botanically, the xylem tissue that forms the bulk of the stem of a woody plant. Xylem conducts sap upward from the roots to the leaves, stores food in the form of complex carbohydrates, and provides support; it is made up of various types of cells specialized for each of these purposes. Among them are tracheids, elongated conduction and support cells; parenchyma (food storage) cells, some of which form rays for transverse conduction; xylem vessels, formed of hollow cells joined end to end; and fiber cells that reinforce these tubes. In the conifers the xylem is made up mainly of tracheids, thus presenting a uniform, nonporous appearance; their wood is called softwood. Deciduous trees have more complex xylem, permeated by vessels, and are called hardwoods, although the description is sometimes inaccurate.
The xylem is formed in the growing season by the cambium; in temperate regions the cells formed in the spring are larger in diameter than those formed in the summer, and this results in the annual rings observable in cross section. The new cells lose their protoplasm as they form the various tissues; the older, nonfunctional cells become plugged up, darken in color, and often accumulate bitter or poisonous substances (tannins, dyes, resins, and gums). This inner wood (the heartwood, as opposed to the functional sapwood) is valued for outdoor construction because of its resistance to moisture and to decay-producing organisms.
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