464 PLANT RESPONSE 



It will be seen from the readings given by specimens i and 

 2, that up to the optimum the total response is approxi- 

 mately constant, while at '^'j° C. there is no expression of 

 energy held latent. The condition of specimen 3 having 

 been at starting somewhat sub-tonic, the total effect is 

 heightened at the optimum, for reasons which have been 

 explained. It has already been said that the proportion of 

 stimulus held latent will be greater with the degree of sub- 

 tonicity of the plant. That this was the condition of 

 specimen 3, then, is demonstrated, not only by the increase 

 of the total effect at the optimum temperature, but also by 

 the fact that at 30° C. so large a proportion of the stimulus 

 as almost exactly one-half is held latent. At -^y^ C. here, as 

 in the other cases, there was no latent component. 



Is the change induced by stimulus always of an 

 explosive character? — It has generally been supposed that 

 stimulus causes response by an explosive chemical change. 

 According to this theory, the stimulus acts as upon a trigger, 

 to release suddenly a large amount of energy previously held 

 latent in the tissue. The response is thus assumed to be 

 always disproportionately larger than the stimulus, and to be 

 brought about by chemical degradation, or dissimilation of 

 the living tissue. The tissue, thus reduced below par, is sup- 

 posed to be restored by the process of assimilation. 



In Chapter X., however — -on Theories concerning Different 

 Types of Response — I adduced considerations, showing that 

 there are cases of responsive phenomena in which this 

 description would not hold good ; that is to say, there are 

 instances in which the response cannot be due to a chemical 

 down change of explosive character. Nor is it always true 

 that response is disproportionally larger than stimulus. The 

 series of experiments which has just been described offers 

 conclusive evidence on this point, of a quantitative character. 

 Further, if the theory of an explosive down change had been 

 the real explanation of response in general, then it is clear 

 that in the case of the response induced by stimulus in grow- 

 ing organs, recovery would have taken place slowly, and 



