144 CHAP. XIII. DIA-PHOTOTROPISM 



Experiment 88. — In a specimen of Ipomoea both the 

 tip and the growing-point were simultaneously stimulated 

 b}^ unilateral light from a loo-candle-power Pointolite. 

 The effect of stimulation of the tip alone would, as already 

 shown, be negative, while that of the growing region would be 

 positive. Since the resultant effect was positive, the response 

 of the growing region, at least in the present case, was 

 relatively the more effective. The positive response occurred 

 20 seconds after the incidence of light and reached a maxi- 



,<:^gp^ 







Fig. 82. Effect on shoot and root of unilateral exposure to light 



(Sanchezia). 



mum after 70 seconds. After this there was a reversal of 

 response from positive to negative, due to transverse con- 

 duction of excitation across the organ from the proximal 

 to the distal side (fig. 81). The extent of this reversal 

 obviously depends (i) on the transverse conductivity and 

 thickness of the root, and (2) on the intensity and duration 

 of the illumination. 



Three characteristic types of response of the root to light 

 are met with : 



1. Positive curvature ; 



2. Neutralisation ; and 



3. Negative curvature. 



Referring to the third type, a pronounced negative 

 curvature occurs under prolonged unilateral stimulation of 



