GROWTH SUBSTANCES FOR PHOTOTROPISM 163 



Gradniann (1930) proposed a complicated theory of growth 

 and of phototropic curvature which, in the main, is in accord 

 with the growth-substance explanation. However, Gradmann's 

 theory assumes the presence of two different growth substances. 

 One of these substances {A) is formed only in the tip and flows 

 in an unlocalized fashion into the basal region. It is identical 

 with the growth substance described above. The other {B) 

 arises along the entire length of the coleoptile and on up into the 

 tip. Each of these substances alone is considered as being 

 ineffective, but together they form a substance {AB) which 

 increases growth. Photo- and geotropic stimuli cause more 

 growth substance B to be formed on the shaded, or under, side; 

 hence more of the compound AB is produced, thus increasing 

 the rate of growth upon this side. With the increase of substance 

 5 in a particular region, a greater use of A follows. A flows to 

 this region of deficit, and the diversion of the stream of growth 

 substance A strengthens the primary effect considerably. With 

 this hypothesis, Gradmann tried to combine Blaauw's theory 

 with the growth-substance explanation, to account for the photo- 

 tropic response. In accord with Blaauw, the unequal origin of 

 B is the primary reaction, and it should be sufficient in itself to 

 bring about a curvature. 



The main objection to Gradmann's theory is that growth and 

 growth changes in the Avena coleoptile can be much more easily 

 explained by the growth-substance explanation alone. The 

 Gradmann postulate is directly refuted by Boysen Jensen's 

 (1933a) experiment in which phototropic curvature was hindered 

 by the application of very great amounts of growth substance 

 although the coleoptile grew vigorously. In spite of the displace- 

 ment of growth substance, a' sufficiently large amount of it 

 remains on the front side to produce a maximum rate of growth 

 permitted by the prevailing conditions. This experiment could 

 not be explained, by Gradmann's theory, because if the curvature 

 came about by unequal distribution of the substance B, then an 

 excess of substance A should not hinder the phototropic curvature. 



DICOTYLEDONOUS STEMS 



The great majority of investigations on the question of photo- 

 tropism have dealt with the Avena seedling, and, without doubt, 

 many of the conclusions reached in these studies concerning 



