i66 



CHAP. XV. PHOTONASTIC PHENOMENA 



Triphasic Response 



Experiment 97. Effect of application of strong light on 

 upper half of pulvinus. — The record given in fig. 92 gives 



Fig. 92. 



Fig. 93. 



Fig. 94. 



Fig. 92. Triphasic response on application of light L above 



(Mimosa). 



Preliminary erectile twitch followed by erectile response {a — b) 

 due to contraction of upper cortex. Reversal of response 

 to negative fall, slow from 6 to c and rapid beyond c. (See text.) 



Fig. 93. Parallel effect of radio-thermal stimulation T applied 



above. 



Fig. 94. Effect of application of strong light L on lower half 



of pulvinus. 

 Erectile movements represented by down-curve and vice versa. 



a striking illustration of the triphasic response. There 

 was at first a short-hved erectile twitch, evidently due to 

 the quick transmission of positive hydraulic impulse which, 

 reaching the most excitable region c, induced expansion and 

 erection of the leaf. This impulse was quickly exhausted. 



