INTERNAL ENERGY AND EXTERNAL STIMULUS 437 



of growth is above the normal, which is due to the fact that 

 the internal energy has been augmented by the absorption of 

 the stimulus applied. 1 A stimulus of longer duration, that is 

 to say of three seconds, was next applied, and the responsive 

 retardation had now a greater amplitude than before, the 

 period of restoration being also longer, that is to say, sixteen 

 minutes. 



Similarities between motile and growth responses. — 

 We have, then, in growth-response an exact parallel to the 

 mechanical responses given by pulvinated or anisotropic 

 organs. The following tabular statement will show the 

 reason of this fundamental parallelism between responses 

 whose modes of indication are so widely different : 



Tabular Statement showing Comparative Effects of Stimulus in 

 Pulvinated and Growing Organs 



In studying the mechanical response of plants we found 

 that direct application of stimulus to the responding organ 

 always produced a response characterised by the negative 

 turgidity-variation, that is to say, a depression of the leaf. 

 We also saw that internal energy, inducing positive turgidity- 

 variation, caused the opposite response, that is to say, an 

 erection of the leaf ; and that an increase of the internal energy 



1 Other records showing the effect of external stimulus on growth will be found 

 in Chapter XXXIV. 



