PLANT HORMONES 



149 



of correlation phenomena were discovered, 

 and often they were supposed to be 

 due to chemical substances. Thus Fitting 

 (1909) could show that a substance present 

 in pollinia induced the typical swellings 

 of the style of orchids after fertilization, 

 and that this swelling was not due to fer- 

 tilization itself. Doposcheg-Uhlar (1911) 

 was able to influence the nature of the 

 buds developing in leaf axils with certain 

 extracts. Boysen Jensen (1911) showed 

 that the transmission of the phototropic 

 stimulus is due to some agent of sub- 

 stantial nature. In all these studies we see 

 indications of the future work on auxins. 

 But the basis for the auxin concept was 

 laid by Paal (1919). He definitely estab- 

 lished the fact that growth in grass coleop- 

 tiles is regulated by a diffusible agent 



per cent amounts of auxin of the order 

 of one ten-millionth of a milligram. The 

 test consists of decapitating oat seedlings, 

 thereby removing the production center of 

 growth substance so that the growing 

 zones of these seedlings become deficient in 

 auxin. Application of growth-promoting 

 substances absorbed in small cubes of agar 

 to one side of the cut surface of such a 

 decapitated oat seedling will lead within 

 one and a half hours to a curvature of the 

 seedling towards the opposite side. This 

 curvature is directly proportional to the 

 amount of applied auxin. A second auxin 

 was also isolated from plant material, and 

 after a chemical analysis Kogl and Erx- 

 leben (1934) were able to establish the fol- 

 lowing two formulas for auxin-a (C18H32O5) 

 and auxin-& (CisHjoOi).^ 



I 



CH3 



I 



CH. • CH„ 



CH, 



I /^\ I 



CH • HC CH • CH • CH, 



CH« 



Auxin-a 



HC === C • CHOH • CH, • CHOH • CHOH • COOH 



CHa 



cm ■ GK, • CH • HC 



CH3 



CHa 





CH • CH • CH2 • CH, 



HC ===== C • CHOH • CH„ • CO • CH, • COOH 



Auxin- fe 



coming from the tip of these coleoptiles 

 and moving down from cell to cell. The 

 phenomenon of transmission of a photo- 

 tropic stimulus across a cut surface, dis- 

 covered by Boj^sen Jensen, was interpreted 

 by Paal as a differential transmission of 

 his growth regulators due to light. 



From then on a rapid development took 

 place, culminating in the chemical isolation 

 of the growth substance, or auxin, by 

 Kogl and Haagen-Smit (1931). The basis 

 for this chemical isolation lay in a good 

 quantitative biological test for growth sub- 

 stances, the Avena test (Went 1928). 

 With this test it is possible to deter- 

 mine with an error of not more than 5 



Auxin-a and auxin-& are extremely diffi- 

 cult to obtain, the Utrecht Laboratory 

 alone so far having succeeded in crystalliz- 

 ing them, and they obtained only slightly 

 more than one gram. Therefore the further 

 discovery of Kogl, Haagen-Smit and Erx- 

 leben (1934) that indole acetic acid acted 

 in essentially the same manner as auxin-a 

 in inducing growth was of the greatest 

 importance, since it made auxins — sub- 



1 It is interesting to point out that in the animal 

 female sex hormones the two naturally occurring 

 products, estrone and estriol, also differ by the ele- 

 ments of water; this is due to the presence of two 

 hydroxyl groups in the one against one keto group 

 in the other. 



