DEVELOPMENT OF THE HORMONE CONCEPT 11 



tile tip." He was led to assume such concentration changes 

 by comparison with Lehman's model of a nerve, in which the 

 electromotive changes of a chain of zinc sulfate concentra- 

 tion cells were compared to the transmission of stimulus. 

 The "material nature" of the transmission seemed not to 

 be purely physical, but "on the other hand, various con- 

 siderations make one think that the transmission is of a 

 chemical nature." 



Under the influence of PfefTer and of the application 

 of zoological concepts, it is clear that Boysen Jensen visual- 

 ized the transmission of the phototropic stimulus as a com- 

 plex chain of reactions. This is exemplified by his interpre- 

 tation of it in terms of the transmission of irritation in a 

 nerve. He assumes that the light causes a differentiation 

 between the bright and dark sides of the coleoptile tip; this 

 constitutes an "irritation" of the dark side, and this irrita- 

 tion "leaves the dark side of the tip to travel down the dark 

 side of the coleoptile." Finally, this irritation "sets free," 

 in the lower part of the coleoptile, an acceleration of growth. 

 This separation into perception, transmission, and reaction 

 made it impossible for him to conceive that all three were 

 reahzed through the same agency. While his experiments 

 prove that the transmission of the irritation is a transmis- 

 sion "of substance or of ions" he did not postulate that 

 this was a special, growth-promoting substance, and it was 

 left to Paal to show that the material nature of the trans- 

 mission was due to a special substance which is active in 

 promoting normal growth. It will be clear to the reader 

 that the above analysis does not at all support Boysen 

 Jensen's claim (1935, 1936) that "the existence of a growth 

 substance in the Avena coleoptile during phototropic curva- 

 ture was demonstrated" by his experiments. 



C. Birth of the Growth Hormone Concept 

 Because of the importance of Boysen Jensen's experi- 

 ment, it was repeated and extended by Paal (1914, 1919), 

 who, after excluding the possibilities of the base being influ- 



