56 



CHAP. VII. THE PHYSIOLOGICAL FACTOR 



record given by the High Magnification Crescograph remains 

 horizontal. Exposure to light upsets the balance, and the 



predominant D reaction is 

 seen in retardation of growth 

 as an up-curve (fig. 12). If, 

 on the cessation of light, 

 growth returned to the 

 normal, the fact would be 

 demonstrated by the up- 

 curve becoming once more 

 horizontal, which is the 

 balanced record of normal 

 growth. Instead of this we 

 find a down-curve, which 

 represents an acceleration of 

 growth above the normal, in- 

 dicative of the A process. 

 After a time the normal rate 

 of growth becomes restored. 

 The dual effects of stimulus 

 of light are seen in the above, 

 the A effect becoming un- 

 masked on the cessation of 

 the stimulus. 



The dual effect of the 

 stimulus of light will be further demonstrated by the electric 

 and by the photosynthetic response (Chapter X.). 



Fig. 1 2 . Direct and After Effect of 

 Light on Growth-elongation of 

 Kysoov 



First part of the balanced record of 

 growth is horizontal; on applica- 

 tion of light of short duration at 

 arrow, there is a retardation of 

 growth (up-curve) .followed by ac- 

 celeration as an after-eff ect(down- 

 curve) . Restoration to normal is 

 seen in the last part of the curve. 



Effect of Chemical Agents 



The effect of a moderate dose of ether on general irrita- 

 bility is well demonstrated by its effect on the mechanical 

 response of Mimosa. In the first part of the record (fig. 13) 

 we observe two uniform responses. Ether-vapour was now 

 introduced into the plant-chamber, with the result of a 

 depression of response. Finally, on substitution of fresh air 



