24 PLANT GROWTH SUBSTANCES 



Structural Requirements for Primary Activity. — Interpreting the pea 

 test as indicative of primary growth activity, a great many substances 

 have been prepared and tested. In general it appears that only compounds 

 with an unsaturated ring system are active. The saturated compounds 

 cyclohexaneacetic and decahydronaphthaleneacetic acids are both in- 

 active. In the Avena test, even saturation of the heterocyclic double bond 

 of indoleacetic acid abolishes activity (5). Further, all the active com- 

 pounds are acetic, propionic, or butyric acid derivatives. Examples are 

 naphthalene- 1 -acetic acid (VI), anthraceneacetic and even phenylacetic 

 acid, but not cyclohexaneacetic acid. The salts are in general less active 

 than the acids, though at the/?H of the cell (5.5-6.0) they are partly con- 

 verted to the free acid. Acids stronger than indoleacetic acid (pK 4.75), 

 however, are more fiilly dissociated, and their apparent activity should 

 be corrected for the extent of dissociation (i). Esters of the acids are in 

 general active, and methyl esters have about the same activity as the 

 acid; ethyl esters, generally less. In the Avena test Kogl and Kostermans 

 (5) found that activity decreased with the increasing size of the alkyl 

 esterifying group and concluded that the esters must be hydrolyzed to 

 the free acids to produce growth. It is probable that such hydrolysis is 

 only necessary for transport and that, for primary activity, the ester 

 is active per se. Amides present a similar problem. In the case of naphtha- 

 leneacetamide, although the activity is lower than that of the free acid, 

 the curve relating activity and concentration is quite different from that 

 of the acid (11), whereas if hydrolysis were involved one would expect 

 the two curves to be parallel. Further, at the optimum concentration 

 no trace of ammonia could be detected in the solution (i i). It is probable, 

 therefore, that amides too are active per se for primary activity. In one 

 or two cases nitriles show activity, which can safely be ascribed to 

 hydrolysis to the acid. 



Spatial configuration evidently plays an important part in determin- 

 ing activity. In phenylacetic acid (VIII), substitution of one of the side- 

 chain hydrogen atoms by a methyl group (IX), or of both of them by a 

 methylene group (X), does not appreciably change the activity,* but 

 substitution of them by two methyl groups (XI), abolishes it completely 

 (3). This suggests steric hindrance and indicates that the carboxyl must 

 bear a certain spatial relation to the ring. Clear evidence for this is given 



*The ^-propyl and ally! derivatives are also active, both being more active 

 than phenylacetic acid, though the isopropyl derivative is almost inactive (14). 



