GROWTH SUBSTANCES FOR PHOTOTROPISM 159 



side (in spite of its displacement) to produce a maximum rate of 

 growth under the prevaihng conditions. Although a displace- 

 ment of growth substance in the coleoptile tip has not been 

 proved directly, it may be supposed, on the basis of these 

 experiments with subapical portions, that growth substance is 

 displaced by the action of unilateral light in the tip. 



Growth-substance Transfer and Electrical Potential. — Proof of the 

 accumulation of growth substance upon the back side of the 

 unilaterally illuminated tip has supplied a link in the chain of 

 phototropic response, which may be considered as the concluding 

 link in the process of induction. There remain for consideration 

 the preceding factors in induction, i.e., how the effect of unilateral 

 light can produce a displacement of growth substance; and the 

 subsequent factors, i.e., how the unequal distribution of growth 

 substance can produce the positive phototropic curvature in the 

 Avena coleoptile. 



A consideration of the first steps in the process of induction 

 leads back to the problem of growth-substance transport in the 

 plant. The difficulties of constructing a plausible theory for the 

 longitudinal transport of growth substance have been discussed 

 in Chap. V. It is equally difficult to explain the transverse 

 transport of growth substance in a satisfactory way. In a 

 discussion of the theory of transverse transport, it should be 

 pointed out that illumination for a mere fraction of a second can 

 produce a curvature. Effective displacement of growth sub- 

 stance naturally cannot take place so rapidly. It must be 

 concluded, therefore, that illumination creates a condition in 

 the coleoptile tip which is the primary cause of the displacement 

 of growth substance. 



Since growth substance is an acid, one might suppose that the 

 unilateral accumulation of it could be brought about by differ- 

 ences in electrical potential induced by illumination of the tip. 

 On this supposition, the back side would have to be electro- 

 positive with respect to the front side. According to the investi- 

 gations of Waller (1929) and Bose (1928), a potential difference 

 actually exists in unilaterally illuminated stems in such a way 

 that the shaded side is positive. Whether the observed differ- 

 ences in potential are sufficient to explain the transverse dis- 

 placement of growth substance is discussed more fully under 

 geotropic curvatures. 



