CHAPTER XIII 



GEOTROPISM 



1. When the stem of certain plants is placed in a 

 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 2!)4, 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 340 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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