The principal food-conducting tissue in vascular plants. Its conducting cells are known as sieve elements, but phloem may also include companion cells, parenchyma cells, fibers, sclereids, rays, and certain other cells. As a vascular tissue, phloem is spatially associated with xylem, and the two together form the vascular system. Less is known of phloem than of xylem, partly because of its lesser direct economic importance and partly because the sieve elements function for a short time (usually one season) and then undergo marked structural and functional changes, such as crushing or sloughing off as a result of periderm formation in the case of woody plants.