INDIRECT STIMULATION OF ROOT I4I 



kept in a proper humid condition by pieces of sponge soaked 

 in water. There are two small openings, one to the right 

 for the passage of light through a narrow horizontal 

 diaphragm D, the other to the left for the passage of the 

 string attached to the recorder. The cork supporting the 

 root can be adjusted up or down, so that either the root-tip 

 or the growing region or both can be exposed to the action 



of light. 



The following experiments were carried out to determine 

 the effect of unilateral photic stimulation (i) on the root- 

 tip, and (2) on the growing region of the root. 



Effect of Unilateral Stimulation of the 



Root-Tip 



Experiment 86. — The tip of the root of Ipomoea was 

 subjected to the unilateral action of light for 70 seconds. 



Fig. 79. Negative response of the root to indirect stimulation 



(Ipomoea). 



Vertical line indicates moment of application, and horizontal 

 arrow within circle, of the withdrawal of light. 



The source of light was a loo-candle-power PointoHte. 

 The curvature induced was found to be negative or away 

 from the source of light, initiated within 15 seconds of the 

 exposure to Hght. The negative movement continued for 

 I minute after the cessation of light, after which there was 

 complete recovery (fig. 79). 



