the movement of plant organs in response to the unilateral action of light, gravity, and other environmental factors as a result of the more rapid growth of cells on one side of a shoot, root, or leaf.
The basis of tropisms and of nastic movements as well is the phenomenon of irritability. The process begins with the plant’s perception of an external stimulus that induces a physiological difference between the two sides of a plant organ. Next there occurs the transmission of a signal to which there is a reaction—a bending caused by the uneven growth rate of the two sides of the organ. The hormonal theory of tropisms, best demonstrated in phototropism and geotropism, is most widely accepted. An oat sprout, illuminated on one side, bends toward the source of light, because its illuminated side grows more slowly than the shaded side. The auxin content turns out to be greater on the shaded, more rapidly growing half; in other words, the phototropic bend is the result of uneven distribution of auxin. In a horizontal stem, auxin accumulates in the lower part, leading to intensified growth of that part of the stem and to an upward bend (positive geotropism). In a horizontal root, auxin also concentrates in the lower part, but the excess of auxin inhibits the growth of the root cells, which are sensitive to auxin. As a result, the root bends down ward (negative geotropism).
V. I. KEFELI