CHAPTER XV 



STEBEOTROPISM 



OUK orientation in space is determined by three groups 

 of tropistic influences, two of which we have already dis- 

 cussed, light and gravitation. The third one is pressure 

 on certain nerve endings of the skin. When the tactile 

 influences on the skin of the soles of the feet are weakened 

 (as is the case in locomotor ataxia), the patient finds it 

 difficult to stand and walk in the dark. When he can use 

 his eyes the difficulty is diminished, since the orienting 

 effect of the retina image can compensate the tactile de- 

 ficiency; just as we have seen that the effect of the loss 

 of the ears in crustaceans can be compensated by the 

 orienting influence of the eyes. 



The role of tactile influences on the orientation of ani- 

 mals is most clearly demonstrable in starfish, flatworms, 

 and many other animals, when put on their backs. The 

 animals "right" themselves, i.e., they turn around until 

 the ventral surfaces or their feet are pressed against 

 solid objects again. As the writer pointed out long ago, 293 

 gravitation has nothing to do with the phenomenon, since 

 starfish will stick to solid surfaces with their tube feet 

 even if by so doing their backs are permanently turned 

 to the center of the earth. Unless the nerve endings on the 

 sole of their tube feet are pressed against a solid surface 

 the animals are restless and the arms move about until 

 the feet are again in contact with solid bodies. This phe- 

 nomenon of orientation the writer called stereotropism. 



Quantitative investigations of this form of tropism are 



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