|HELIOTROPISM: THE TURNING MOTIONS OF PLANTS.|
AS its derivation would indicate, heliotropism means "turning toward the sun," and is the technical name applied to all such phenomena in the vegetable kingdom. It was well known to the ancients that plants exhibited a remarkable sensitiveness to light, for Aristotle mentions it, and indeed, in its more apparent forms, it is conspicuous even to the naïve observer of to-day. The sunflower, or tournesol, as the French name it, follows the daily course of the sun with its disk-like inflorescence; plants, potted and placed in a window, bend toward the light, unless, perchance, the plant is an ivy, in which case it bends away from the source of illumination; trees and shrubs in the edge of a thicket or forest may be seen to slant toward the open, and in general it may be said that there is a plant which does not respond more or less distinctly to the directive action of light. Exceptions, as shown by Darwin and by Edouard Morren in his treatise on insectivorous plants, are for the most part carnivorous species like Dionea, Drosera, and Nepenthes—the Venus's-flytrap, sundew, and pitcher-plant, respectively—and twining plants. The reason why these plants should not fall under the rule will be apparent when the uses of heliotropism are discussed. Parasitic and the so-called saprophytic plants of the lower orders—those which live upon once-living matter—are commonly insensible to heliotropic stimulus, and, in short, all plants devoid of the great light-product—chlorophyl—manifest in this direction either weak irritability or none at all.
Heliotropism, it must be remembered, is not confined to plants as individuals, but is manifested by the different organs in varying degrees. Tendrils, for example, are either distinctly heliotropic, or far more commonly apheliotropic, as Darwin calls it—that is, negatively heliotropic; leaves are transversely or diaheliotropic—in other words, they tend to place themselves perpendicularly to the incident rays; stems, flower-peduncles, even roots, each in its own way, reply to the stimulus of lateral light. In passing, it should be mentioned that a plant has but one way of responding to conditions without, and this is by curvature. Even the sleep of leaves, the spontaneous movements of the sensitive-plant, or of that singular pulse, the Hedysarum gyrans, in which the two lateral leaflets keep up an incessant jerking motion; the reaction to a cut, bruise, or wound of any kind—as may be seen in an injured tendril; the effect of ether or chloroform, and indeed of natural forces such as electricity, gravity, or heat-vibrations, is in every