Xylem
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The xylem is the principal water-conducting tissue of vascular plants. It consists of tracheary elements, tracheids and wood vessels and of additional xylem fibres. All of them are elongated cells with secondary cell walls that lack protoplasts at maturity. Bordered pits are typical for tracheids, while wood vessels are marked by perforated or completely dissolved final walls. The xylem also takes part in food storage, support and the conduction of minerals. Xylem and phloem together form a continuous system of vascular tissue extending throughout the plant.
The word “xylem” is derived from classical Greek ξυλον, "wood", and indeed the best known xylem tissue is wood. The xylem transports sap from the root up the plant: xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well.
This transport is not powered by energy spent by the tracheary elements themselves, which are dead at maturity and no longer have living contents. Two phenomena cause xylem sap to flow:
- The soil solution (see soil) is more dilute than the cytosol of the root cells. Thus, water moves osmotically into the cells, creating root pressure. Root pressure is very variable between different plants; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus [1].
- By far the most important cause of xylem sap flow is transpirational pull. The reverse of root pressure, this is caused by the transpiration of water. In larger plants such as trees, the root pressure and transpirational pull work together as a pump that pulls sap from the soil up to where it is transpired.
Xylem can be found:
- in vascular bundles, present in non-woody plants and non-woody plant parts
- in secondary xylem, laid down by a meristem called the vascular cambium
- as part of a stelar arrangement not divided into bundles, as in many ferns.
Note that, in transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only. The most distinctive cells found in xylem are the tracheary elements: tracheids and vessel elements. However, the xylem is a complex tissue of plants, which means that it includes more than one type of cell. In fact, xylem contains other kinds of cells in addition to those that serve to transport water.
Evolution of xylem
Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the Silurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks. The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first-formed xylem) of all living groups of plants. Several groups of plants later developed pitted tracheid cells, apparently through convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the metaxylem (following the protoxylem.
In most plants, pitted tracheids function as the primary transport cells. The other type of tracheary element, besides the tracheid, is the vessel element. Vessel elements are joined by perforations into vessels. In vessels, water travels by bulk flow, like in a pipe, rather than by diffusion through cell membranes. The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the angiosperms. However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae), and their secondary xylem is described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms. Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis being that vessel elements originated in a precursor to the angiosperms and were subsquently lost.
See also
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References
- Campbell, Neil A. and Jane B. Reece. 2002. Biology, 6th ed., published by Benjamin Cummings.
- Carlquist, S. and E.L. Schneider. 2002. The tracheid–vessel element transition in angiosperms involves multiple independent features: cladistic consequences. American Journal of Botany 89:185-195 (link to abstract here).
- Cronquist, A. 1988. The Evolution and Classification of Flowering Plants. New York, New York: The New York Botanical Garden.
- Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
- Muhammad, A.F. and R. Sattler. 1982. Vessel Structure of Gnetum and the Origin of Angiosperms. American Journal of Botany 69: 1004-1021 (available online here).