DEVELOPMENT OF THE HORMONE CONCEPT 13 



tiles, the agar being mixed with various tissue extracts, but 

 in no case did any of these promote growth, only inhibi- 

 tions being observed. The same failure to extract the 

 growth-promoting substance from coleoptile tips was expe- 

 rienced by Nielsen (1924) and by Seubert (1925). However, 

 Seubert was able to prove that agar containing sali\'a, dia- 

 stase, and malt extract caused a promotion of growth. This 

 was the first evidence that growth-promoting substances 

 exist outside the plant. 



Stark (1921), Stark and Drechsel (1922), Gradmann 

 (1925), and Brauner (1922) attempted to explain tropisms 

 in terms of special stimulus substances, or ''Tropohor- 

 mones," but with little success, and shortly afterwards 

 Cholodny (1924, 1926, 1927) and Went (1928) developed 

 the view of Paal and attributed all tropisms to asymmetric 

 distribution of the normal growth-promoting substance 

 {cf. X B). 



D. Isolation of the Growth Hormone 

 Success in obtaining the active substance from the coleop- 

 tile tip was finally achieved by Went (1926, 1928 ^). Using 

 the findings of Paal and adapting the method of Stark 

 (see III B), he placed coleoptile tips upon blocks of agar, 

 and then placed the agar on one side of the stumps of de- 

 capitated coleoptiles. The result was a curvature away 

 from the agar block (negative curvature). His fundamental 

 improvement in technique was to measure this curvature, 

 which was proven to be proportional, within limits, to the 

 concentration of the active substance. This test, the '^ Avena 

 test," was then used to determine some of the properties 

 of the substance, which was shown to be thermo- and photo- 

 stable, as well as readily diffusible. From the diffusion rate 

 its approximate molecular weight was determined. Went 

 interpreted the normal growth of the coleoptile in terms of 

 the action of this growth substance in conjunction with 

 other limiting factors. For the asymmetric growth involved 



> Published November, 1927. 



