146 GROWTH HORMONES IN PLANTS 



In conclusion, it can be said concerning the course of photo- 

 tropic curvatures that (1) there are three (or five) different types 

 of reaction, two (or three) positive and one (or two) negative 

 curvatures; (2) the curvature begins in the tip and proceeds in a 

 basipetal direction; (3) the average rate of growth is not changed 

 during the primary positively phototropic curvature. During 

 the second phototropic curvature there is, however, according to 

 du Buy and Nuernbergk, a generally lessened rate of growth. 

 Apparently there is no agreement concerning the precise influence 

 of light upon the second positive curvature. 



The Blaauw Theory. — Among the different theories which 

 have been proposed to explain the phenomena of phototropic 

 curvature, perhaps the best known is that of Blaauw. The 

 Blaauw theory has been discussed so often and so thoroughly in 

 phototropic literature that it seems unnecessary to treat it here in 

 great detail. This theory has been regarded occasionally as an 

 explanation for all curvature phenomena, but actually it was 

 applied for a time only to the phenomena of phototropism, and 

 even here decisive proof was not obtained. The main point of 

 the theory is that phototropic curvature in the separate regions of 

 a plant organ comes about by an unequal rate of growth which 

 is caused by an unequal distribution of light in the organ. Each 

 part of the plant grows separately, according to the amount of 

 light with which it is provided. Blaauw (1919) has described his 

 theory in the following words: "The light-growth reaction is the 

 primary phenomenon; phototropism is the secondary one which 

 necessarily follows from it, when locally different light-growth 

 reactions arise from locally different conditions of illumination." 

 The ideas of Blaauw coincide, in the main, with the theory pro- 

 posed earlier by de Candolle (1832). The application of the 

 Blaauw theory to phototropism in the Avena coleoptile is difficult, 

 because of the transmission of the stimulus which causes a 

 phototropic curvature to take place in a region of the organ which 

 is not directly illuminated. 



Evidence Supporting the Blaauw Theory. — The test of the valid- 

 ity of the Blaauw theory of phototropic curvature in the Avena 

 coleoptile begins with the determination of whether or not a 

 phototropic curvature can be derived from the summation of the 

 light-growth reactions of the illuminated and shaded sides under 

 conditions of unilateral lighting. Sierp (1921) investigated the 



