phloem
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.
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phloem