phloem

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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

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