The New Student's Reference Work/Heliotropism

Heliot′ropism (in plants) is the sensitiveness of a plant to a change in the direction of NSRW Heliotropic Mustard.jpg
A young plant of mustard grown in water, showing heliotropic curvature caused by one-sided illumination. n, n, level of water; b, older part of stem; a, younger part where curvature occurs; c, c, primary leaves; d, sharp curve of root; e, the growing point. The arrows show the direction of the light.
light, to which it responds by placing its organs again in a definite position with respect to it. In many plants this is accomplished by a change in the rate of growth of some part, producing curvature and carrying attached parts into the new position. Thus, if a geranium plant be taken from a greenhouse and set before a window, the leaves are carried into a position facing the window by curvature of their stalks. This is the usual response of leaves of this kind, while radial leaves, as those of the onion, direct their tips toward the light. Some stems bend so as to direct the apex toward the light, and some roots turn away from it. Other plants accomplish like changes by means of the motor organs (which see), in which changes of turgor (which see) produce the curvatures that move the attached parts.