xylem

xylem A tissue that transports water and dissolved mineral nutrients in vascular plants. In flowering plants it consists of hollow vessels that are formed from cells (vessel elements) joined end to end. The end walls of the vessel elements are perforated to allow the passage of water. In less advanced vascular plants, such as conifers and ferns, the constituent cells of the xylem are called tracheids. In young plants and at the shoot and root tips of older plants primary xylem is formed by the apical meristems (see protoxylem; metaxylem). In plants showing secondary growth this xylem is replaced in most of the plant by secondary xylem, formed by the vascular cambium. The walls of the xylem cells are thickened with lignin, the extent of this thickening being greatest in secondary xylem. Xylem contributes greatly to the mechanical strength of the plant: wood is mostly made up of secondary xylem. See also fibre. Compare phloem.

xylem Transport tissue of a plant, which conducts water and minerals from the roots to the rest of the plant and provides support. The most important cells are long, thin tapering cells called xylem vessels. These cells are dead and have no cross-walls; they are arranged in columns to form long tubes, up which water is drawn. As water evaporates from the leaves (transpiration), water is drawn across the leaf by osmosis to replace it, drawing water out of the xylem. This suction creates a tension in the xylem vessels, and the side walls are reinforced with rings or spirals of lignin, a rigid substance, to prevent them collapsing. Tiny holes in the walls of the xylem vessels (pits) allow water to cross from one tube to another. In trees, the xylem becomes blocked with age, and new xylem forms towards the outside of the trunk to replace it. The core of dead, non-functioning xylem remains an essential part of the support system. See also phloem; vascular bundle