CHAPTER VII 



THE CHEMISTRY OF THE AUXINS 

 A. Early Work 



The first indication that growth-promoting substances 

 occur in cjuantity outside the plant was given by the experi- 

 ments of Seubert (1925) who found that malt extract, 

 sahva, diastase, and pepsin contained a substance active 

 in causing negative Arena curvatures. On boiling these 

 preparations for 15 minutes most of the acti\dty remained, 

 but in spite of this she concluded it was connected with the 

 enzymic activity. Gorter (1927) also found growth-promot- 

 ing activity in numerous enzyme preparations. Went (1928) 

 in his experiments on the auxin secreted by the coleoptile 

 tip showed, however, that it is stable to boiling as well as 

 to hght. He also determined its molecular w^eight (see sec- 

 tion D). 



Nielsen (1928) then found that the medium on which 

 Rhizopus suinus or Ahsidia ramosa, two pathogenic fungi, 

 had grown, was rich in a substance active in producing 

 Avena curvatures. He made a preliixiinary study of the 

 purification of the substance from Rhizopus cultures. At 

 first he believed it was only produced on solid media, but 

 this was later shown by Bonner (1932) to be due to the 

 necessity of aerating the cultures. Nielsen (1930) found 

 the substance to be soluble in ether but readily inactivated 

 by the traces of peroxide normally present. By extracting 

 with purified ether he obtained a syrup of which 1/60 y ^ 

 caused a definite curvature in Avena. Dolk and Thimann 

 (1932), using liquid Rhizopus culture medium, then found 

 that the substance is extracted by ether only from acid 

 solution, i.e. it is itself an acid. By shaking out into ether 

 and analyzing extract and residue for auxin, its partition 



1 One y = 0.001 milligram. 



105 



