DEVELOPMENT OF THE HORMONE CONCEPT 9 



stimulus" (or as we should say now, a stimulating material) 

 (1888, p. 132). Beijerinck designates this protein as a 

 "growth enzyme." Later (1897) he extended this \'iew to 

 the development of organisms in general, and stated that 

 "form is determined by liquid substances, which move freely 

 through considerable numbers of cells in growing tissues" 

 (1897, p. 203). 



Except for these observations of Beijerinck no direct 

 evidence for the existence of such special substances as 

 Sachs had postulated was obtained for nearly 40 years. On 

 the other hand, at about the same period, the existence of 

 polarity in correlation phenomena was proven, both for 

 whole organs, and for each separate cell of a transplant, by 

 Vochting (1878, 1884, 1892, 1908). His work, however, is 

 primarily concerned with inherent morphological polarity, 

 rather than with its physiological basis. 



After the time of Sachs and Vochting most of the studies 

 on correlation laid emphasis on nutritional factors. Goebel 

 (1908), for instance, in discussing quantitative correlations, 

 says "of the numerous organ initials, many remain unde- 

 veloped because the building materials, which they need 

 for their development, go to others which can ' attract ' these 

 materials more powerfully." Similar \'iews, invoh-ing also 

 the nitrogen content of the plant, i.e. "the carbon: nitro- 

 gen ratio," have been generally held by American workers 

 (c/. e.g. Kraus and Kraybill, 1918). While their experiments 

 show that there is a parallelism between a given carbon : ni- 

 trogen ratio and a given type of growth, no causal relation 

 has been shown to exist. 



B. Tropisms 



About 1880 it began to be realized that tropisms were to 

 be regarded as a special kind of correlation phenomenon. 

 This aspect of tropisms was particularly emphasized by 

 C. Darwin (1880). Both for roots and shoots he was able to 

 show that the effects of light and gra\'ity are perceived by 

 the tip, and that the stimulus is transmitted to the lower 



