110 PHYTOHORMONES 



C4H9.CH C— CHOH.CH2.CHOH.CHOH.COOH 



/ 



CHo auxin a (active) 



C4H9.CH CH 



C4H9.CH C=CH.CH2.CH0H.CH0H.C00H 



/ 

 CH2 



\^ pseudo-auxin a (inactive) 



C4H9.CH CH 



I 

 OH 



Whether this change has any physiological significance re- 

 mains to be seen. In the case of the lactone, in which the 

 change is accompanied by loss of H2O, and introduction of a 

 second double bond in the side-chain, the same change 

 may be brought about by ultra-violet irradiation. 



C. Discovery of Indole-3-Acetic Acid as an Auxin 



Subsequently another active substance was found. This 

 was the result of the development of a charcoal absorption 

 method for removing the active substance from urine, which 

 led to the working up of still larger volumes (Kogl, Haagen 

 Smit, and Erxleben, 1934). Under these conditions it ap- 

 peared that the bulk of the activity in the charcoal eluate 

 could not be purified by the methods previously found 

 satisfactory. The active substance was largely destroyed 

 on attempting to lactonize, and other methods involving 

 only precipitation and solution were therefore substituted 

 in the later stages. These led readily to the separation 

 of another active substance which turned out to be iden- 

 tical with indole-3-acetic acid (usually called hetero-auxin), 



C— CHo.COOH. 



/CH 



H 



