644 PLANT RESPONSE 



by epinastic. It has already been shown that autonomous 

 response in general can take place only when the internal 

 energy or the sum total of the latent stimulating factors is 

 above par. This is equally true of the autonomous response 

 of growth itself. In the case of autonomous epi- or hypo- 

 nastic pulsations, therefore, a certain amount of internal 

 energy is essential for their initiation. This is in some cases 

 supplied by other forms of stimulus, but there may be others 

 in which light is the critical factor. In order, then, to study 

 the directive action of light on dorsi-ventral organs under 

 normal tonic conditions, we must be able to determine the 

 characteristic influence which will be exerted by the stimulus 

 of incident light in modification of the already existing 

 movement of the organ. This inquiry is therefore exactly 

 parallel to our previous study of the action of light in 

 modifying the existing growth-movements of radial organs. 

 In that case the variations induced in the ordinary rate of 

 movement afforded us a measure of the effect of the stimulus 

 of light. In the case of dorsi-ventral organs such as leaves, 

 similarly, the effect of light can be correctly inferred only by 

 observing the variations which it induces in their existing 

 movements. The manner in which this is done will be 

 described presently. 



2. Effect of gravity. — In long-continued experiments on 

 the curvatures induced by light, the observed movement is 

 also modified in part by the influence of geotropism. This 

 geotropic action in leaves is by some investigators believed 

 to be of two types, negative and positive. Others, again, 

 regard it as dia-geotropic. I shall, however, adduce con- 

 siderations which will show that the upper and lower halves 

 of dorsi-ventral organs exhibit differential excitability to geo- 

 tropic as to other forms of stimulus. In order to neutralise 

 the geotropic action, and thus study the heliotropic effect 

 alone, Francis Darwin mounted the plant on a rotating 

 klinostat. It is true that in a strictly radial organ the 

 geotropic effect is successfully neutralised by rotation on the 

 klinostat, since in this case the geotropic sensitiveness of the 



