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