402 PLANT RESPONSE 



erection of the leaf at a rate of '12 mm. per minute. Warm 

 water was now poured on the root, thus suddenly increasing 

 the internal activity of the plant. Now, if it be true that 

 recovery is brought about by this factor of internal energy, 

 then the increase of internal energy ought to produce a 

 sudden augmentation of the rate of the recovery. That this 

 is the case will be seen from the record (fig. 166), where it will 

 be noticed that the enhanced rate of recovery from this point 

 is r8 mm. per minute, that is to say, fifteen times the normal 

 rate. 



Antagonistic actions of internal energy and external 

 stimulus. — We thus see that it is the internal energy of 

 the plant — vaguely known as a favourable tonic condition — 

 which actively determines the recovery of the organ. This 

 will explain the fact which I have mentioned elsewhere, that 

 in summer, when the internal energy is considerable, the leaf 

 of Mimosa recovers from the effect of stimulus in about six 

 minutes, whereas in winter, when the internal energy is low, 

 the same process may take as long as eighteen minutes. We 

 thus see that as regards mechanical response the external 

 stimulus and internal energy act antagonistically. Local 

 external stimulus induces a diminution of turgidity, while 

 internal energy causes an increase of turgidity. Thus when 

 the internal turgidity is very great it opposes the mechanical 

 response to external stimulus. This we saw in the case of 

 over- turgid leaves of Mimosa, and in those of Artocarpus 

 during the rainy season, which, though excited, did not exhibit 

 mechanical response to stimulation (pp. 49, 58). 



This will be clearly understood also from an attentive 

 consideration of the experiment, the record of which is given 

 in fig. 166. In that case, had the warm water which in- 

 creased the internal activity been applied earlier at the root, 

 that is to say during the application of external stimulus, 

 the induced internal turgidity would then have been so great 

 as to arrest the responsive down movement of the leaf. The 

 amplitude of the response to external stimulus would thus 

 have undergone diminution or even abolition. 



