CHAPTER XIII 



1. When the stem of certain plants is pi 

 horizontal position, the apex grows vertically upward and 

 the root downward. The downward growth of the root 

 is called positive, the upward growth of the apex nega- 

 tive geotropism. The writer has observed a similar phe- 

 nomenon in a hydroid, Antennularia antennina 294> 30 

 and his observations were confirmed by Miss Stevens. 553 

 Animals as well as plants, therefore, show the phenomenon 

 of geotropism. 



These phenomena have given rise to a strange dis- 

 cussion, namely: What constitutes the "stimulus" in the 

 case of geotropism? When a galvanic current is sent 

 through a motor nerve the muscle answers with a con- 

 traction only when the current is made or broken, but 

 not while a constant current is flowing through the nerve. 

 The older physiologists were not able 'to form a mental 

 picture of what happened in this case, and they cut the 

 knot by invoking a verbalism, namely by calling the mak- 

 ing or breaking of a current a ' ' stimulus. ' ' This perhaps 

 innocent verbalism then led to the less harmless dogma 

 that only a rapid change could act as a " stimulus. ' ' Thus 

 Jennings 253 and Mast 346 took it for granted that phe- 

 nomena of orientation by light could only be produced 

 by rapid changes in the intensity of light and not by 

 constant illumination, since they had the a priori convic- 

 tion that only a rapid change in the intensity of a gal- 

 vanic current or of light is a "stimulus." The same diffi- 



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