..... Click the link for more information. , or xylem, and the phloem (see bark bark, 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. ; stem stem, 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
..... Click the link for more information. ). The growth in diameter of tree trunks is wholly dependent on the division of cambium cells. Other meristematic tissues include cork cork, protective, waterproof outer covering of the stems and roots of woody plants. Cork is a specialized secondary tissue produced by the cork cambium of the plant (see meristem, bark).
..... Click the link for more information. cambium, which divides to produce waterproofing and protective cork tissue at the surface of the stem and root; and intercalary meristems, modified apical meristems found in different positions than either apical or lateral meristems, e.g., in the stem nodes of grasses. See also differentiation differentiation, in biology, series of changes that occur in cells and tissues during development, resulting in their specialization. This, in turn, permits a greater variety of organisms.
..... Click the link for more information. , in biology.
meristem
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meristem [′mer·ə‚stem]
Meristem
formative tissue, a plant tissue characterized by prolonged cell-division activity. Some cells of the meristem—the initiating cells—remain in the embryonic phase of development and, by dividing, provide uninterrupted growth of the plant body. Other cells of the meristem gradually become differentiated, forming various derivative tissues (integumentary, conducting, mechanical, basal).
Meristems originate from the protomeristem of the embryo, which develops into the apical and lateral meristems. The apical meristems—the growing points of shoots and roots—are formed very early in the embryo. The formation of the cotyledons and later of the leaf rudiments on the growing point of the shoot results in differentiation of the lateral meristems into the procambium and cambium. Meristematic tissue is partially preserved in some parts of the plant body during plant growth, for example, in the roots (Pericycle—rhizogenic meristem), the nodes of the shoot, and the medullary rays of the stem. Intercalary meristem is temporarily preserved in the buds, the inter-nodes of the shoot (Gramineae), and the bases of the petioles.
Because almost all living, mature tissues (except the sieve tubes) possess the capacity to divide, new, or secondary, meristems, such as Phellogen (forming cork tissue) and callus, may arise in the plant. Meristem cells differ from the cells of permanent tissues in that they are smaller, densely arranged, and almost cubical in shape (only cells of the procambium and cambium are elongated). They usually have a thin primary membrane and a dense protoplast, in which the nucleus and nucleolus occupy a central position. The endoplasmic network is poorly developed; there are many ribosomes. The mitochondria and dictyosomes are only slightly differentiated. The vacuoles are very small, and the plastids are in the form of protoplastids. Meristem cells are characterized by high metabolic activity.
REFERENCES
Esau, K. Anatomiia rastenii. Moscow, 1969. (Translated from English.) Lehrbuch der Botanik fur Hochschulen, 30th ed. Jena, 1971.How to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit webmaster's page for free fun content.
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