phloem (bast) A tissue that conducts food materials in vascular plants from regions where they are produced (notably the leaves) to regions, such as growing points, where they are needed. It consists of hollow tubes (sieve tubes) that run parallel to the long axis of the plant organ and are formed from elongated cells (sieve elements) joined end to end and closely associated with companion cells. The end walls of these cells are broken down to a greater or lesser extent to allow passage of materials. In young plants and in newly formed tissues of mature plants primary phloem is formed by the activity of the apical meristem (see protophloem; metaphloem). In most plants secondary phloem is later differentiated by the vascular cambium and this replaces the earlier formed phloem in older regions. See also mass flow. Compare xylem.
phloem Vascular tissue for distributing dissolved food materials in plants. Phloem tissue contains several types of cells. The most important are long, hollow cells called sieve-tube cells. Columns of sieve tubes are joined end to end, allowing passage of materials from cell to cell. The sieve tubes are closely associated with ‘companion cells’, which have dense cytoplasm and many mitochondria, and are thought to produce the energy needed to transport the food substances (see active transport). Phloem may also contain fibres that help to support the tissue. See also mitochondrion; xylem
phloem A tissue comprising various types of cell, which transports dissolved organic and inorganic materials over long distances within vascular plants, by mechanisms that are still not understood fully. It can be distinguished from xylem by the general absence of thickened cells and by the presence of cells containing areas resembling a sieve.
phlo·em / ˈflōˌem/ • n. Bot. the vascular tissue in plants that conducts sugars and other metabolic products downward from the leaves.
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