialized tissue, called xylem, which provides

mechanical support and transports water, mineral nutrients and phytohormonal signals in

the plant. Although it is the most abundant biological tissue on earth, much remains to be

learned about the structure, function, development and evolution of xylem and of the

genes that regulate the processes.

Introduction

The earliest land plants were short, herbaceous plants thatevolved from primitive, water-living ancestors. For theseplants, the change from a predominantly aquatic to aterrestrial environment was accompanied by the need foradditional structural support to keep the plants uprightand the need for more efficient transport of water to theaboveground parts of the plants. Larger plant sizes alsoincreased the need for co-ordination between remote plantparts. The development of specialized vascular tissues tofulfil these requirements played an important role in theevolution and adaptation of plants to the terrestrialenvironment.As the early land plants filled more and more terrestrial

niches, the selective advantage of increased propaguledispersal associated with increase in height, and latercompetition for sunlight, increased the selection pressurefor plants that could grow taller than other plants. Themost successful plants were able to support more weight,transport water further and sustain growth for more thanone season. The dramatic result of this evolutionaryprocess is evident in the rapid increase of plants withsecondary vascular tissues and arborescent growth form ina rather short evolutionary timespan (380–350 millionyears ago).The stems and roots of modern plants are highly

specialized conductive organs that can transport water,nutrients, photosynthetic products and chemical regula-tory signals. These organs contain two types of conductivetissue: phloem and xylem. Phloem is the tissue thattransports photosynthetic products and plant growthregulators (phytohormones) mainly from the leaves tothe rest of the plant. Xylem is the tissue that transportswater, mineral nutrients and phytohormones from theroots to the leaves and other plant organs. Whileherbaceous plants do contain xylem, it is a tissue that ismost prominent in woody plants, especially trees. Most ofour knowledge of xylem structure and function is based onwoody plants. The most important functions of xylem

include: (1) transport of water and mineral nutrients, (2)mechanical support and (3) storage of nutrients and water.

Xylem Structure and Variability

The cell types that make up xylem tissue show greatvariability across different plant groups, from species tospecies and even within the same plant. This section willfocus on the structure and variability of xylem producedduring primary and secondary growth in different plantgroups.

Xylem cell types

The structural features of xylem are determined by the size,shape and distribution of xylem cell types and, inparticular, by the shape and thickness of their cell walls.

Cell wall structure affects cell type and characteristics

Almost all plant cells produce primary cell walls. Themajor component of most primary walls in xylem is adisorganized network of cellulose fibrils, which allows thewall to stretch and expand as the cell grows. The secondarywall is deposited on the inner side of the primary wallduring and after the cell has elongated or enlarged. Thecellulose fibrils in the secondary wall are arranged in aregular fashionwith alternating layers at fixed angles to themain axis of the cell (Figure 1). This reinforces the plant cell,while preserving the elastic nature of the primary wall.Most of the cell types in xylem can be distinguished basedon the shape and features of the secondary cell wall.

Xylem parenchyma cells store water, mineral nutrientsand carbohydrates, and respond to wounding

The cells responsible for most of the storage function ofxylem are called parenchyma cells. Many xylem parench-yma cells have secondary lignified walls, particularly in

Article Contents

Introductory article

. Introduction

. Xylem Structure and Variability

. Xylem Functions: Water Transport and Structural

Support

. Xylem Differentiation and Cell Wall Biosynthesis

. Origin and Evolution of Xylem in Plants

. Genetic Manipulation of Xylem Formation

1ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

wooden plants. In other cases, these cells have thin,primarywallswith areas of plasmodesmata, called primarypit fields, through which cell-to-cell movement of waterand mineral nutrients can take place. Mature xylemparenchyma cells in active xylem tissue retain a functionalprotoplasm and can store carbohydrates in the form ofstarch. These cells also play an important role in woundhealing by forming callus and can differentiate toregenerate functional xylem cells.

Sclerenchyma cells provide mechanical support, defenceand water transport

The cells involved in mechanical support and defence arespecialized sclerenchyma cells. Fibres are long, narrowsclerenchyma cells, mostly with thick secondary walls(Figure 2b and c). They are mainly involved in themechanical support function of xylem and defence againstpathogens and herbivores.The conducting cells of xylem are called tracheary

elements. There are two types of tracheary elements:tracheids (Figure 2a) and vessel elements (Figure 2d ande). Vessel elements are connected end-to-end through largeperforations in their end walls to form a vessel. Tracheidsare connected through large, circular bordered pits that areconcentrated at the tapered ends (in the radial walls) of thecells (Figure 2a). Mature vessel elements and tracheids haveno cellular contents and consist mainly of thickenedsecondary walls.

In most tracheary elements, almost the entire innersurface of the primary wall is covered by secondary wall,except for small areas called pits. In the lateralwalls of suchvessel elements, and the walls of tracheids (mostly radialwalls), the pits occur in pit-pairs with the pits ofneighbouring cells precisely aligned (Figure 3). A pitmembrane, comprised of the primary walls of adjacentcells, separates the pits of each pit-pair. The inner apertureof the pit is often narrowand reinforced by extra secondarywall material to form a border. The outer aperture of eachpit, which is bounded by the pitmembrane, is usually widerto allow maximum conductance of water across the pitmembrane. In most conifers, the central part of the pitmembrane is thickened and lignified to form a torus(Figure 3). The torus is usually slightly larger than theaperture of the pit border and is impermeable towater. Theouter part of themembrane (themargo) is digested to leavea porous network of cellulose fibrils through which watercan move easily. Under certain circumstances, the toruscan block one of the two inner apertures of the pit-pair andprevent the movement of water and air through the pit. Intracheids, this may serve to isolate cavitated tracheids andprevent the spread of embolisms.The end walls of vessel elements are modified into

perforation plates (Figure 2d and e). Most vessel elementspossess simple transverse perforation plates with only onelarge perforation, but compound perforation plates withtwo or more perforations occur. Simple perforationsprovide the least amount of resistance to water flow and,therefore, maximum conductance. Some primitive angios-perm families have slanted scalariform perforation plates.

Primary growth

Primary xylem occurs in separate vascular bundles

Primary growth refers to the primary plant body that isformed through cell production by the apical meristems ofthe plant. In most but not all monocots (monocotyledons)and herbaceous dicots (dicotyledons), almost the entireplant body is the product of primary growth. In woodyplants, this represents the innermost layers of xylem alongthe stem, branches and roots. The xylem tissue of young,unthickened stems and roots usually occurs in separateprimary vascular bundles along with the phloem tissue. Indicots, the primary vascular bundles are typically arrangedin a peripheral cylinder, while in monocots, the vascularbundles are scattered throughout the parenchymatousground tissue of the plant body. The primary xylem instems usually consists of early differentiating protoxylem,located on the inner side of the xylem, and latedifferentiating metaxylem on the outer side of the xylem.In most dicot and gymnosperm stems, a lateral

meristem, called the vascular cambium, separates theprimary xylem and phloem of each vascular bundle. Thislayer of cells develops as an extension of the procambium,

S3 (60°–90°)

S2 (10°–30°)

S1 (50°–70°)

Secondarywall

Primary wall

Middle lamella

Figure 1 Drawing of the secondary thickened wall of a mature trachearyelement showing the orientation of cellulose microfibrils in the differentlayers of the wall. Note the designation of the secondary wall layers and theaverage microfibril angle of each layer: S1 is the outermost layer, S2 is themiddle layer and S3 the innermost layer. Most of the wall thickness isdetermined by the thickness of the S2 layer (the relative thicknesses are:primary wall, 1%; S1,10 to 20%; S2, 40 to 90% and S3, 2 to 8%) . Modifiedafter Cote WA (1967) Wood Ultrastructure: An Atlas of Electron Micrographs.Seattle: University of Washington Press.

Xylem Structure and Function

2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

strands of meristematic cells beginning just below thegrowth tip of the stem (and root). The vascular cambiumwill later give rise to secondary xylem and secondaryphloem.Although some do have thickeningmeristems, thevascular cambium is absent in monocots.

Secondary growth

The secondary xylem of woody plants constitutes themajor part of the stem, i.e. thewood. This sectionwill focuson various aspects of wood structure that are directlyrelated to the development and organization of secondaryxylem.

The development of secondary xylem

Secondary xylem is formed by the vascular cambium

All gymnosperms and woody dicots undergo secondarygrowth, which results in an increase in the diameter of thestem, branches and roots. The onset of secondary growth ischaracterized by the activation of cell division in thefascicular vascular cambium, i.e. the meristematic layerinside the vascular bundles. These cell divisions are

Simple pit

(e)

(d)

Perforation platePerforation

Borderedpits

(c)

(b)

(a)

Figure 2 Drawing showing the relative sizes and shapes of some xylem cell types: (a) conifer tracheid with circular bordered pits, (b) fibre tracheid withbordered pits, (c) libriform fibre with simple pitting, (d) vessel element with scalariform perforations and (e) vessel element with a simple perforation. Notethat conifer tracheids (3 to 5 mm) are usually much longer in relationship to fibres (0.8 to 2.3 mm) and vessel elements (0.2 to 1.3 mm).

Middle lamella

Secondary wall

Primary wall

Border

Inner aperture

Torus

Margo

Secondary wall ofadjacent cell

Figure 3 Structure of a bordered pit in the secondary wall of a conifertracheid showing the modification of the pit membrane to a torus andmargo. Note the loose network of cellulose fibrils that forms the margo andthe secondary thickening of the central region to form the torus. Inangiosperms, the pit membranes of bordered pits are usually not modified.



co-ordinated with cell divisions in the adjacent interfasci-cular region to produce a continuous cylinder of vascularcambium.Usually, secondary xylem is formed on the innerside and secondary phloem on the outer side of thecambium (Figure 4).

Fusiform and ray initials give rise to the axial and radialcomponents of xylem

The vascular cambium consists of fusiformand ray initials.Fusiform initials divide longitudinally to give rise to theaxial components of secondary growth, i.e. trachearyelements, fibres and axial parenchyma towards the insideand phloem cells towards the outside of the stem. Rayinitials divide to form ray cells that run radially across thesecondary vascular tissue. Rays serve to transport water,dissolved gases and organic nutrients radially in wood. Assecondary growth proceeds, the cambial cylinder increasesin diameter through lateral division of fusiform initials.

Earlywood, latewood and growth rings

The cambium of many woody plants exhibits periodicactivity. In the spring and early summer (in temperateregions), conditions are conducive to active growth andrelatively wide tracheary elements with thin walls areproduced. Later in the summer and autumn, relativelynarrow tracheary elements with thick walls are formed(Figure 4). These two types of xylem, called earlywood andlatewood, are most commonly observed as concentriccircles on the transverse section of the stem and are formedas a result of changes in the activity of the vascularcambium. When the activity of the vascular cambium iscontrolled by annual seasons (one ring is formed per year),

these circles are true annual rings and the age of the stemcan be deduced from the number of rings. In many regionsof the world, particularly the tropics, growth rings do notalways represent annual increments.More thanone (or lessthan one) growth ring can be formed per year, for examplewhen several dry and wet periods occur within a year.

Heartwood and sapwood

Wood cells have a limited lifetime in which they canactively transport water. After a variable number of years,cavitation occurs in most of the vessels and tracheids andthe rest of the xylem cells in the growth ring die. These cellsare then filled with resinous materials and polyphenols,and constitute the inner, often darker part of the woodystem called heartwood. The outer, water-conducting partof the stem is called sapwood. Inmany species, as sapwoodis converted to heartwood, air-filled vessels in the sapwoodare often sealed off by the intrusive growth of surroundingparenchyma cells. These intrusions are called tyloses and,together with the resinous materials, serve to preventfungal growth in the empty vessel lumens. The outer,conducting part of the stem is called sapwood.

Dicot versus conifer wood

Woods are commonly classified as either hardwoods orsoftwoods. Hardwoods are angiosperm (dicotyledonous)trees, while softwoods are gymnosperm (conifer) trees.These two terms do not accurately express differences inthe hardness or density of the wood, but are useful for thedescriptionof the basic structural differences betweendicotand conifer wood.

Earlywood Latewood M M C M

Earlywood vesselLatewood vessel

Secondary xylemPhloemCambial

zone

Primary xylemPith

(a)

(b)

Ray

Figure 4 Drawing of cross sections of young woody stems showing the cambial zone and secondary xylem development. (a) Dicot wood. (b)Conifer wood. Note the abrupt change in the size of tracheids from earlywood to latewood. M, differentiating xylem and phloem mother cells;C, cambial initial.



Dicot wood contains vessels, fibres, parenchyma andtracheids

Dicot wood contains a greater number of cell types thanconifer wood and the structure of dicot wood, therefore,varies more than that of conifer wood. The cell types ofdicot wood include vessel elements, fibres and parenchymacells. Tracheids are rare in dicots, but occur in some speciessuch as oaks and chestnuts. The cell types in dicot wood arealso more diversified in function; vessels and tracheidstransport water, fibres provide structural support andparenchyma cells perform storage and regeneration func-tions. The feature that most distinguishes dicot wood fromconifer wood is the presence of large-diameter vesselelements that disrupt the regular organization of the radialcell files derived from the cambial initials (Figure 4a). Twotypes of fibres are common in dicot wood: fibre tracheidsand libriform fibres (Figure 2b and c). Fibre tracheids havethickwallswith bordered pits. Libriformfibres have simplepits.Dicotwoodgenerally contains larger rays than coniferwood and in most dicot species the rays consist only of rayparenchyma.

Conifer wood consists mostly of regular files of tracheids

Conifer wood is relatively simple in structure. The mostdistinctive features of conifer wood include: the regularorganization of the radial files of tracheids, the absence ofvessels and fibres and the small amount of woodparenchyma (Figure 4b). The long, tapered tracheids formthe predominant cell type and fulfil both the mechanicaland conductive functions of conifer wood. Themajority ofparenchymacells in coniferwoodare present in rays and, insome conifers such as the Pinaceae, in axial and radial resinducts. Conifer rays consist primarily of ray parenchymaand, in some conifers, a smaller amount of ray tracheids.Resin ducts are large intercellular spaces surrounded bythin-walled parenchyma cells that excrete resin into theduct. The resin is believed to seal wounds and protect theplant against fungi and herbivores.

Reaction wood

Woody plants respond to bending induced by externalforces, such aswind and gravity, bymaking reactionwood.Conifers produce reaction wood on the side of the branchor stem where the tissues are compressed (usually theunderside) and it is therefore called compression wood. Indicots, reaction wood forms on the side under tension(usually the upper side) and it is called tension wood.Compression wood has thicker cell walls, higher lignincontent and is darker than normal conifer wood. Tensionwood is characterized by the presence of gelatinous fibres,low lignin content and high cellulose content. The purposeof reaction wood is to reorient bent stems and branches toallow optimal light exposure of the tree canopy.

Secondary thickening in monocots

The majority of monocots are herbaceous, which meansthat the primary xylem has to fulfil all the requirements ofwater transport that the plant may encounter throughoutits lifetime. However, somemonocots do undergo thicken-ing of the primary stem. In bamboos and other monocotspecies with wide stems, a broad region of mitotic activity,called the primary thickening meristem, is responsible forradial and tangential expansion of the primary stem. Veryfew examples exist of truly woody monocots. In woodymonocot genera such as Yucca and Dracaena, the activityof a secondary thickening meristem in the outer cortex ofthe stem is responsible for anomalous secondary growth.Arborescent monocots such as palms undergo diffusesecondary growth through the division of cells in theground parenchyma of the stem.

Xylem Functions: Water Transport andStructural Support

Water transport

A gradient of water potential drives water transport

Despite a large amount of researchon this topic, the precisemechanismofwater transport in plants is still debated. Theexperimental evidence strongly suggests that water trans-port in plants is driven by a gradient of water potential thatexists between the air surrounding the leaves at one end andthe water that surrounds the roots at the other. These twoextremes are connected by the xylem, which supports awater column that extends from the roots to the leaves. Airusually has a very negative water potential (even when thehumidity is very high). As the leaves of the plant lose waterto the air, thewater potential becomesmore negative insideleaf cells. This causes water to gradually move from xylemcells to leaf cells. The water molecules inside the watercolumns of the capillary xylem elements are pulledupwards by cohesion forces when water molecules at thetop of the columns move out into the aerial parts of theplants. This is knownas the cohesion–tension theory of sapascent.

Adhesion, cohesion and tension forces act on the watercolumn

The upward movement of the water column is counter-acted by three forces: (1) the weight of the water column,(2) adhesion of water to the cell walls of tracheary elementsand (3) adhesion of the water to soil particles. The upwardmovement of the water molecules in each trachearyelement will cause tension in the water column, causing itto become narrower. During times of high transpiration,the negative pressure inside tracheary elements can becomestrong enough to cause these cells to collapse inward.



Vessel elements and tracheids possess secondary thickenedwalls that serve to reinforce the walls and prevent inwardcollapse under the tremendous forces produced inside thetracheary element.

Water columns can break

The ability of tracheary elements to allow movement ofwater is called conductance. The conductance of atracheary element is related to the fourth power of theradius of the element (known as the Hagen–PoiseuilleLaw). This means that a slight increase in diameter of theelementwill significantly increase the conductance. Indeed,this seems to have been a major driving force in theevolution of tracheary elements. However, under certaincircumstances it is beneficial to possess tracheary elementswith small diameters. In such elements, hydrogen bondingof water molecules to the wall (adhesion force) serves toreinforce and strengthen thewater column. If the tension inthewater columnbecomes sufficiently strong, however, thewater column can break (cavitate) and an embolism (‘airbubble’) will form in the element. This problem is moreserious in large-diameter elements than small-diameterelements. In tracheids, the embolism can expand to fill thewhole cell, but the surface tension of water will prevent itfrom passing through the pit membrane. In vessels,embolisms can spread from element to element throughthe perforations that link consecutive vessel elements. Thewhole vessel will then become dysfunctional for watertransport.

Tracheids and vessel elements are adapted for optimalconductance

Tracheids are generally much longer than vessel elements.This reduces the number of pit membranes that a watermolecule has to cross on its way to the leaves. Tracheidsalso have long-tapered ends to allow themaximumnumberof pit-pairs between consecutive cells. Vessel elements aremuch shorter than tracheids, but they are connected end-to-end to form long vessels.Gymnosperm tracheids tend tobe wider than those of angiosperms, where most of thewater transport occurs through large-diameter vesselelements. Angiosperms combine the structural andwater-conducting benefits of small-diameter and large-diameter tracheary elements. Most of the water volume istransported by large-diameter vessels whenwater is readilyavailable, while small-diameter vessels and tracheids (insome dicots) are usedwhen thewater column is under greattension and greater protection against cavitation isrequired.

Structural support

The aerial parts of all terrestrial plants require mechanicalsupport. This is provided in large part by xylem tissue in thestem and branches. The mechanical support function ofxylem is most prominent in the stems of trees, whichinclude some of the largest living organisms on earth.

Cell walls form the basic unit of structural support

The basic unit of structural support in plants is themechanical support provided by the cell wall of each cell inthe plant body. Cell walls consist mostly of cellulosemicrofibrils. Cellulose fibrils can be very strong; strongerthan steel, silk or nylon. This makes cell walls strongenough to resist internal forces (turgor) as well asexternally applied forces (tension). Additional rigidityand compressive strength is provided by lignin, especiallyin tissues (such as xylem) that accumulate lignin.

Xylem contains several cell types with structural supportfunctions

Fibres provide most of the mechanical support in dicotxylem. The structure of the fibre walls allows this cell typeto support weights of up to 15–20 kgmm2 2. Moreimportantly, fibres are elastic enough to retain theiroriginal length after subjection to tension forces of thismagnitude. Vessels and tracheids also contain secondarythickened walls and therefore contribute to structuralsupport in xylem. In conifer wood, all the structuralsupport is provided by tracheids.

Wood is a complex material

The woody stems of large trees provide the mostspectacular examples of structural support in plants.Wood in living tree stems is structurally complex, withseveral levels of organization.At themolecular level, woodis comprised of crystalline cellulose embedded in a matrixof hemicellulose and lignin, a highly crosslinked phenolicpolymer. The cellulose fibrils in the secondary wall aredeposited in layers, each layer with a different preferredmicrofibril angle (Figure 1). At the cellular level, the xylemcells in wood are arranged in cylinders parallel to the longaxis of the stem. Finally, above the cellular level, thegrowth rings form concentric layers of wood tissue withdifferent wall and lumen dimensions. This makes wood alayered structural composite, which is muchmore complexthan reinforced concrete.

Xylem Differentiation and Cell WallBiosynthesis

The developmental process in which procambial andcambial initials differentiate into mature xylem cells iscalled xylogenesis. This process can be as short as four days



in primary xylemand from14–21 days in secondary xylem.Xylogenesis typically includes the following phases: (1) celldivision and enlargement, (2) cell wall thickening, (3)lignification and (4) programmed cell death. Cell wallbiosynthesis is an integral part of xylem formation. Thebasic chemical components and organization of xylem cellwalls are known, but little is known of the mechanisms bywhich they are synthesized and organized to form thehighly complex cell wall. This section will outline thephases of xylem differentiation and the formation of xylemcell walls, which form themajor part ofmature xylem cells.

Xylem cells are derived from apical and lateralmeristems

Tracheary elements are derived from either the procam-bium (primary xylem) or vascular cambium (secondaryxylem). The differentiation of cambial initials into xylemelements is thought to be initiated by plant hormones. Theimmature xylem cells have dense protoplasm, smallvacuoles and thin primary walls (Figure 4). Soon after celldivision, these cells undergo cell elongation and an increasein the size of the vacuole and nuclei.

Most xylem cell walls undergo secondarythickening

The deposition of secondary walls begins sometime beforetracheary elements and fibres reach their full size. Thecellulose, lignin, hemicellulose and protein components ofthe secondary wall are synthesized and deposited coopera-tively during secondary wall thickening. The onset ofsecondary wall thickening is associated with the formationof arrays ofmicrotubules under those regions of the plasmamembrane where active secondary wall deposition willtake place. Microtubules may play a role in defining thepattern of secondary walls by guiding dictyosome-derivedvesicles with cell wall material to the sites of deposition onthe cell membrane. Cellulose microfibrils are produced atthe membrane surface of the cell by complex rosettestructures, which consist of several different proteins. Themovement of these rosette complexes in the plasmamembrane may also be directed by microtubules.

The cell walls and intercellular regions ofxylem cells are lignified

Following secondary thickening of the xylem cell walls,lignin is deposited between the newly formed trachearyelements and within their walls. The area between the cells,called themiddle lamella, and the primarywalls are rapidlylignified, followed by a more gradual lignification of thesecondary walls. Lignin is a very complex, crosslinked,three-dimensional polymer of aromatic phenolic mono-mers, called cinnamyl alcohols. The lignin monomers are

delivered to the cell wall via Golgi and endoplasmicreticulum-derived vesicles and polymerized into lignin bywall-bound enzymes. The aromatic nature of the ligninmonomers makes lignin hydrophobic. Lignin, therefore,provides a hydrophobic inner surface to the cell wall thatfacilitates water transport. The three-dimensional natureof the lignin polymer provides rigidity and compressivestrength to the cell wall, while the chemical stability oflignin provides protection against pathogens.

Tracheary elements undergo programmedcell death

At the completion of secondary wall deposition andlignification, tracheary elements undergo autolysis, anexample of programmed cell death in higher plants. Soonafter the initiation of secondary thickening, hydrolyticenzymes (DNAases, RNAases and proteases) start accu-mulating in the vacuole. The autolytic process is initiatedwhen the tonoplast ruptures, causing the hydrolyticenzymes to spill out into the cytoplasm. This leads to thecomplete degradation of the cell contents and partialdigestion of the unprotected regions of the primary wall.Only regions covered by lignified secondary wall materialare protected from degradation. The end walls ofdifferentiating vessel elements are degraded at the perfora-tion sites to allow direct cell-to-cell movement of water andnutrients. Only regions covered by lignified secondary wallmaterial are protected from degradation. Pit membranesare often partially degraded to leavemats of cellulose fibrils(Figure 3). This enhances the movement of water throughpit-pairs, which is the only way water can enter and leavetracheids.

Origin and Evolution of Xylem in Plants

Vascular plants (Tracheophyta) are characterized by thepresence of xylem tissue with lignified cell walls. Modernvascular plants are ferns, gymnosperms and angiosperms.Mosses, liverworts and hornworts (Bryophyta) do notcontain xylem. Tracheid-like cells, called hydroids, arepresent in certain bryophytes, but lignified cell wallthickenings are absent in these plants. This section willoutline the major trends of xylem evolution in vascularplants.

Evolution of primary xylem

Tracheids were present in the first vascular land plants

It is widely accepted that the first land plants evolved fromgreen algae (Chlorophyta) and that these plants wereadapted to aquatic or semiaquatic environments. Theevolution of conducting tissue was closely associated with



the adaptation of plants to fully terrestrial environments.The acquisition of xylem tissue allowed plants to supplywater andmineral nutrients to those parts of the plant bodyexposed to the desiccating environment of the air. One ofthe earliest known fossilized land plants, Cooksonia(present as early as 420 million years ago in the Mid-Silurian Period), had tracheids with annular secondarythickenings.

Vessel elements and fibre tracheids evolved fromtracheids

Vessel elements evolved independently from tracheids inseveral groups of flowering plants, i.e. the conversion oftracheid end walls to perforation plates had a polypheleticorigin. Fossil evidence of the early evolving vessel elementsis very scarce. It is assumed that vessel elements evolvedfrom tracheidswith scalariformly reinforcedwalls and thatthese cells gave rise to the short vessel elements withtransverse, simple perforation plates and wide lumens.Fibres also evolved independently in many angiospermfamilies. Fibre tracheids evolved very early in angiospermhistory, while libriform fibres with simple pits appearedlater.

Evolution of secondary xylem

Woody plants appeared early in the history of land plants

The ability to produce secondary vascular tissues evolvedsoon after the appearance of the first vascular land plants.Bifacial cambiumwas present in the Progymnospermopsi-da in the Devonian Period (approximately 370 millionyears ago). It is still highly debatable whether the firstangiosperms that evolved from the progymnosperms werewoody or herbaceous plants. It appears however that mostpresent day herbaceous angiosperms are able to formsecondary tissues, although most usually flower and dieearly, precluding much secondary growth.

Secondary xylem increased the lifespan of plants

The ability to produce secondary xylem had profoundconsequences for early vascular plants. It greatly increasedthe lifespan of plants by allowing plants to essentially forma newwater-conducting system each year that replaced thenon-functional xylem elements from previous years. Theincrease in lifespan enabled the existence of taller plantsand increased the need for long-distance conductance andmechanical support. The major trends of xylem evolution(the shift towards vessel elements, simple perforations andlibriform fibres) are thought to be associated with theincreased efficiency of water transport in xylem and, to alesser degree, the increased demand for mechanicalsupport in plants.

Genetic Manipulation of XylemFormation

The content and composition of xylem cell walls affect thecommercial value of many biological materials, such aswood and plant fibres, as well as many food crops, such asfodder, cereals, fruits and vegetables. The potential toimprove the properties of these plants has motivatedstudies dedicated to the modification of xylem cell walls.

Xylem properties are specified by a largenumber of genes and proteins

The properties of wood and the xylem in herbaceous plantsresult from the content, composition and location of xylemcells and theirwalls. Except for thewall, tracheary elementsretain little or no material of the living cells from whichthey are derived. The composition and structure of xylemcell walls are determinedby the coordinated expressionof alarge number of genes and proteins during xylogenesis.Variation in the developmental programme and levels ofexpression of individual genes determine the variation incell wall architecture within and between different species.Therefore, knowledge of the genes involved in this processand the mechanisms by which they are controlled couldlead to the ability to manipulate the properties of xylem.Although the general composition and structure of

xylem cell walls is known, very little is known of theorganization and biosynthesis of cell wall components.Xylem cell walls contain hundreds of proteins and enzymesinvolved in the formation of the primary cell wall, whichprovide the framework for the synthesis of the secondarywall. Biosynthesis of the secondary wall involves preciselyregulated formation of cellulose microfibrils, assembly ofhemicellulose–cellulose complexes and polymerization ofa network of the phenolic polymer lignin. Work isprogressing rapidly to identify important genes andproteins in these processes, but only a few genes have beenstudied sufficiently to establish their specific roles.

New technologies allow rapid progress in thegenetic manipulation of xylem

Studies of model plant systems, such as Arabidopsis,Zinnia, tobacco and maize have been important inidentifying specific genes and proteins involved in cell wallformation. Genetic and biochemical studies of cotton andforest trees have identified some important genes for theformation of cellulose and lignin. Most recently, manylaboratories have decided to use high-throughput auto-mated techniques to identify all of the expressed genes ofhigher plants and to learn their function. This approach,called genomics, is expected to rapidly advance theknowledge of the genes and proteins forming the primary



and secondary cell walls of xylem.With this knowledge, themodification of xylem in commercially important plantswill become a process of rational design.

Further Reading

Boudet AM, Lapierre C and Grima-Pettenati J (1995) Tansley Review

no 80: Biochemistry and molecular biology of lignification. New

Phytologist 129: 203–236.

Carlquist JS (1975) Ecological Strategies of Xylem Evolution. Berkeley,

CA: University of California Press.

DelmerDPandAmorY (1995)Cellulose biosynthesis.PlantCell 7: 987–

1000.

Fahn A (1990) Plant Anatomy, 4th ed. New York: Pergamon Press.

Fukuda H (1996) Xylogenesis: initiation, progression and cell death.

AnnualReview ofPlantPhysiology andMolecularBiology 47: 299–325.

Higuchi T (1997) Biochemistry and Molecular Biology of Wood. Berlin:

Springer-Verlag.

Ingrouille M (1992) Diversity and Evolution of Land Plants. London:

Chapman & Hall.

Mauseth JD (1988) Plant Anatomy. Menlo Park, CA: Benjamin/

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