Phloem

Phloem, in higher plants, vascular tissue that conducts sugars and other synthesized food materials from the regions of manufacture in the plant to those of consumption and storage. Phloem is found in the vascular bundles, the longitudinal strands of conductive tissue, in association with the water-conducting tissue, or xylem. The vascular bundles constitute major structural units in herbaceous stems and are the veins in leaves. In the vascular cylinder traversing the center of the buttercup root, for example, the xylem forms a star-shaped central core, and bands of phloem are present in the grooves of this core. Typically, the xylem is on the side of the vascular bundle closest to the pith, although other arrangements are not uncommon. In the older portions of a plant the soft cells of the phloem are crushed as new phloem is formed in the growing process and pushed outward. This new phloem is formed by the action of the cambium, or growing zone, a layer of cells that separates the xylem and phloem and produces phloem cells toward the outside of the plant.

Phloem consists of two types of conducting cells, the characteristic type known as sieve-tube elements and another type called companion cells. Sieve-tube elements are elongate cells that have end walls perforated by numerous minute pores through which dissolved materials can pass. Such sieve-tube elements are connected in vertical series known as sieve tubes. Sieve-tube elements are alive at maturity, although their nuclei disintegrate before the element begins its conductive function. Companion cells, which are smaller, have nuclei at maturity and are living; they are found adjacent to the sieve-tube elements and are believed to control the process of conduction in the sieve tubes.

Phloem may contain bast fibers. These fibers are very strong and in certain plants are the source of such commercial fibers as flax and jute, used to make linen fabric and burlap and sacking.