138 TROPISMS 



leaf determines the rate of geotropical curvature of a 

 horizontally placed stem. The only way in which the 

 mass of the leaf could have such an influence is through 

 the mass of substances it sends into the stem, so that this 

 case of geotropism is a function of mass action. There 

 are indications that the way contact with a solid in- 

 fluences the behavior of living matter is also through the 

 influence on the rate of certain chemical reactions. The 

 writer observed that the stolons of a hydroid, Aglao- 

 phenia, have a tendency to adhere to solid surfaces and 

 not to leave them any more if they once reach them, and 

 that as soon as such a stolon reaches a solid surface, 

 e.g., a piece of a glass slide, its growth is accelerated con- 

 siderably. It was very astonishing to notice how much 

 more rapid the growth of roots of Aglaophenia was when 

 they were in contact with a solid body than when they 

 grew in sea water. The rate of growth is the function of 

 a chemical mass action (Loeb 543 ). 





