GROWTH SUBSTANCES FOR PHOTOTROPISM 155 



distribution of growth substance in the illuminated tip should be 

 represented by line III in Fig. 47. 



Theories of Beyer, Cholodmj, chi Buy, and Nuernhergk. — The 

 total concentration of growth substance present in the coleoptile 

 during phototropic curvature, in relation to the normal concentra- 

 tion, is slightly increased according to the hypothesis of Boysen 

 Jensen (as represented in hne V). On the other hand, according 

 to the hypothesis of Went (Hne III), the amount of growth sub- 

 stance is shghtly decreased. The investigations of Beyer, 

 Cholodny, and du Buy and Nuernbergk have shown that the 

 average rate of general growth during phototropic curvature is 

 not demonstrably changed, and one must conclude, therefore, 

 that none of the foregoing hypotheses is entirely correct but 

 rather that the growth-substance distribution in the tip during 

 phototropic curvature is proportionately decreased on the 

 illuminated side and increased on the shaded side, as represented 

 by line IV in Fig. 47. 



It has been shown that different types of phototropic curvature 

 may occur in response to different amounts of light. The ques- 

 tion arises as to whether or not the distribution of growth sub- 

 stance in the tip can be correlated with the two different positive 

 curvatures. Since, according to du Buy and Nuernbergk, the 

 rate of growth is unchanged in both curvatures, it may be con- 

 cluded that the distribution of growth substance is substantially 

 the same in both cases. What the growth-substance distribution 

 may be during the negative curvatures, remains to be investigated. 



The Displacement of Growth Substance. — The origin of the 

 unequal distribution of growth substance during phototropic 

 induction remains an important point for further discussion. 

 Various suggestions have been offered to explain the growth- 

 substance distribution as presented diagrammatically in line IV in 

 Fig. 47. One might conclude that the rate of formation of growth 

 substance in the tip is governed by the application of light, so 

 that at certain light intensities the amount present is increased, 

 while at other intensities it is decreased. However, such a 

 notion would seem to refer us back to Blaauw's theory which has 

 been proved inadequate as an explanation for phototropic curva- 

 ture in the Avena coleoptile. 



The assumption of an unequal distribution of growth substance 

 brought about by an exchange of material between the light and 



