| 单词 | cambium | ||||||
| 释义 | cambiumcam·bi·umC0047400 (kăm′bē-əm)cambium(ˈkæmbɪəm)cam•bi•um(ˈkæm bi əm)n., pl. -bi•ums, -bi•a (-bi ə) cam·bi·um(kăm′bē-əm)
cambiumcambium(kăm`bēəm), thin layer of generative tissue lying between the bark and the wood of a stem, most active in woody plants. The cambium produces new layers of phloem on the outside and of xylem (woodwood,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 ..... Click the link for more information. ) on the inside, thus increasing the diameter of the 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. ..... Click the link for more information. . In herbaceous plants the cambium is almost inactive; in monocotyledonous plants it is usually absent. In regions where there are alternating seasons, each year's growth laid down by the cambium is discernible because of the contrast between the large wood elements produced in the spring and the smaller ones produced in the summer. These are the annual rings, by which the age of a tree can be established. A tree dies when it is "ringed," or girdled, i.e., cut through the cambium layer. The cork cambium, which lies outside the phloem layer, produces the cork cells of barkbark, outer covering of the stem of woody plants, composed of waterproof cork cells protecting a layer of food-conducting tissue—the phloem or inner bark (also called bast). ..... Click the link for more information. . Cambiumthe formative tissue, or meristem, primarily in the roots and stems of dicotyledonous and gymnospermous plants. The cambium is in the shape of a monostichous concentric cylinder (in cross section, it appears annular). As a result of cambial activity, the axial organs grow in thickness. Secondary phloem is produced toward the outside of the axis, and secondary xylem toward the inside. Cambium originates from the cells of the procambium, which lie between the primary phloem and primary xylem. The pericycle plays an important role in the formation of the cambium in roots. It appears that there is no cambium in leaves; however, if there is any, its activity terminates at an early stage in leaf development. Cambial cells are prosenchymatous in form—elongated, pointed (with beveled ends), and flat. They extend the length of the plant. Cambial cell walls are soft and consist of cellulose; the primary pit fields contain plasmodesmata. Cambial cells divide, becoming phloem cells (toward the outside of the cambium) or xylem cells (toward the interior of the plant). Usually, much more xylem is formed than phloem. As a result of the division of some cambial cells, tiny cells are formed; these make up phloem-xylem rays. The phloem forms on one side of the ray; the xylem, on the other. During the autumn and winter, cambial activity ceases (intemperate zones). The periodic activity of the cambium resultsin the formation of annual rings. Depending on the character ofcell division, cambium is divided into storied and nonstoriedtypes. In some dicotyledons (such as beets), additional cambiallayers form consecutive rings toward the periphery (polycam-bium formation). O. N. CHISTIAKOVA cambium[′kam·be·əm]cambium cambiumcam·bi·um(kam'bē-ŭm),cambium
Words related to cambium
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