GROWTH SUBSTANCES FOR GEOTROPISM 



179 



Table 12. — Amount of Growth Substance Given Off by Geotropically 

 Stimulated Avena-coleoptile Tips in Comparison with Controls 



substance given off from a coleoptile tip is not changed by 

 geotropic stimulation. 



The Unequal Distribution of Growth Substance. — Just as photo- 

 tropic curvature is caused by an unequal distribution of the growth 

 substance present in the coleoptile tip, so geotropic curvature 

 must be also. Dolk (1929a) showed that the geotropic stimulus 

 alters the distribution of growth substance in Avena coleoptiles if 

 they are placed in a horizontal position. By placing them in 

 such a position for 30 minutes, then removing the tip and extract- 

 ing the growth substance from its upper and lower sides, he was 

 able to show that the lower side gives off considerably more than 

 the upper side (Table 13). With a similar technique, Navez and 



Table 13. — Amount of Growth Substance Given Off from the Upper 

 AND Lower Sides of Geotropically Stimulated Avena-coleoptile 



Tips 



Robinson (19326) found that a shift in distribution of growth 

 substance in the Avena tip is brought about by change in position 

 of the coleoptile. 



