150 GROWTH HORMONES IN PLANTS 



ties, produce equal light-growth reactions. Different wave 

 lengths, when applied in suitable intensities, also produce equal 

 curvature reactions (first positive, negative, and second 

 positive)." In spite of agreement of this sort, it must be said 

 that phototropism and light-growth reaction in Avena are two 

 fundamentally different processes. The reader is referred to 

 Biinning (1929) for further discussion of the matter. 



Conclusions in Regard to the Blaauw Theory. — It is highly 

 probable that the light-growth reaction which appears in unlocal- 

 ized illumination also exists in unilateral illumination and there- 

 fore may have some significance in the production of phototropic 

 curvature. In view of the available data on the subject, it is 

 certain that Blaauw's theory is not sufficient to explain the 

 observed changes in the rates of growth during the phototropic 

 response of the Avena coleoptile. It can be shown that photot- 

 ropism is connected with the transverse transport of a growth 

 substance, Mobile such a phenomenon is not concerned in Hght- 

 growth reactions which appear with unlocalized illumination. 



The Growth-substance Explanation. — The historical develop- 

 ment of the growth-substance explanation of tropisms has been 

 sketched in some detail in the first chapter. Sachs (1882a) early 

 postulated the existence of formative substances which were 

 supposed to control growth and development in plants. Definite 

 evidence in favor of special growth substances was not forthcom- 

 ing until some thirty or forty years later when, in connection with 

 certain studies on phototropism, it was demonstrated that a 

 growth-promoting substance is dispersed from the tip of the 

 Avena coleoptile. (See Figs. 1 and 2 for a graphical story of the 

 discovery of growth substances.) 



The Relation of Growth Substance to Phototropism. — In a paper 

 entitled "Das Problem des Phototropismus und sein Ende," 

 Blaauw (1919) wrote: 



The problem of phototropism in itself has become empty. Further 

 theoretical observations on this problem will only keep us still further 

 from the investigation of the actual and therefore significant phenomena 

 of growth. Surely there is no problem in phototropism itself, since it is 

 a pure growth phenomenon. Growth, however, as a phenomenon of 

 life, is a problem of great depth. 



The growth-substance explanation has been proposed for both 

 phototropic and geotropic phenomena. The application of this 



