xylem

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Transport tissue found in vascular plants, whose main function is to conduct water and dissolved mineral nutrients from the roots to other parts of the plant. The water is ultimately lost by transpiration from the leaves (see leaf). Xylem is composed of a number of different types of cell, and may include long, thin, usually dead cells known as tracheids; fibres (schlerenchyma); thin-walled parenchyma cells; and conducting vessels.

Transpiration is the loss of water, in the form of water vapour, from leaves of a plant to the air outside the leaf. The loss of water from the leaves drives the movement of water and minerals in the xylem. This is important because many plant cells need the minerals as nutrients. Xylem tissue is usually found close to the other transport tissue in plants, phloem, which transports sugars and amino acids (see protein). In non-woody plants phloem and xylem are found in bundles (called veins if they are in a leaf).

In most angiosperms (flowering plants) water is moved through these vessels. Most gymnosperms and pteridophytes lack vessels and depend on tracheids for water conduction. Non-woody plants contain only primary xylem, derived from the procambium, whereas in trees and shrubs this is replaced for the most part by secondary xylem, formed by secondary growth from the actively dividing vascular cambium. The cell walls of the secondary xylem are thickened by a deposit of lignin, providing mechanical support to the plant; see wood.

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