114 PHYTOHORMONES 



destroyed by alkali but not by acid; auxin b is destroyed by 

 both. The auxin of coleoptiles is sensitive to alkali, and that 

 from fungi to acid. The facts, therefore, are consistent with 

 the view that the auxin of coleoptiles is auxin a. It is, 

 indeed, probable that other higher plants contain the same 

 auxin. ^ 



E. Activity of Compounds Related to Indole- 



AcETic Acid 

 Since the discovery of the activity of indole-3-acetic 

 acid, a large number of other substances, more or less active 

 as auxins, have been recognized. Kogl and Kostermans 

 (1935) prepared a large number of derivatives of indole-3- 

 acetic acid and tested them by the Avena curvature method. 

 They regarded substances with an activity 0.04 per cent 

 of that of indole-3-acetic acid, or less, as inactive. Later, 

 some of these substances have been retested by Avena curva- 

 ture and other methods, and their small activities deter- 

 mined (see VIII G). Kogl and Kostermans first considered 

 the importance of the carboxyl group, and therefore pre- 

 pared a series of esters. It was found that the acti\dty 

 decreased with increasing size of the esterifying radical 

 thus : 



Activity in AE per mg. 

 Indole-3-Acetic Acid: Free acid 25. 10^ 



Methyl ester 10. 10« 



Ethyl ester 3. 10" 



n-Propyl ester 1. 10" 



Iso-Propyl ester 0.1. 10"^ 



It seems likely that the esters are hydrolyzed in the plant, 

 so that they owe their activity to the acid produced from 



1 It should be remembered that Stark and Drechsel (1922) believed that the 

 "phototropic hormones," which have now been identified with auxin, were differ- 

 ent in different species. They found that when coleoptile tips of one species of 

 grass were placed on decapitated stumps of another species, the number of stumps 

 which reacted was smaller the greater the systematic difference between the species 

 of the tip and the stump. Not only has this view never been confirmed, but the 

 entire weight of evidence has always supported the complete non-specificity of 

 the auxins. Recently Soding (1936) has reported that apparent differences in activ- 

 ity of auxins from different plant sources were due merely to quantitative varia- 

 tions in amount. There is therefore nothing against the view that the same auxin 

 occurs throughout all the higher plants. 



