156 GROWTH HORMONES IN PLANTS 



dark side of the coleoptile tip is supported by certain of Boysen 

 Jensen's investigations (1928). When the tip of an Avena 

 coleoptile was split and a thin glass plate was inserted into this 

 incision parallel to the direction of light, normal phototropic 

 curvature resulted in response to unilateral illumination. On the 

 other hand, the curvature was very weak when the plate was 

 oriented perpendicularly to the direction of light. If both halves 

 of the tip, without the insertion of a glass plate, were held together 

 closely either with a platinum spiral or with a thin glass tube, a 

 phototropic curvature took place even when the incision was 

 oriented perpendicularly to the direction of the light beam. The 

 result of this experiment can be explained only on the assumption 

 that an exchange of material between the two halves of the tip 

 takes place during the photic induction. 



The question arises, What kind of substance is transported 

 during the induction? At least two possibilities present them- 

 selves : It could be assumed that the growth substance itself was 

 displaced in a transverse direction during the action of unilateral 

 light. Or, on the theory that a constant formation of growth 

 substance takes place in the coleoptile tip, it could be assumed 

 that the growth substance itself was not transported, but rather 

 another substance which in turn influences growth-substance 

 production. The displacement of such a substance should then 

 cause a lessened formation of growth substance upon the front side 

 and an increased formation of growth substance upon the back 

 side. Since both of the above were possibilities, the author did 

 not venture to draw definite conclusions from his investigations, 

 though the first of these two assumptions was without doubt the 

 simpler. 



At practically the same time as the publication of Boysen 

 Jensen's (1928) experiments, Cholodny (1927) and Went (1928a) 

 expressed the idea that the unequal distribution of growth sub- 

 stance in the coleoptile comes about by a growth-substance 

 displacement. Cholodny proceeded with his investigations 

 on the assumption that tropic stimuli do not influence the produc- 

 tion of growth substances. Went supported his ideas with 

 experiments which seemed to show that after phototropic induc- 

 tion, the amount of growth substance given off by the lighted side 

 of the tip was lessened, while that of the darkened side was 

 increased. The results of his experiments are given in Table 10 



