160 



Fundamentals of Auxin Action 



Fig. 71. Hirschfelder models of cis- and han^-cinnamic acids (left and right re- 

 spectively). C/5-cinnamic acid with the side-chain out of the plane of the ring is an 

 auxin, while irfl/?5-cinnamic acid with the side-chain in the plane of the ring is an 

 anti-auxin. 



form is not. In this case the activity cannot be explained by the Veld- 

 stra scheme, for both isomers must exist with the carboxyl in the same 

 plane as the ring (Thimann, 1951). 



,COOH 



HOOCCH CHCOOH 



II II 



trans 



XX 



XXI 



Optical isomerism can also strongly influence auxin activity. Per- 

 haps the clearest case of such an influence is in 2,4-dichlorophenoxy-a- 

 propionic acid (XXII), which exists in D and in L forms. The D 

 isomer is an active auxin and the L is essentially inactive (Thimann, 

 1951). Comparison of optical isomers of a variety of acids has shown 

 that in all of the cases studied the D form was more active than the L 

 (Matell, 1953), and in many cases the L isomers are anti-auxins 

 (Aberg, 1953). 



CHi 

 lOC'COOH l/"^tCOOH 



r 



H 



V 



i 



XXII . XXIII 



