598 PLANT RESPONSE 



the tip, the stimulus of light is not necessarily restricted 

 in the area of its action. Again, whereas the stimulation 

 caused by statolithic particles is moderate, that caused by 

 light may be of any degree of intensity. The fact that 

 there is no true extended analogy between the action of 

 light and that of gravitation, is seen from the fact that, while 

 gravitation in the case of the root induces a movement 

 opposite to that induced in the stem, in the case of light this 

 is not always so ; for though a few roots turn away from 

 light, in others there is either no resultant movement, or 

 movement towards the light. Again, while the shoot makes 

 a definite curvature with reference to the direction of gravity, 

 in the case of light we shall observe that though under 

 moderate stimulation it turns towards it, yet it will sometimes 

 under stronger stimulation be found to move away. The idea 

 that positive and negative heliotropic curvatures are due to 

 two distinct sensibilities could not be better disproved than 

 by the fact, which will be demonstrated shortly, that the same 

 organ can be made under different conditions of illumination 

 to exhibit the two opposite effects. 



Discarding, then, the theory of any specific sensibility, we 

 shall now proceed to show how the movement away from the 

 stimulating light, the so-called negative heliotropic curvature, 

 is brought about. 



Theoretical considerations. — From the movements al- 

 ready demonstrated (p. 535) as taking place in plant-organs 

 in response to stimulus unilaterally applied, we can see the 

 possibility of such movement becoming negative, or away 

 from stimulus, under three different conditions : 



(i) Under longitudinal transmission of the indirect effect 

 of stimulus, when, for example, moderate stimulus is applied 

 to the tip of either shoot or root. 



(2) Under transverse conduction of the direct excitatory 

 effect of stimulus to the distal side of a radial organ, the 

 proximal side being fatigued by excessive stimulation ; and 



(3) Under the transverse transmission of excitation to the 

 distal side of an anisotropic organ, the distal side being the 



