GROWTH SUBSTANCES FOR PHOTOTROPISM 167 



negative instead of a positive curvature resulted under these 

 conditions. According to Oehlkers (1926), the curvature is not 

 brought about by any concentration of the Hght upon the back 

 side but rather by the fact that the Hght rays penetrate farther 

 in the back than in the front. 



Castle (1930, 1933a, h) has investigated the Avhole question 

 thoroughly. His computation of the path of light rays in the 

 sporangiophore led to the conclusion that the light in the back 

 half, owing to refraction, is 1.26 times as great as it is in the front 

 half of the organ. When the absorption coefficient is not greater 

 than 6, more light is absorbed in the back half of the cell than in 

 the front. From this, Castle concluded that the difference in the 

 amount of light absorbed causes the back side to grow more 

 strongly than the front side ; hence bending occurs toward the light. 



Up to the present time, nothing is known concerning the signif- 

 icance of growth substance for the phototropic curvature of 

 sporangiophores. It can be demonstrated easily that Phy- 

 comyces forms growth substance in culture, and it is possible, 

 therefore, that this substance may be concerned in the photo- 

 tropic curvature exhibited by the fungus. Heyn (1935) has 

 extracted a growth substance from the sporangiophores of 

 Phycomyces nitens which, on the basis xjf the coefficient of diffu- 

 sion, appears to be 3-indole acetic acid. 



SUMMARY 



The observations of de Candolle (1832) led to an appreciation 

 of the fact that differential growth is involved in the phototropic 

 and geotropic responses of plants. These tropisms may be 

 regarded as specialized cases of normal growth, where unequal 

 stimulation on the two sides of an organ leads to greater enlarge- 

 ment on one side. The bending of plant organs toward light is 

 the result of more rapid growth on the shaded side than on the 

 side toward the light. 



Since plants elongate less rapidly in the light than in darkness, 

 Blaauw and others concluded that light must have a retarding 

 effect upon growth. Subsequent detailed investigations of 

 phototropic curvature in the Avena coleoptile and organs of other 

 plants showed that (1) bending can be induced in the darkened 

 part of an organ only when some other portion is illuminated; 

 (2) the depressing action of light upon growth is not sufficient to 



