Xylem

Xylem, woody tissue, found in higher plants, that conducts water and inorganic salts throughout the plant and provides it with mechanical support. In leaves, flowers, and young stems, xylem is present in conjunction with phloem in the form of conducting strands called vascular bundles. In roots there is a central core of xylem (see Root). Xylem that derives from the shoot and root-growing points is called primary xylem. In addition, new xylem, called secondary xylem, may be added by the activity of the cambium, which actively divides cells situated between the xylem and phloem. This division gives rise to new xylem cells toward the center in roots and toward the outside in most stems. Some plants have little or no secondary xylem, in contrast to woody plants in which indefinite amounts of secondary xylem is the botanical term equivalent to wood. See Stem.

Xylem may contain three types of elongated cells: tracheids, vessel elements, and fibers. At maturity, when functioning in conduction, all of these cells are dead. Tracheids are elongated cells with thick walls characterized by small, sharply defined thin areas known as pits. Vessel elements are specialized tracheids in which the end walls have one or more pores; a vertical series of vessel elements forming a continuous tube is known as a vessel. Fibers, which are specialized tracheids with much-thickened walls, function only slightly in conduction, but serve to increase the strength of xylem.

The xylem of plants that are earlier than most in evolutionary development, such as ferns and conifers, consists of tracheids. In most flowering plants, the xylem also contains well-developed vessels and fibers. Because sequences in the specialization of all these tissue elements can be observed quite clearly, the study of xylem provides important clues to the evolutionary pathways of higher plants.

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