CHEMISTRY OF THE AUXINS 117 



well be the source of the auxin a in the urine, as has been 

 made very probable by feeding experiments: it was found 

 that the auxin content of the urine increased greatly after 

 ingestion of salad oil or butter, but not after ingestion of 

 pure protein, sugar, or hydrogenated cocos fat (Kogl, Haagen 

 Smit, and Erxleben, 1933a). The possibility that the auxin 

 precursor in the plant is an ester of auxin a was discussed 

 in IV 5 (p. 65). 



