362 CHAP. XXXII. THE CRITICAL ANGLE 



to the right was gradually increased, and the hitherto 

 quiescent spot of light exhibited a sudden deflection to the 

 right at 30°. On return to the vertical the excitatory reaction 

 disappeared. Inchnation to the left gave a sudden deflection 

 to the left at — 35°. The critical angle is therefore 32-5°. 



Experiment 194. Determination of the critical angle 

 for the stem of Tropaeohim. — The critical angle for the stem 

 was next determined, the procedure adopted being the same 

 as in the last case. The minimum angle at which response 

 occurred to right-handed rotation was 33° : rotation to the 

 left elicited reaction at —30°. Hence the true critical 

 angle for the specimen was 31*5°. Five other determina- 

 tions were made with other specimens, and the mean critical 

 angle obtained was 32-7°, which is very nearly the same as 

 the critical angle for the petiole. 



Experiment 195. Critical angle for the stem of Com- 

 melina bengalensis. — With this plant an inchnation of 30° 

 to the right induced the excitatory reaction ; inchnation in 

 the opposite direction induced reaction at — 33°. The true 

 critical angle is thus 31-5°. In a second specimen the mean 

 value of the critical angle was found to be 31°. 



Table XXXII. — The Critical Angle for Various Plants. 



