Chemical Nature of Growth Regulators 1 57 



Whereas in most instances greatest auxin activity is obtained with 

 side-chains two carbons in length, the interesting benzoic acid auxins 

 are exceptions (Bentley, 1950). The entirely different ring substitution 

 requirements of the benzoic acid auxins have been discussed above. 



The presence of various substituent groups on the acid side-chain 

 can also have a strong influence on auxin activity. For example, start- 

 ing with the auxin, phenylacetic acid (IV), the substitution of a 



CARBON ATOMS IN SIDE-CHAIN 



Fig. 70. A, the effects of different lengths of side-chains of 2,4-dichlorophenoxy 

 acids upon auxin activity in the tomato test. B, the effects of different lengths of 

 side-chains of phenoxy acids upon the breakdown to phenol after exposure to flax 

 seedlings. Auxin activity expressed as the inverse of the minimal dosage required for 

 cell elongation (calculated from Synerholm and Zimmerman, 1947); the phenol pro- 

 duced is expressed as micrograms recovered from 2.5 x 10"* M of the acids after 10 

 days (Fawcett et al, 1952). 



