The Dia-Heliotropic Attitude of Leaves. 



173 



leaflets became reduced by the torsion. Hence it is obvious that it is not 

 the advantage of the plant, but the inevitable physiological reaction, that 

 determines the movement. Stimulation of the leaflets of the left sub-petiole 

 induced a left-handed torsion. When the leaflets of both the sub-petioles 

 were illuminated by vertical light, the two resulting torsions balanced each 

 other. While in this state of dynamic balance, if the intensity of light on 

 one of the sides, say the left, be diminished by interposition of a piece of 



Fig. 11. — The upper figure is a diagram of stimulation of nerve-ending of Helianthus. 

 The record below shows that stimulation beyond the cut gives (a), no response ; 

 while stimulation at b, induces right-handed torsion. 



paper, the balance is at once upset, and we find a right-handed torsion. It 

 is thus seen that equilibrium is only possible when the entire leaf-surface 

 (consisting of the two rows of the leaflets) is equally illuminated ; and that 

 would be the case when the surface is perpendicular to the incident light. 

 The dia-heliotropic attitudes of leaves is thus brought about by distinct 

 nervous impulses, initiated at the perceptive region actuating the different 

 effectors. 



In the case of leaves with four sub-petioles, illumination of the extreme 



