The dia-heliotropic attitude of leaves as determined by transmitted nervous excitation

Abstract

The leaves of plants adjust themselves in various ways in relation to the incident light. The heliotropic fixed position is assumed by means of curvatures and torsions of the motor organ which may be the pulvinus, or the petiole acting as a diffuse pulvinoid. In some cases the motor organ alone is both perceptive and responsive; in others, the leaf blade does exert a directive action, the perceptive lamina and the motor organ being separated by an intervening distance. This directive action of the lamina has been found by Vochting in Malva verticillata, and by Haberlandt in Begonia discolor, and in several other plants. In connection with this it should be borne in mind that this characteristic does not preclude the possibility of the motor organ being directly affected by the stimulus. In a nerve-and-muscle preparation, the muscle is excited, not merely by indirect but also by direct stimulus. As regards the heliotropic adjustment of leaves, the stimulus of light acts, in the cases just mentioned, both directly and indirectly, the indirect stimulation being due to some transmitted effect from the perceptive lamina. We may regard the coarse adjustment to be brought about by direct, and the finer adjustment by indirect stimulation. Certain leaves thus assume a heliotropic fixed position so that the blades are placed at right angles to the direction of light, the directive action being due to certain transmitted reaction, yet unknown. No explanation has, however, been forthcoming as regards the physiological reaction to which this movement must be due. Suggestions have been made that the dia-heliotropic position of leaves is of obvious advantage, since this position assures for the plant the maximum illumination. But such teleological consideration offer no explanation of the definite physiological reaction. It is, moreover, not true, as I shall show in the course of this paper, that there is anything inherent in the plant-irritability by which the surface of the leaf is constrained to place itself perpendicular to the incident light.