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The Dia-Heliotropic Attitude of Leaves as determined by 

 Transmitted Nervous Excitation. 



By Sir Jagadis Chunder Bose, F.R.S., Director, Bose Institute, Calcutta. 

 Assisted by Satyendea Chandra Guha, M.Sc, Eesearch Student, 

 Bose Institute, Calcutta. 



(Received May 25, 1921.) 



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 

 considerations 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. 



I have for many years been engaged in pursuing investigation on the 

 subject, and have recently succeeded in discovering the fundamental reaction 

 to which the directive movement is due. I shall be able to show that the 

 VOL. xciii. — B. N 



