TRANSLOCATION

in plants, movement of water and dissolved substances within the vascular system. Soil minerals and substances synthesized by root tissues move in very dilute solution in water through the xylem (fluid-conducting tissues) from the roots to the leaves but, exceptionally, also in the opposite direction. The speed of upward transport varies up to 50 m (150 feet) or more per hour. Foods and other substances synthesized by or applied to the leaves move in all directions in the phloem (vascular tissues of the inner bark). Speeds of phloem transport normally range from about 20 to 100 cm (8 to 40 inches) per hour. Certain growth-regulating substances move only in a polar manneri.e., away from the shoot tip. Mechanisms suggested to account for transport of substances in the phloem include simple diffusion, spreading along interfaces, cytoplasmic streaming, active transport, and the one most generally accepted, the pressure flow hypothesis. (See mass flow.)