phloem

phloem, plant vascular tissue that conducts foods made in the leaves during photosynthesis to all other parts of the plant. Phloem is composed of various specialized cells called sieve elements, phloem fibres, and phloem parenchyma cells. Together with xylem (tissue that conducts water and minerals from the roots to the rest of the plant), phloem is found in all vascular plants, including the seedless club mosses, ferns, and horsetails, as well as all angiosperms (flowering plants) and gymnosperms (plants with seeds unenclosed in an ovary).

Sieve tubes, which are columns of sieve tube cells having perforated sievelike areas in their lateral or end walls, provide the main channels in which food substances travel throughout a vascular plant. Phloem parenchyma cells, called transfer cells and border parenchyma cells, are located near the finest branches and terminations of sieve tubes in leaf veinlets, where they also function in the transport of foods. Companion cells, or albuminous cells in non-flowering vascular plants, are another specialized type of parenchyma and carry out the cellular functions of adjacent sieve elements; they typically have a larger number of mitochondria and ribosomes than other parenchyma cells. Phloem, or bast, fibres are flexible long sclerenchyma cells that make up the soft fibres (e.g., flax and hemp) of commerce. These provide flexible tensile strength to the phloem tissues. Sclerids, also formed for sclerenchyma, are hard irregularly shaped cells that add compression strength to the tissues.

Primary phloem is formed by the apical meristems (zones of new cell production) of root and shoot tips; it may be either protophloem, the cells of which are matured before elongation (during growth) of the area in which it lies, or metaphloem, the cells of which mature after elongation. Sieve tubes of protophloem are unable to stretch with the elongating tissues and are torn and destroyed as the plant ages. The other cell types in the phloem may be converted to fibres. The later maturing metaphloem is not destroyed and may function during the rest of the plant’s life in plants such as palms but is replaced by secondary phloem in plants that have a cambium.