Development of Knowledge of Auxins 7 



really systematic study. He discovered that the hormone could be col- 

 lected in an agar block by diffusion. This was not only the first separa- 

 tion of the hormone from a plant, but it also presented a technique 

 for obtaining the hormone from a great variety of plant materials. 

 Then, utilizing this technique, he worked out a quantitative test for 

 auxin. Using the oat coleoptile, about which so much information 

 had now been accumulated, he established a test so accurate and re- 

 producible that it still stands today as the best auxin assay technique, 

 the Avena test. Thus there arose simultaneously a general theory* of 

 the growth hormone, a technique for obtaining the hormone from 

 plants, and a technique for its quantitative assay. 



With such a sound basis on which to proceed, it is hardly surpris- 

 ing that a great deal of very productive work followed. In a short 

 eight-year period, from 1928 to 1936, three auxins were isolated, 

 characterized and identified, the quantitative relationships of auxin 

 to tropisms of roots and shoots were established, and at the end of this 

 period half of the major functions of auxin in growth and develop- 

 ment as we now know them had been already discovered. 



In series of remarkable inquiries, Kogl (1933) and co-workers in 

 Holland found two materials, strongly active in the Avena test, which 

 they named auxin a and auxin b. When isolated and characterized 

 these materials were found to have the molecular formulas given 

 below. Auxin a was first isolated from human urine, and auxin b from 

 corn germ oil. These compounds are principally of historic interest, 

 for they have never been positively isolated from growing plant tissue. 

 For this reason they are not considered to be growth hormones by 

 most physiologists today. There were two lines of evidence which led 

 to the early assumption that auxins a and b were growth hormones. 

 First, their molecular weights are similar to that of the diffusible 

 auxin obtained from oat coleoptiles and some other plant materials 

 (e.g. Went, 1928). Auxin a has a molecular weight of 328, which is 

 quite close to Went's figure for diffused auxin from Avena, 372. 

 Second, tests for stability show that both auxin a and auxin diffused 

 from coleoptiles are stable to warm acid and break down in warm 

 alkali. The poor reliability of these criteria in the positive identifica- 

 tion of growth hormones is discussed in chapter III. 



CH3 OH- OH OH CH3 OH 

 CH3CH2CH, nCHCHjCHCH COOH CHjCHgCK ^ ch CHgC CHjCOOH 



c'M 



CHjCHgCK^ CH3CH2CH 



CH3 CH 



auxin b 



