GROWTH SUBSTANCES FOR GEOTROPISM 171 



This interpretation of geotropic curvatures, established by 

 Frank, was then rounded out by Sachs (1873-1882), as the result 

 of numerous investigations which form the foundation of our 

 knowledge of geotropic processes. Sachs evolved the general 

 concept that any valid explanation of geotropism must explain 

 both positive and negative curvatures. His idea may be stated 

 as follows: A theory of geotropism is satisfactory only when it 

 explains both positive and negative curvatures at the same time ; 

 when it shows why the same external cause produces the opposite 

 effect in structurally similar cells and organs, i.e., the promotion 

 of growth upon the underside in the stem and inhibition of growth 

 upon the underside in the root. The effect of gravity upon root 

 and stem can be explained only by the assumption "that the 

 inner organization (even though submicroscopic) of the different 

 regions determines the different manner of reaction to the same 

 external stimuli." 



Sachs formulated a theory to account for opposite reactions of 

 the root and stem in the plant. The geotropic processes he 

 viewed as stimulation phenomena whose course is determined 

 partly by external force and partly by the organization of the 

 plant organ concerned. If the course of a stimulus phenomenon 

 is determined by the internal organization of the plant, then the 

 final response must be the result of the combined action of various 

 single reactions in a chain. The problem is to analyze the chain 

 of reactions which begin with the direct effect of gravity upon 

 the plant cells and end with a positively or negatively geotropic 

 response. The contribution of the theory of growth substance 

 to the analysis of geotropism will be taken up for different plants 

 separately, in the same manner as has been done for phototropic 

 curvatures. 



THE AVENA COLEOPTILE 



The Avena coleoptile is negatively geotropic, because when 

 displaced from the vertical position it grows away from the center 

 of the earth. This geotropic curvature is the result of unequal 

 rates of growth on the upper and lower sides of the displaced 

 organ. 



Stimulation and Response. Distribution of Geotropic Sensitiv- 

 ity. — In the case of phototropism, a local induction of the stimulus 

 can be obtained easily by screening the remaining regions of the 



