764 



PLANT GROWTH 



10 



always hold. Exceptions are provided, among others, by the weak but real ac- 

 tivity of 1,2,3,4-tetrahydronaphthoic acid, XIV, and the quite strong activity of 

 2,3,6-trichlorobenzoic acid, XV, and some other chlorinated benzoic acids. An 

 explanation for this latter case has been suggested, based on the idea that the acid 



.CH 

 ^CH 



COOH 



<^^ COOH 



HC 



trans 



-C — COOH 

 H 



Cinnamic acid 



XVI 



a-(3-indole)- 

 propionic acid 



XVII 



CHgCOOH 



S-Carboxymethyl- 



N,N-dimethyl- 



dithiocarbamate 



XVIII 



auxin activity. It may be that resonating double bond structure gives the molecule 

 a flat configuration similar to that conferred by a ring. Several related compounds 

 are active also (Veldstra, 1955; Fawcett et al., 1956a, b). 



Many compounds with the general structure of auxins, but without actual 

 growth-promoting activity, may still strongly affect the growth responses caused 

 by auxins. In some cases, e.g. 2,4,6-trichlorophenoxyacetic acid, XIX or a-indoleiso- 

 butyric acid, XX, they act as antagonists, and there is evidence, based on kinetic 



