A. J. HAAGEN-SMIT 9 



in the Avena test. This material proved to be thermostable, and soluble 

 in water, ether, and alcohol. It was soon evident that auxin production 

 was shown not only by the two fungi used by Nielsen, but also by other 

 fungi, bacteria, and yeasts. In the isolation in Kogl's (25) laboratory in 

 Holland several of these organisms were used as producers of starting 

 materials, but attempts to isolate the growth substance from their media 

 were abandoned when it was found that urine causes strong bending of 

 the Avena coleoptile. After several purification steps the active agent 

 was obtained in chemically pure form and proved to be a nitrogen-free 

 substance (C18H32O5). 



Diet experiments showed that auxin secretion was directly connected 

 with food intake, and when the secretion was found to be increased 

 considerably by corn oil, this was used for isolation purposes (24). The 

 auxin found was identical with the one from urine, but in addition a 

 second nitrogen-free substance with auxin activity was obtained. These 

 were distinguished by the names auxin-a and auxin-b. Using a different 

 extraction procedure with urine, a third substance active in the Avena 

 test was isolated (28). This substance proved to be 3-indoleacetic acid, 

 a compound discovered by Salkowski (51) about 75 years before, during 

 his investigations of protein decomposition. Indoleacetic acid was then 

 isolated from yeast (29), and in this country Thimann (56) proved by 

 isolation that the organism with which Nielsen had begun his studies, 

 Rhizopus suinus, also produced indoleacetic acid. 



During the years immediately following the discovery of the action 

 of indoleacetic acid as a growth hormone, it was generally assumed that 

 this substance was formed only in lower organisms. This belief was 

 supported by its isolation from Rhizopus and yeast and from the evidence 

 obtained using indirect means of identification. However, evidence 

 gradually accumulated that indoleacetic acid could be present in higher 

 plants too. The final proof was supplied by its isolation from wheat and 

 corn by extraction under mild alkaUne conditions (2,19), and from 

 immature corn (18). 



As a result of Larsen's (37) investigations, indoleacetaldehyde must 

 be added to our list of naturally occurring auxins, although it is recog- 

 nized that its activity is due to its conversion into indoleacetic acid in 



the plant. 



The auxins-a and -b were shown to have the following structures 

 (23,24). (See Fig. i). Auxin-a is a trihydroxy acid, whereas auxin-b 



