In shoots, the polar transport is always basipetal, that is, from the shoot tip and leaves downward in the stem However, once this downward-directed stream moves beyond the base of the root (the root-shoot junction) and toward the root tip, its direction is described as acropetal (toward the apex). The velocity of polar auxin transport—2 to 20 centimeters per hour—is faster than the rate of passive diffusion. Although transport of auxin is characteristic of shoot apical meristems and young leaves, most of the auxin synthesized in mature leaves seems to be transported throughout the plant nonpolarly via the phloem, at rates considerably higher than those of polar transport.