The Did- Heliotropic Attitude of Leaves. 



157 



negative movement by the contraction of the more excitable lower half of 

 the organ. This is the reason why the dia-heliotropic adjustment is less 

 perfect under strong sunlight. 



We obtain parallel reaction with Helianthus ; here the petiole acts as an 

 extended pulvinoid. Light applied from above causes an erectile movement ; 

 when applied below it causes a more energetic down movement. As the 

 transverse conductivity of the petiole is feeble, the positive heliotropic 

 response, induced by light acting from above, is rarely reversed into negative- 



3. The Mechanism of Heliotropic Curvature. 



A few words may now be said of the mechanics of curvature by which the 

 stem of Helianthus bends towards light (fig. 1,5). All forms of stimuli, including 

 that of light, induce a diminution of turgor and consequently contraction, 

 and retardation of the rate of growth of the directly excited side. But 

 this is not the only factor in bringing about the positive curvature. I have 

 shown (4) that while the effect of direct stimulus at the proximal side of the 

 stem induces diminution of turgor and contraction, its effect on the distal 

 side, where it acts indirectly, is the very opposite, namely, an increase of 

 turgor and expansion. The positive curvature is thus due to joint effects 

 of direct and indirect stimulus at the two opposite sides. I have been able 

 to demonstrate the induced increase of turgor at the distal side by experi- 

 menting with the stem of Mimosa. The stimulus of light is applied at a 

 point directly opposite to the motile leaf, which by its movement indicates 

 the change of turgor, the induced increase of turgor being indicated by an 

 erection, and diminution of turgor by a fall of the leaf. Application of light 

 at a point on one side of the stem was thus found to induce an increase of 

 turgor at its diametrically opposite point. 



Parallel experiments which I have recently carried out with Helianthus 

 gave identical results. Arc light was continuously applied at a point 

 opposite the indicating leaf ; this induced an increase of turgor, as exhibited 

 by a continuous erection of the leaf. We thus find that while direct stimu- 

 lation induces a diminution of turgor at the proximal side, indirect stimulation 

 causes an increase of turgor at the distal side. The positive heliotropic 

 curvature is thus due to the joint effects of contraction of the proximal and 

 expansion of the distal side. 



4. The Diurnal Movement. 



The daily periodic movements of the leaf Mimosa and of Helianthus 

 exhibit a further similarity which is remarkable. I have shown elsewhere (5) 



