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xylem(zī`ləm): see stemstem,
supporting structure of a plant, serving also to conduct and to store food materials. The stems of herbaceous and of woody plants differ: those of herbaceous plants are usually green and pliant and are covered by a thin epidermis instead of by the bark of woody plants.
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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
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The principal water-conducting tissue and the chief supporting system of higher plants. This tissue and the associated phloem constitute the vascular system of vascular plants. Xylem is composed of various kinds of cells, living or nonliving. The structure of these cells differs in their functions, but characteristically all have a rigid and enduring cell wall that is well preserved in fossils.
In terms of their functions, the kinds of cells in xylem are those related principally to conduction and support, tracheids; to conduction, vessel members; to support, fibers; and to food storage, parenchyma. Vessel members and tracheids are often called tracheary elements. The cells in each of the four categories vary widely in structure. See Parenchyma
Xylem tissues arise in later stages of embryo development of a given plant and are added to by differentiation of cells derived from the apical meristems of roots and stems. Growth and differentiation of tissues derived from the apical meristem provide the primary body of the plant, and the xylem tissues formed in it are called primary. Secondary xylem, when present, is produced by the vascular cambium. See Lateral meristem
In the trade, softwood is a name for xylem of gymnosperms (conifers) and hardwood for xylem of angiosperms. The terms do not refer to actual hardness of the wood. Woods of gymnosperms are generally composed only of tracheids, wood parenchyma, and small rays, but differ in detail. Resin ducts are present in many softwoods. Woods of angiosperms show extreme variation in both vertical and horizontal systems, but with few exceptions have vessels.
a tissue of terrestrial plants that serves to conduct water and mineral salts upward from the roots through the plant. Xylem is distributed as a solid ring or in conducting, or fibrovas-cular, bundles. It consists of conducting cells proper (tracheae [vessels] and tracheids), mechanical cells (libriform), and xylem and ray parenchyma. The walls of all xylem cells lignify. True xylem is characteristic of all pteropsids, gymnosperms, and flowering plants. The xylem of perennial stems and roots is predominantly called wood. Primary xylem arises from the procambium. The first element of the primary xylem to appear is the proto-xylem, which consists of tracheids and vessels with ringed and spiral thickenings of the walls. The metaxylem, which has scalariform and pitted thickenings, forms somewhat later. Secondary xylem is formed by the cambium.
O. N. CHISTIAKOVA