Chemical structure and biological activity 



involved in the mode of action of these substances, it is very improbable that 

 such action can be accomplished by several different chemical reactions; 

 therefore we are forced to conclude that the different active substances are 

 converted within the plant into one certain form. Thus, either the aldehyde 



CI 



0-CH2-C00H 



0-CHaCOOH 



CI 

 fi^^O-CH^COOH 



0-CH2-C00H 



< 



CHz-COOH 



CH2-C00H 



/ 



n m 



Figure 6. The chemical formulae of dichloropyrocatechol-acetic acid (/), 4:6-dichlororesorcin- 

 biscarboxymethyl ether (II), and phenylimidodiacetic acid {III). These substances were found to be 

 practically inactive in the paste test. 



CI 



:CH-COOH 



CI 



Bis-2:'(-dichlorophenoxyacetic 

 acid 



I I I I L 



10'* 10^ 10^ 10^ 10° lo" 



Figure 7. The molecular model 

 of bis-2 : A-dichlorophenoxyacetic 

 acid with the concentration-action 

 curve and formula. Abscissa: \o^ 

 concn. Ordinate: change in length 

 of coleoptiles as per cent of control. 



% 



and nitrile are converted to the acid, or more probably, the aldehyde goes to 

 the acid and the acid into the nitrile, the latter being the only reacting form 

 which acts directly. 



There is, however, another possible explanation of the mode of action; 

 we can assume, as we did in earlier publications, that the action of a growth 



146 



