764 PLANT GROWTH lO 



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 



group has to be spaced, not from the ring itself, but from specific combining posi- 

 tions on it. This is part of the general concept held by many workers that the 

 auxin molecule combines with its substrate (whatever that may be) at several 

 specific points; in other words, it is the shape of the molecule as a whole, as well as 

 the location and reactivity of the main combining groups, that determines biolo- 

 gical effectiveness. Strong support for this viewpoint is given by the differing 

 activity of geometrical and optical isomers. In acids with unsaturated side-chains 

 it is the m-forms which are active, e.g. cinnamic acid, XVI, and several of its 

 derivatives, all of which have real (though not very high) activity in the r/j"-form, 

 while the lrans-{ovxi\% are inactive. In acids with an asymmetrical carbon atom 

 in the sidechain it is the D-forms which are the most active, e.g. a-(3-indole)- 

 propionic acid, XVII. The L-forms do have some activity in most cases, but it is 

 always less than the d, and it varies down almost to zero (Matell, 1953). 



Recently it has been shown that compounds without a ring may have weak but 

 real activity, especially in the slit pea curvature test, which is usually the least 

 specific. S-Carboxymethyl-N, N-dimethyldithiocarbamate, XVIII, was prepared in 

 the course of a study of fungicides (Van der Kerk et al., 1955) and shown to have 



CH3 



COOH L^ \) 



CIS trans 



Cinnamic acid a-(3-indole)- S-Carboxymethyl- 



propionic acid N,N-dimethyI- 



dithiocarbamate 

 XVI XVII 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-trichlorophenoxyaceticacid, XIX or a-indoleiso- 

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



