158 GROWTH HORMONES IN PLANTS 



more growth substance was conducted through the lower than 

 through the upper half. Although it is possible that unilateral 

 changes in permeability may be produced by the action of gravity, 

 whereby differences in the conduction of the substance might 

 arise, still these experiments show decidedly that the growth 

 substance can actually be displaced transversely by the action of 

 gravity. 



The effect of unilateral light upon the distribution of growth 

 substance in the coleoptile stump of Avena has been investigated 

 by Boysen Jensen (1933a). When an Avena coleoptile was 

 decapitated, and the cut surface covered with a block of growth- 

 substance agar, the growth substance proved to be unlocalized 

 in its downward flow. When the upper part of such a plant was 

 unilaterally illuminated, and the lower portion was darkened, a 

 positive phototropic curvature arose not only in the upper, lighted 

 portion but also in the lower, darkened portion. It would appear, 

 therefore, that in the shaded part of the coleoptile, more growth 

 substance flows downward along the back side than upon the 

 front side. The difference in the conduction of growth substance 

 upon the front and the back side of the lighted portion of the 

 coleoptile could be explained by the destruction of the growth 

 substance upon the front side, by the lowering of the permeability 

 upon the front side, or by the displacement of growth substance 

 from the front to the back under the influence of light. Displace- 

 ment of growth substance was directly proved by a comparison 

 of the phototropic curvature in coleoptile stumps with and with- 

 out growth substance present. Some decapitated coleoptiles 

 were covered with growth-substance agar and some with plain 

 agar blocks for a sufficient period of time to permit the intake of 

 substances present in the agar. Then the blocks were removed, 

 and the coleoptile stumps were illuminated with continuous 

 light. It was found that in coleoptile stumps receiving growth 

 substance, the phototropic curvature appeared about 2 hours 

 earlier than in those without growth substance. This result can 

 be explained by the assumption that the growth substance present 

 in the coleoptile is displaced in a transverse direction by the 

 unilateral effect of light. In some instances, the growth-sub- 

 stance content of the coleoptile stump can be so great that a 

 phototropic curvature does not appear, the reason being that an 

 ample supply of the growth substance remains upon the front 



