52 



CHAP. VII. THE PHYSIOLOGICAL FACTOR 



stimulus and response ; (2) the dependence of maximum 

 response on the intensity of stimulus ; (3) the effect of 

 temperature variation ; (4) the D effect of shock-stimulus ; 

 (5) dual effect of the stimulus of light ; (6) phenomenon of 

 alternating response ; (7) effect of chemical agents ; and (8) 

 effect of tonic condition on response. I will in subsequent 

 chapters show that photosynthetic activity is modified under 

 external variations in a manner parallel to the modification of 

 other physiological activities such as growth and movement. 



Photosynthetic and Phototropic Curves 



It has been shown that photosynthesis is feeble when the 

 light-intensity is low ; that it increases rapidly in the median 



range where the rate of increase 

 is uniform ; that the curve then 

 reaches a turning-point, after 

 which it tends to become hori- 

 zontal. Examination of the 

 phototropic curve obtained with 

 the leaf of Eryihrina indica (fig. 

 9) shows that its characteristics 

 are very similar to those of the 

 photosynthetic curve (fig. 7). 



Dependence of Maximum 



Response on the Intensity 



of Stimulus 



I have shown that the maxi- 

 mum photosynthesis is not abso- 

 lute but relative, and that it 

 depends on the intensity of 

 stimulus. Parallel results are 

 obtained with other modes of 

 For instance, we take 



Fig. 9. TheTropic Curve under 

 the Action of Light (Erythrina 

 indica) 



response. 



Mimosa and subject it to tetanising electric shocks of in- 

 creasing intensity. Under moderate intensity of stimulus 



