KENNETH V. THIMANN 33 



place in two steps, the first or preparatory reaction being pH insensitive 

 and being capable of accomplishment by a wider variety of structures 

 than the growth reaction can be. In general, substances which have all 

 but one of the structural requirements of an auxin have preparatory 

 activity (17), and these are called hemi-auxins. They include 2-bromo-3, 

 5-dichlorobenzoic acid (carboxyl adjacent to ring), cyclohexancacetic 

 acid (saturated ring), thiazoleacetic acid (5-membered ring) and vinyl- 

 acetic acid (no ring). There are many other examples. In some ways 

 2,3,5-triiodobenzoic acid, "TIBA" (XXXIII), behaves similarly both 

 in the pea test and in other growth reactions, but when the concentration 

 ratio between this substance and the auxin becomes large then growth 

 inhibition takes place (12). This was interpreted by Thimann and Bonner 

 In terms of competition between the TIBA and the auxin for the same 

 active centers in the cell. Several workers have found that TIBA pro- 

 motes flowering and have explained this in terms of Its Inhibition, at 

 high concentrations, of the action of auxin in the plant. It is evident that 

 the structure of TIBA comes within the grouping of hemi-auxins men- 



/1-C5H,,— CH-/7-CjH„ »z^ ^CHCOOH 



ioOH H^C^^CH, 



XXX III xxxivo xxxiy b 



tioned above. However, Veldstra, under the term "synergism," describes 

 a similar phenomenon brought about by quite unrelated compounds, 

 especially dl-w-amylacetlc acid (XXXIV), (14). Although it Is true that 

 the related amine, di-7z-amylmethylamlne, has no synergistic activity, 

 and hence that the carboxyl may be needed, still the relationship be- 

 tween XXIV and the typical auxin structure is somewhat remote.* (If 

 the formula is written as shown at XXXIVb it suggests perhaps a relation- 

 ship with phenyl-a-propionic acid or auxin-a.) Even if such exceptions 

 should show that Went's generahzatlon is only partly correct, it still 

 seems fair to visualize the enhancement phenomenon as due in some way 

 to the ability of related substances to combine with a given structure. 



The Mechanism of the Ac/ion of Auxin: A Theory.— h remains only to 

 consider what light all this sheds on the mechanism of auxin action. The 

 discussion of growth as a metabolic process will take place in other papers 



*A recent direct comparison, using a sample of XXXIV kindly supplied by Dr. 

 Veldstra, shows that TIBA is the more active of the two. 



