CHAPTER XLIV 



EFFECT OF IWISIBLE RADIATION AND EMANATIONS 



Effect of temperature and its variations — Demonstration of fundamental effect of 

 thermal radiation on growth — Response to successive uniform stimuli of 

 thermal radiation — Effect of continuous unilateral stimulation — Effect of 

 electrical waves on growth — Response of Mimosa to electric radiation — 

 Action of high frequency Tesla current. 



We have now studied the curvature effects induced in plants 

 by those ethereal vibrations that lie within narrow limits, and 

 are known as visible light. There are, however, other vibra- 

 tions outside this range, the ultra-violet and the infra-red. 

 The excessively quick vibrations beyond violet are known to 

 produce very marked heliotropic effects. But below the red, 

 again, we have comparatively long waves which give rise to 

 thermal, and others, still longer, to electrical radiation. 



In studying the curvature-effects on plants of invisible 

 radiations of low frequency, it is necessary to distinguish 

 carefully between the action of radiation as such and the 

 subsidiary effect of temperature. 



Effect of temperature and its variation. — In this in- 

 vestigation it becomes especially important to distinguish 

 the temperature from the radiation-effect, and I shall pre- 

 sently describe a very decisive experiment by which the 

 effects of the two may be clearly distinguished. The effect 

 of temperature up to the optimum is, as we have seen, to 

 increase the internal energy of the plant, in consequence of 

 which there is an acceleration of the rate of growth ; but 

 variation of temperature acts as an external stimulus, and 

 would thus be effective in inducing a transient retardation of 

 growth. A part of the stimulus, however, is held latent, as 



