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In vascular plants, xylem is the tissue that carries water up the root and stem. In trees, it constitutes wood — hence the word is derived from the Greek word for "xylo" (Ξυλο) for wood. 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 (at least in dicots) is composed of vessel elements and 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.

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. Three 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.
• Capillary action helps sap to flow up the narrow tracheids.
• 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 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.