422 PLANT RESPONSE 



latent in the tissue (p. 314), which we have designated as 

 the internal energy. We have thus succeeded in defining 

 the actual nature of those ' inner stimuli ' to which the 

 phenomenon of growth is usually vaguely ascribed. 



From what has been said, it is clear that the responsive 

 peculiarities of the growing region are not per se in any 

 way different from those of any other excitable tissue, the 

 apparent contrast between negative and positive turgidity- 

 variations — that is to say, between responsive contractions and 

 responsive expansions — having been shown to depend upon 

 the fact that in one case we see the direct, and in the other 

 the indirect, effects of excitation. If this be so, it follows that 

 the direct application of external stimulus to a growing tissue 

 ought to have the normal effect of excitatory contraction. 

 In other words, while the action of the so-called ' inner 

 stimuli,' or internal energy, gives rise, as explained above, to 

 responsive expansions, the direct effect of external local 

 stimulation must be the production of responsive contractions. 

 That this is the case, will be shown in the next chapter. 



Summary 



Growth is a phenomenon of multiple response. 



Each of these multiple growth-responses consists of a 

 sudden elongation, due to a pulse of increased turgidity, 

 followed by an incomplete recovery. The difference between 

 elongation and recovery is the irreversible growth-effect. 



Such responses, when very rapid, appear as continuous. 



In ordinary excitatory response there is a pulse of 

 diminished, and in growth-response a pulse of increased, 

 turgidity. Both are, however, effects of excitatory reaction. 



When a tissue is locally excited, it gives a response of 

 negative turgidity-variation, that is to say, of contraction. 

 This is the direct effect of stimulus. 



When the source of stimulation is behind, and the inter- 

 vening tissue does not conduct excitation, then the excitatory 

 expulsion of water finds expression in a positive turgidity- 



