Xylem Xylem The Concert in Central Park Category="Plant anatomy"Category="Plant physiology"Category="Tissues"In vascular plants, the xylem is the tissue that carries water up the root and stem. The word is derived from classical Greek xúlon, "wood, timber". This is appropriate, since Wood is composed almost entirely of xylem tissue.

Xylem is one of the two (main) types of transport tissues in plants, Phloem being the other one. The xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well.

In non-woody plants and plant parts phloem and xylem are found in vascular bundles.
In woody plants, secondary xylem, commonly called Wood, is laid down by a Meristem called the Vascular cambium: see Wood.

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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.

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Reference

Campbell, Neil A. and Jane B. Reece. 2002. Biology, 6th ed., published by Benjamin Cummings.