364 CHAP. XXXII. THE CRITICAL ANGLE 



angle is reached, and that then there is an abrupt excitatory 

 reaction, can only be satisfactorily explained on the theory 

 of the sudden fall of heavy particles from the base on to the 

 side of the perceptive cells. 



Summary 



The excitatory reaction under the stimulus of gravity is 

 reduced disproportionately with the diminution of the angle 

 of inclination. This indicates the approach of some hiatus 

 or discontinuity. By producing the curve of excitation 

 backwards, it cuts the abscissa at about 31-5°, at which 

 angle the abrupt excitatory reaction should be reduced 

 to zero. 



The critical angle for geotropic excitation has been 

 found in a large number of plants to be about 31-8°. 



The effect of repetition of inclination is found to lower 

 the critical angle. 



The abrupt excitatory reaction induced beyond the 

 critical angle can only be attributed to the sudden fall of 

 heavy particles from the base on to the side of the sensitive 

 cells. 



