Xylem Structure and Function

Abstract

Xylem is the specialised tissue of vascular plants that transports water and nutrients from the plant–soil interface to stems and leaves, and provides mechanical support and storage. The water‐conducting function of xylem is one of the major distinguishing features of vascular plants. Water is the primary solvent for plant nutrition and metabolism, and is essential for photosynthesis, turgor and for transport of minerals, hormones and other signalling molecules. The water‐conducting xylem cells provide an internal hydrophobic surface facilitating water transport as well as mechanical strength. The xylem cells also support the weight of the water transported upward in the plant and the weight of the plant itself. For many trees, xylem is wood, which has been an essential raw material for human societies since antiquity, providing structural material, fuel and fibre. Modification of xylem by genetic engineering may lead to better energy sources, improved raw materials and wood products.

Key Concepts:

  • Xylem is the tissue of vascular plants that transports water and nutrients from the soil to the stems and leaves.

  • Xylem plays an essential ‘supporting’ role providing strength to tissues and organs, to maintain plant architecture and resistance to bending.

  • The water‐transporting cells of mature xylem are dead, and therefore the transport of water is mostly a passive process with a very small active root pressure component.

  • In most woody plants, xylem grows by the division and differentiation of cells of a bifacial lateral meristem, the vascular cambium, which produces secondary xylem and phloem.

  • Xylem is wood, one of the world's most abundant and valuable renewable raw materials.

  • The morphology, frequency and distribution of xylem cell types determine its biological, physical and chemical properties, and the resulting properties of wood.

  • The chemical and physical properties of wood determine its role in the carbon cycle through its carbon storing capacity and its resistance to decay when alive or dead, and in various sediments.

  • The chemical and physical properties of wood are themselves determined by the composition and interactions of the three polymers, cellulose, hemicellulose and lignin.

  • The difference between a woody and herbaceous plant depends on the expression of a small number of regulatory genes controlling the duration and quantity of xylem formation.

  • Genetic engineering has great potential to develop xylem with desired qualities for specific purposes.

Keywords: vascular plants; secondary cell walls; plant water transport; cambial differentiation; xylogenesis; wood formation

Figure 1.

The organisation of xylem, cambium and phloem in a segment of a cross section of wood from the trunk of a conifer. Layers representing tissues of bark, phloem and cambium are indicated. Xylem is shown as wood with annual growth rings of alternating layers of springwood (earlywood) and summerwood (late wood). Radial files of cells (Rays) are also indicated.

Figure 2.

Drawing of an expanded segment of typical conifer xylem cut from a cross section, showing vertical early wood and latewood tracheids. Light earlywood and darker latewood tracheids produce visible growth rings (as seen in Figure ) bordered pits are shown on the radial surface of the tracheids. A section of a ray is shown, as is a vertical resin duct lined by epithelium.

Figure 3.

Cellular organisation in pine wood. A cross section of wood from pine xylem shows the diameter and cell wall thickness of cell types. Earlywood, with thin walls and large lumens, is indicated by green arrows, a blue arrow indicates latewood cells. The light blue arrow indicated the seasonal growth boundary of summerwood and springwood determined by the onset of dormancy. The red arrows indicate horizontal rays, and the yellow arrows indicate resin ducts. Photomicrograph by S. Lev‐Yadun.

Figure 4.

Flow of water, nutrients and storage materials through the vascular system. Phloem transports products of photosynthesis, primarily sucrose, from the leaves to the stem tissues and the roots. Xylem transport through sapwood moves water and nutrients from the roots to stems and leaves. Heartwood represents xylem that is no longer involved in transport. Rays represent a horizontal transport system that carries nutrients, storage materials, and extractives radially.

Figure 5.

Relative sizes and shapes of some xylem cell types: (a) conifer tracheid with circular bordered pits, (b) fiber tracheid with bordered pits, (c) libriform fiber with simple pitting, (d) vessel element with scalariform perforations and (e) vessel element with a simple perforation. Note that conifer tracheids (3–5 mm) are usually much longer in relationship to fibers (0.8–2.3 mm) and vessel elements (0.2–1.3 mm).

Figure 6.

(a) Structure of a bordered pit in the secondary wall of a conifer tracheid showing the modification of the pit membrane to a torus and margo. Note the loose network of cellulose fibrils that forms the margo and the secondary thickening of the central region to form the torus. In angiosperms, the pit membranes of bordered pits are usually not modified. (b) Section of pine wood showing bordered pits on radial surface of tracheids. Photomicrograph by S. Lev‐Yadun.

Figure 7.

Vessels and numerous pits in the walls of the vesselsofRicinuscommunis (castor bean) are seen in this longitudinal tangential section of the trunk. Photomicrograph by S. Lev‐Yadun.

Figure 8.

Pine wood in tangential longitudinal sections. (a) showing bordered pits (blue arrow), and rays (red arrow)and (b) Pine radial resin duct (green arrow). Section also shows rays and pits. Photomicrographs by S. Lev-Yadun.

Figure 9.

Diversity of wood in dicotyledons. (a) Diffuse porous wood of ash (Fraxinus) with a many large vessels. The cross section shows several growth rings containing smaller vessels in the latewood. (b) Cross section of wood from coral trees (Erithrina) showing dark bands of fibers and a lower frequency of large vessels. Photomicrographs by S. Lev-Yadun.

Figure 10.

Large aggregate rays in an oak, Quercus ithaburensis in a cross section of a mature trunk. Two huge aggregate rays are visible (arrows) and in between them many early‐ and narrower latewood vessel members, as well as narrow uniseriate rays are seen. Photomicrograph by S. Lev‐Yadun.

Figure 11.

Cross section of young root from pea. Primary xylem in the young root of Pisum. The larger metaxylem vessels are in the center. Photomicrograph by S. Lev‐Yadun.

Figure 12.

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

Figure 13.

Cambium of castor bean in cross section. Figure X (L–Y). A cross section in a young stem of the castor bean, Ricinus communis. The red lignified secondary xylem in the bottom and the brick‐like cambial cells are seen (arrows). Secondary phloem occupies the upper part. Photomicrograph by S. Lev-Yadun.

Figure 14.

The secondary thickened wall of a mature tracheary element is shown in this drawing of the location and orientation of cellulose microfibrils in the different layers of the wall. Note the designation of the secondary wall layers and the average microfibril angle of each layer: S1 is the outermost layer, S2 is the middle layer and S3 the innermost layer. Most of the wall thickness is determined by the thickness of the S2 layer (the relative thicknesses are: primary wall, 1%; S1, 10–20%; S2, 40–90% and S3, 2–8%). Modified after Côté WA (1967) Wood Ultrastructure: An Atlas of Electron Micrographs. Seattle: University of Washington Press.

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Further Reading

Albersheim P, Darvill A, Roberts K, Sederoff R and Staehelin A (2011) Plant Cell Walls, New York: Garland Science. 430 pp.

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Zimmermann MH (1983) Xylem Structure and the Ascent of Sap. Berlin: Springer‐Verlag.

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Myburg, Alexander A, Lev‐Yadun, Simcha, and Sederoff, Ronald R(Oct 2013) Xylem Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001302.pub2]