phloem(redirected from Mass flow hypothesis)
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phloem(flō`ĕm): see 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).
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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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The principal food-conducting tissue in vascular plants. Its conducting cells are known as sieve elements, but phloem may also include companion cells, parenchyma cells, fibers, sclereids, rays, and certain other cells. As a vascular tissue, phloem is spatially associated with xylem, and the two together form the vascular system. See Xylem
Sieve elements differ from phloem parenchyma cells in the structure of their walls and to some extent in the character of their protoplasts. Sieve areas, distinctive structures in sieve element walls, are specialized primary pit fields in which there may be numerous modified plasmodesmata. Plasmodesmata are strands of cytoplasm connecting the protoplasts of two contiguous cells. These strands are often surrounded by callose, a carbohydrate material, that appears to form rapidly in plants when they are placed under stress.
Typical sieve cells are long elements in which all the sieve areas are of equal specialization, though sieve areas may be more numerous in some walls than in others. In contrast, a sieve-tube member has some sieve areas more specialized than others; that is, the pores, or modified plasmodesmata, are larger in some sieve areas. Parts of the walls containing such sieve areas are called sieve plates.
Companion cells are specialized parenchyma cells that occur in close ontogenetic and physiologic association with sieve tube members. Some sieve-tube members lack companion cells. The precise functional relationship between these two kinds of cells is unknown.
Parenchyma cells in the phloem occur singly or in strands of two or more cells. They store starch, frequently contain tannins or crystals, commonly enlarge as the sieve elements become obliterated, or may be transformed into sclereids or cork cambium cells.
Phloem fibers vary greatly in length (from less than 0.04 in. or 1 mm in some plants to 20 in. or 50 cm in the ramie plant). The secondary walls are commonly thick and typically have simple pits, but may or may not be lignified.
(bast), a complex tissue in higher plants that conducts organic matter to different organs. It also performs storage, frequently mechanical, and, in part, secretory functions. The presence of a variety of elements is characteristic of phloem in view of its many functions: sieve elements, storage and crystal-containing phloem parenchyma cells, phloem (bast) fibers and sclereids, and radial parenchyma of medullary rays. In some plants, the phloem has latex vessels and resin ducts.
Sieve elements in angiosperms consist of sieve tubes, which are a longitudinal series of cells (segments) communicating by “sieve pores,” thin portions of walls with through openings. In the protoplasts of the sieve elements, the nuclei die early and the cytoplasm becomes completely permeable to organic matter and mineral salts dissolved in water; the membrane is of cellulose and often thickened. The rate of translocation of plastic substances in the phloem is 70-150 cm/hour. The adsorbing capacity of protoplasm plays an important part in the translocation of matter in the phloem. The sieve tubes usually function only during a single growing season, at the end of which the sieve pores become plugged with callose. Communication between the cells ceases and they become compressed (obliterated) by the secondary phloem tissues growing within. The sieve tubes function for as long as two to eight years only in some perennial plants (for example, in grape and linden). The sieve tubes of angiosperms are associated with companion cells. These cells arise from the same mother cell as the associated sievetube member and have large nuclei and dense protoplasm.
Storage tissue in the phloem consists of cells of axial phloem and ray parenchyma. The phloem parenchyma is formed by elongated cells (fusiform parenchyma) or short cells joined into a vertical strand (strand parenchyma). The medullary ray cells are formed by special initiating cambial cells, and they may be elongated in a radial or vertical direction. Living phloem cells store starch, oils, and other organic substances as well as resins and tannins. The phloem parenchyma may serve as a depot of calcium oxalate crystals. Latex vessels in phloem—for example, in the family Compositae and the spindle tree—contain rubber and gutta while the resin ducts (in many conifers) have sap. The thickwalled bast fibers are the main elements that perform a mechanical function. Sclereids, frequently formed from parenchyma cells in the zone of nonconducting phloem, are found in some plants. A distinction is made between primary phloem, which originates from procambium, and secondary phloem, which is produced by the activity of cambium. In woody plants the annual rings of phloem are less distinct than in xylem because of the rapid agerelated changes that take place in the phloem’s structural elements.
REFERENCESRazdorskii, V. F. Anatomiia rastenii. Moscow, 1949.
Iatsenko-Khmelevskii, A. A. Kratkii kurs anatomii rastenii. Moscow, 1961.
Esau, K. Anatomiia rastenii. Moscow, 1969.
R. P. BARYKINA
a tissue of higher plants that serves to conduct organic substances to various organs. Together with the xylem, the phloem constitutes the vascular bundles. In archegoniates the conducting elements of the phloem are represented by sieve cells; in angiosperms they are represented by sieve-like tubules with satellite cells. The phloem also contains parenchymatous cells and, in many plants, mechanical elements.