THE MECHANISM OF THE ACTION 



133 



-C— CH2 COOH 



N^ 

 H 



.CH 



II 



-C— CH2 COOH 



.CH 



C^ 



H2 



o^ 



III 



-CH 



II 

 .C— CH2 COOH 



The substance III was found to have no activity in pro- 

 ducing curvature in Avena, but on the other hand it pos- 

 sessed moderate activity in promoting straight growth of 

 Avena sections and still more in producing pea stem curva- 

 tures. Hence a substance may be active in growth promo- 

 tion without causing any curvature on Avena. A similar 

 case, though less extreme, is presented by the indene deriva- 

 tive II, which has only small activity in Avena curvature, 

 but has much greater activity in producing straight growth 

 of Avena sections, while on pea stem curvatures its activity 

 is up to 14 of that of indole-acetic acid. There are, of course, 

 different ways of explaining such action. One is that these 

 substances act only by their effect on the pH, their acidity 

 setting free the auxin in the plant as we have seen above. 

 This, however, was ruled out for straight growth by first 

 exposing the sections to acid till their auxin supply was 

 practically used up and their growth had ceased, and then 

 placing them either in a solution of the test substance or 

 again in acid. The controls in acid now scarcely elongated 



