GROWTH SUBSTANCES FOR PHOTOTROPISM 161 



light. In the absence of dye, these roots were normally not 

 phototropic. This response was attributed to the light absorp- 

 tion of the dye in a manner similar to that exhibited by the 

 naturally occurring porphyrins in plants that are generally 

 photodynamic. 



On the other hand, the presence of certain dyes in plants has 

 in some instances been found to destroy the phototropic response 

 without having any great effect upon the average rate of growth 

 (Boas, 1933; Schweighart, 1935). Boas found that seedlings 

 of Lolium perenne treated with dilute eosin exhibited no photo- 

 tropic curvature in response to unilateral illumination. The 

 eosin seemed to affect both sensitivity to the stimulus and the 

 distribution of the growth substance, though, obviously enough, 

 the mechanism is still imperfectly understood. 



The growth substance which is active in phototropic cur- 

 vatures is identical with the growth substance of normal growth. 

 By unilateral illumination of the coleoptile tip, the rate of 

 grow^th-substance dispersal is scarcely changed, but a displace- 

 ment of it in a transverse direction takes place with the result 

 that its concentration is greater upon the shaded than upon the 

 illuminated side. The flow of growth substance into the basal 

 region on the front side of the coleoptile is decreased, and that 

 upon the back side is increased; therefore, the rate of subsequent 

 growth on these two sides is roughly proportional to the amounts 

 of controlling growth substance present. The average rate of 

 growth over-all is either not changed or only slightly altered. 



Blaauw's theory assumes that the individual parts of the 

 coleoptile grow at rates inversely proportional to the differential 

 amounts of light and that the phototropic curvature results from 

 separate reactions of the individual regions. According to the 

 growth-substance explanation, or theory, the coleoptile tip 

 reacts as a whole, and a difference between the lighted and 

 shaded sides of the organ is created by a displacement of growth 

 substance in the unilaterally illuminated tip. Herein lies the 

 fundamental difference between the two theories. The growth- 

 substance theory properly interpreted is capable of explaining 

 all the observed facts of phototropic response in the Avena 

 coleoptile. 



Other Theories on Phototropism. — The theory of Fitting 

 (1907), mentioned in the introduction, holds that a polarization 



