Primary vascular system

The arrangement of conducting elements which serves for two-way transportation of substances between different parts of a plant. The conducting elements are of two principal kinds: xylem, which is mainly responsible for the conduction of water together with dissolved inorganic substances upward from the root to other plant organs; and phloem, which is mainly responsible for the conduction of food materials (assimilates), a flow which may take place in either direction. In the shoot region of the plant, xylem and phloem are usually associated into vascular bundles. In the root, however, they usually alternate with one another on different radii. See also Phloem; Xylem.