NEGATIVE HELIOTROPISM 613 



2. (a) Stronger or long-continued stimulus, on account 

 of its internal diffusion by transverse transmission, causes 

 neutralisation of effect. In other words, the organ, after 

 first moving towards the light, returns to its original position 

 at right angles to it, or assumes the dia-heliotropic attitude. 

 This neutralisation, depending as it does on transverse con- 

 duction of stimulus, can only take place when the stimulus is 

 very strong, or when the organ is highly conducting. The 

 first of these considerations explains the fact that while 

 moderate stimulation causes positive curvature, stronger 

 stimulation has no resultant effect. The effect of the second 

 factor (the higher conductivity of the tissue), which is brought 

 about by a warmer season, is seen when the transversely 

 transmitted effect causes neutralisation. The apparent 

 absence of heliotropic effect in various tendrils is due not 

 to any want of sensibility, but to this neutralisation by 

 transverse conduction of the effect of stimulus. 



(b) The transverse transmission of stimulus to the distal 

 side is sometimes attended by oscillatory responsive move- 

 ments, owing to periodic or alternate fatigue. 



3. With still stronger unilateral stimulation, the organ 

 becomes for the time being anisotropic, owing to the fatigue 

 of the proximal side. The internally diffused stimulus then 

 induces negative curvature, through the relatively greater 

 excitability and contraction of the distal half of the organ. 

 The negative heliotropic movement of the tendril of Vitis 

 is explained by these considerations. This tendril, under 

 moderate unilateral stimulus, exhibits positive heliotropic 

 movement ; but stronger stimulation, in consequence of 

 transverse transmission and unilateral fatigue, gives rise to 

 negative heliotropic movement. 



The statement that the different responsive curvatures 

 brought about by light are not due to different sensibilities 

 possessed by different organs, is proved by the fact that the 

 same organ exhibits continuous changes, from positive to 

 negative through neutral, under different intensities of 

 stimulation. 



