Misplaced Pages

This is an old revision of this page, as edited by Ceyockey (talk | contribs) at 05:07, 4 March 2005. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

In vascular plants, xylem is the tissue that carries water up the root and stem. In trees, it constitutes wood; the word is derived from Greek ξύλον xúlon, "wood, timber". Together with phloem, xylem is one of the two transport tissues of plants. The cell walls of xylem cells derive most of their strength from lignin, a chemical compound produced only by plants.

Xylem (in angiosperms) is composed of vessel elements and tracheids (gymnosperm xylem consists only of tracheids). Vessel elements are similar in structure to the sieve-tube members of phloem, but they lack companion cells, and have perforated sides as well as pores at the ends. Tracheids are much narrower cells, with tapered and perforated ends, constituting most of the volume of the xylem tissue. Both tracheids and vessel elements are dead at maturity.

A xylem vessel element is an elongated cell that dies once it has functionally matured. When the interior of a xylem vessel element disintegrates, the thickened cell wall remains, forming a nonliving passage for the flow of water. Vessel elements form in plant parts that no longer elongate. Vessel elements are usually wider, shorter, thinner walled, and less tapered than tracheids. Vessel elements are aligned end to end, forming long micropipes, the vessels of xylem. The end walls of vessel elements are perforated, enabling water to flow through vessels.

The xylem sap consists mainly of water and inorganic ions, such as nitrate, although it can contain a number of organic chemicals as well.

Xylem sap always moves from the roots to the leaves. It travels by bulk flow, like water in a series of pipes, rather than by diffusion through cells. 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. Even under optimal conditions, root pressure can only lift water a couple of feet.
• By far the most important cause of xylem sap flow is transpirational pull. This is the reverse of root pressure, caused by the transpiration of water from leaves. 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 the leaves where it is transpired.

In perennial plants, xylem is laid down in multiple phases. Primary xylem is one of the tissues left behind by the apical meristem. Secondary xylem is laid down by vascular cambium on the outside of the xylem column.

Reference

• Campbell, Neil A. and Jane B. Reece. undated. Biology, 6th ed., published by Benjamin Cummings.

This anatomy article is a stub. You can help Misplaced Pages by expanding it.

• v
• t
• e

• Plant physiology
• Trees
• Anatomy stubs