140 PHYTOHORMONES 



herewith. This table (Table XII) is founded on those of 

 Kogl and Kostermans (1935) and Haagen Smit and Went 

 (1935), with additions from various sources. 



The conclusions to be drawn from this table are in some 

 respects modified from those touched on in VII E. Tenta- 

 tively we may say that: 



The primary growth-promoting activity is connected with 

 the presence of: ^ the double bond, or aromatic unsatura- 

 tion; 2, a carboxyl group, free, or if esterified, readily hydro- 

 lyzable; 3, a ring system, either 5-membered (auxin a and h), 

 aromatic (naphthyl or phenyl), or a combination of both 

 (indole, indene, etc.); 4, a minimum distance of at least one 

 C atom between the carboxyl group and the ring; 5, a very 

 definite steric structure, since in the one case studied the 

 cis-compound is active, the trans-compound not. 



The chemical changes which do not directly influence the 

 primary growth reaction, but modify the secondary prop- 

 erties of a growth-promoting substance, are: 



1. length of the acid side-chain, 



2. methyl substitution in the nucleus, 



3. substitution in the side-chain, 



4. the structure of the nucleus itself. 



A more detailed discussion and a hypothesis relating 

 structure to activity has been given by Went, Koepfli, and 

 Thimann (1937). 



Although this is only a beginning, we are already in a 

 position to predict what substances should possess activity. 

 Presumably an exact knowledge of the active grouping will 

 help in determining the type of the reaction involved. 



