358 CHAP. XXXII. THE CRITICAL ANGLE 



of all angles of inclination above zero. But if the fall 

 of solid particles be the efficient cause, there will be a 

 hiatus in this relation, for there will be no excitatory- 

 response at angles smaller than the critical. Even at a 

 slightly greater angle than the critical, some of the particles 

 may remain adherent, and the excitation will then be dis- 

 proportionately lower than what is demanded by the law 

 of sines. It is only after the critical angle has been con- 

 siderably exceeded that the relation of sines will be found 

 to hold good, at least approximately. 



The above considerations will now be subjected to the 

 test of experiment to discover, in the first place, if there is 

 any discontinuity in the responsive reactions at angles 

 below 45°. 



Excitatory Reaction at 35°, 45°, and 60° 



From the results of experiments detailed in the previous 

 chapter it was found, for angles of 45° and 90°, that geo- 

 electric excitation is approximate^ proportional to the sine 

 of the angle of inclination. In order to observe whether 

 this relation holds good at other angles, a series of observa- 

 tions was made with six different specimens of the petiole 

 of Tropaeolum at 35°, 45°, and 60°. The results are given 

 in the following table : 



Table XXXI. — -Excitatory Reactions at 35°, 45**, and 60°. 



