GROWTH SUBSTANCES FOR GEOTROPISM 



187 



Table 15). By removing the tip of the Avena coleoptile immedi- 

 ately after geotropic stimulation, Dolk (19296) found that the 

 opposing reaction was greatly retarded. Li this way, he demon- 

 strated that a continuous supply of growth substance is necessary 



Table 15. — Comparison of the Amounts of Growth Substance 



Extracted from the Upper and Lower Halves of Avena Coleoptiles 



Which Had Been Stimulated Geotropically and Allowed to 



Recover 



for the reaction to take place. It appeared after 150 minutes, 

 which is the same length of time required for renewed appearance 

 of growth substance in this region. 



During the opposing growth curvature which leads to straight- 

 ening of the organ, the concave side grows more rapidly than the 

 convex. If one does not assume a shift in the distribution of 

 growth substance, making concentration for a time greater upon 

 the concave than upon the convex side, then one must explain 

 how an equal distribution of growth substance can bring about an 

 unequal rate of growth. Went (1928a) had hypothecated a cell- 

 stretching material which, together with growth substance, is 

 necessary for growth. Dolk assumed that more of it is used on 

 the convex side and, therefore, that the concave side where more 

 stretching material remains should grow more rapidly with an 

 equal supply of growth substance. It should be mentioned, too, 

 that the individual cells go through a long period of growth and 

 that the stage of development of the cells also influences the rate 

 of growth of the organ as a whole. On the convex side, the cells 

 may be in a later stage of development than upon the concave 



