CHEMISTRY OF THE AUXINS 111 



This substance had been isolated from fermentations in 

 1885 by E. and H. Salkowski, and was shown by them to be 

 also present in urine. The activity of indole-3-acetic acid is 

 of the same order as that of the Ci8 compounds auxin a and 

 h and is not due to an impurity, because the synthetic 

 product has the same activity. 



Kogl and Kostermans (1934) isolated the same substance 

 from yeast plasmolysate and showed that the indole-acetic 

 acid obtained represented a considerable part of the original 

 activity of the yeast. 



At the same time the active substance from Rhizopus cul- 

 tures, originally called by Nielsen ''Rhizopin, " was worked 

 up in a somewhat different way by Thimann (1935). He was 

 able to carry out a large number of purification stages with- 

 out any loss in activity. The impossibility of lactonization, 

 together with the sensitivity of the substance to acid previ- 

 ously found (Dolk and Thimann, 1932), made it probable 

 that it was identical with indole-acetic acid rather than with 

 auxin a or h. After purification, first by shaking out at 

 different controlled pH, and then by extraction with vari- 

 ous solvents, the free acid was distilled in very high vacuum 

 (10"^ mm.) in a special still, and the minute amount of 

 crystalline material finally obtained gave all the reactions 

 characteristic of indole-acetic acid. Comparison with a syn- 

 thetic sample confirmed the identity. 



The mode of formation of indole-acetic acid by these 

 microorganisms is almost certainly by the oxidative deami- 

 nation of tryptophane. Thus the yield of auxin is determined 

 by the amount of tryptophane present in the peptone used for 

 culture (Thimann, 1935). It is also proportional, in a given 

 culture, to the extent of aeration (Thimann and Dolk, 1933), 

 which is explained by the entry of oxygen into the reaction: 



