GROWTH SUBSTANCES FOR PHOTOTROPISM 165 



The results of these experiments make it probable that growth 

 substances, at least in certain cases, are also concerned in the 

 phototropic response of stems. 



Growth Substance and Phototropism in Seedling Axes. — Van 

 Overbeek (1933), as mentioned before, studied the significance of 

 growth substance for normal growth and the photogrowth 

 reaction in Raphanus sativus. He demonstrated the formation 

 of growth substance in the cotyledons and its subsequent move- 

 ment into the hypocotyl. When the upper end of a hypocotyl 

 cylinder was completely covered with growth substance and 

 unilaterally illuminated, more growth substance was extracted 

 from the basal regions on the dark side than on the hghted side 

 (Fig. 2). This shows that it was displaced by the action of 

 unilateral light, just as it is in the Avena coleoptile. It 

 seems reasonable to conclude that the growth substance dis- 

 tributed from the cotyledons is displaced toward the back of the 

 hypocotyl by unilateral light. 



In addition, van Overbeek showed that the light-growth reac- 

 tion was very strong in dark-adapted plants. With general 

 illumination he found a growth retardation which amounted to 

 over 50 per cent. This phenomenon could be explained neither 

 by changes in growth-substance transport nor by the destruction 

 of growth substance through the action of light, and van Over- 

 beek assumed that the tonus of the seedling stem with reference 

 to growth substance must be changed by the application of light. 

 The importance of this phenomenon for phototropism in seedlings 

 is clear. Under conditions of unilateral illumination, the light 

 intensity on the lighted side was 5.2 times greater than that on the 

 shaded side of the Raphanus hypocotyl. Since the rate of growth 

 is retarded so greatly by the application of light, then unilateral 

 illumination must be sufficient to produce a phototropic curvature 

 in accord with Blaauw's theory. Boysen Jensen (1936) extracted 

 with chloroform the growth substance from the front and back 

 sides of unilaterally illuminated seedling axes of Phaseolus. 

 During phototropic curvature the growth substance was more 

 concentrated on the side away from the light. 



Van Overbeek (1932) was able to show that the quantity of 

 growth substance transported through a portion of Raphanus 

 hypocotyl was about the same in the light as in the dark. The 

 presence of the tip was not essential for bringing about a lateral 



