CHAPTER XII 



THE MECHANISM OF PHOTOTROPIC CURVATURE 



Attention was mainly directed in the previous chapter to 

 the effect of direct stimulation on the proximal side of the 

 organ. It was shown that the positive phototropic curvature 

 is attributable to the contraction of the stimulated side. 



The question now arises whether the stimulus of light, 

 acting on the proximal side of the organ, also induces 

 a reaction at the distal side ; if so, whether this helps or 

 opposes the positive curvature. It has been shown in a 

 previous chapter that unilateral mechanical stimulation 

 induces not only contraction and retardation of growth 

 of the proximal side, but also expansion and acceleration 

 of growth of the distal side, the resulting curvature being 

 due to these conjoint effects. 



Does photic stimulation induce effects parallel to those 

 produced by mechanical stimulation ? And what are the 

 effects, if any, of the impulse transmitted from the stimulated 

 to the unstimulated side ? 



The Effects of Direct and Indirect Stimulation 



In order to analyse the effects of stimulation, it has been 

 necessary to devise a number of independent methods of 

 investigation, the concordant results of which lead to a 

 convincing conclusion. The questions to be answered are : 

 Is the effect of stimulation to generate a single impulse or 

 two impulses ? Is the impulse modified in transmission by 



