Xylem

In vascular plants, the 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 the phloem, the xylem is one of the two types of transport tissues in plants. The cell walls of xylem cells derive most of their strength from lignin, a chemical compound produced only by plants.

Structure

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 the phloem, but they lack companion cells and do not 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. They 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.

Xylem cells are also known as tracheary elements . This name was applied by Marcello Malpighi after noticing similarities between the tracheae of insects and xylem cells.

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.

Contents

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

Mechanism

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.

Reference

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