CHAPTER XXXII 



THE CRITICAL ANGLE FOR GEOTROPIC EXCITATION 



The results of investigations described in the present chapter 

 afford independent evidence that the falling starch-grains 

 are the effective agents in geotropic excitation. The facts 

 to be presently described will be better understood from the 

 following illustration. If some sand-grains be placed on 

 a flat board which is gradually tilted, the particles start 

 sHding down only after a certain critical angle has been 

 reached. If the board is rough, this critical angle will be 

 large ; if it is smooth, the angle will be small. Moreover, 

 by the scouring action of the sand, the rough surface may 

 become smoothed down after numerous repetitions of the 

 experiment, the result being a reduction of the critical angle. 

 If geotropic stimulation is effected by the fall of the 

 starch-grains, it would be expected that : 



1. At a small angle of inchnation the grains will not be 

 immediately displaced ; therefore no excitation will ensue. 



2. When the angle of inclination is gradually increased, 

 the grains will shde down as soon as the critical angle is 

 exceeded. This fall of particles and the resulting pressure 

 on the protoplasm will constitute a stimulus and give rise 

 to an excitatory response. 



3. The critical angle will probably be lowered to a certain 

 extent, on repetition of the process, by the reduction of 

 frictional resistance to the fall of the particles. 



4. Were the weight of the fluid contents of the cell in the 

 higher plants the only means for stimulation by gravity, 

 the excitatory reactions would be proportional to the sines 



