152 PHYTOHORMONES 



explanation of tropisms to more general functions of auxin 

 in correlative processes and to the growth process itself. 



A. Historical Introduction 



The early work on tropisms has been reviewed by Wiesner 

 (1878, 1880), Pringsheim (1912), F. A. F. C. Went (1929, 

 1931), Rawitscher (1932), du Buy and Nuernbergk (1935) 

 and need not be further discussed here. We can thus pass 

 over the first period, consisting first of simple mechanical 

 explanations, and then of their gradual replacement by 

 more and more elaborate stimulus-concepts, none of which 

 had a really concrete physical basis. This period of complex- 

 ity of thought was accompanied by simpHcity of experi- 

 mental approach, and work was of a purely qualitative 

 nature. The experiments of Blaauw (1909, 1914, 1915, 1918) 

 ushered in a new era,^ that of quantitative work; this was 

 associated with greater simplicity of theories. He showed 

 that, within limits, the response is proportional to the total 

 energy received by the plant, and he envisaged tropisms 

 simply as a phenomenon of differential growth. Thus: 

 "Whenever light causes a growth reaction, unequal dis- 

 tribution of the light will cause unequal growth, which we 

 call phototropism " (Blaauw, 1918, p. 171). 



Almost simultaneously with the first publication of 

 Blaauw, the "stimulus" concept of tropisms received a 

 blow from another direction. Boysen Jensen (1910, 1911, 

 1913) found that the transmission of the light stimulus 

 does not involve any vital process, but can take place by 

 simple diffusion. He concluded from his experiments, which 

 have been discussed in H B, that the path of the phototropic 

 stimulus is along the shaded side, that of the geotropic 

 stimulus along the lower side, of the plant. The stimulus 

 could cross a cut surface, but would not pass a sheet of 

 mica; however, a slice of rattan inserted in the cut did not 



1 The editors of the Zeitschrift fiir Botanik received Blaauw's paper "Licht und 

 Wachstum, J" (1914), with the following unusual reservation: "We publish this 

 paper although we do not agree in every respect with the author's theoretical con- 

 ceptions." 



