CHEMISTRY OF THE AUXINS 113 



stimulate the growth of Aspergillus; this action has been 

 shown by Nielsen and Hartelius (1932) to reside in the 

 ether-insoluble fraction, and it is entirely unconnected with 

 auxin. The active substance has been termed Wuchsstoff B 

 (not to be confused with auxin b). This and much other 

 evidence shows that the auxins in general have little or 

 nothing to do with the substances stimulating growth of 

 fungi, yeasts, and bacteria, about which there is a large 

 literature. 



D. Identity of the Native Plant Growth Hormone 

 Now that it has been shown that such chemically different 

 compounds, all active on Avena, are widely distributed in 

 nature, it becomes of interest to know which of these is the 

 native growth hormone in the various higher plants. While 

 the final proof can only be given by isolations, Kogl, Haagen 

 Smit, and Erxleben (1934) have given good evidence by in- 

 direct methods that the active substance of the Avena coleop- 

 tile is auxin a. Firstly, the molecular weight may be de- 

 termined by the diffusion method, using agar blocks which 

 are assayed on Avena before and after diffusion; from the 

 results the diffusion constant, D, is directly calculated, and 

 since D^^Jl = ca. 7.0 (Oholm, 1912) the molecular weight, 

 M, is thus obtained. In Went's experiments (1928) this 

 molecular weight was found to be 376 for the auxin coming 

 from A vena, and Kogl, Haagen Smit, and Erxleben obtained 

 the same value for pure auxin a by this method (actual mol. 

 wt. C18H32O5 = 328). The auxins from Zea Mays coleoptiles, 

 the regenerated tip of Avena coleoptiles, and the root tip of 

 Vicia Faba all give about the same value (Heyn, 1936). On 

 the other hand the auxin from Aspergillus gave 169, from 

 Rhizopus 176 and 190, and from yeast 193 (Kostermans, 

 1935). These, together with Heyn's value for the auxin from 

 Phijcomyces, are all close to 175, the theoretical value for 

 indole-acetic acid. Secondly, the sensitivity to pH provides 

 a differentiation of another type ; indole derivatives in gen- 

 eral are destroyed by warm acid, but not by alkali ; auxin a is 



