160 GROWTH HORMONES IN PLANTS 



Recently, Koch (1934) has demonstrated the transport of 

 growth substance across the Avena coleoptile toward the positive 

 side in an artificially produced electric field. Similarly, Kogl 

 (1933, Mitt. VI) and Ramshorn (1934) have shown the electrical 

 transport of growth substance in Avena saliva and other species 

 of plants. There are various obstacles in the way of an electrical 

 explanation. Not all organs that contain growth substances 

 react phototropically. A theory adequate to explain the 

 transverse transport of growth substance must take into con- 

 sideration the negative as well as the positive responses. 



Conclusion in Regard lo the Growth- sub slance Explanation. — 

 If an unequal distribution of growth substance has taken place 

 in the tip itself, a curvature must of necessity follow in the 

 lower portions of the organ, since growth-substance transport 

 in the basal region takes place almost exclusively in a longitudinal 

 direction. According to du Buy (1933), the following three 

 factors must be taken into consideration for the evolution of a 

 phototropic curvature: (1) the production of growth substance, 

 (2) the transport of growth substance, and (3) the effect of 

 growth substance. Actually, the methods that one has for the 

 measurement of these individual components are still far from 

 perfect. Even in the simplest case of unilateral illumination 

 of the tip, the quantitative relationship between the curvature 

 and the distribution of growth substance (not to mention the 

 amount of light applied) does not hold. When the entire 

 coleoptile is illuminated, the difficulties become still greater. 

 The phototropic curvature is determined not only by the amount 

 of growth substance given off from the tip but also by its dis- 

 placement throughout the basal region. Other factors must be 

 considered also, such as the effect of light upon the activity of 

 the growth substance and modifications in the inherent capacity 

 for response. 



The importance of fight absorption for phototropism may be 

 emphasized by brief reference to the peculiar action of certain 

 dyes introduced experimentally into the fiving tissues of plants. 

 Blum and Scott (1933) have demonstrated the photosensitizing 

 effect of dilute erythrosin upon wheat roots grown in a nutrient 

 solution. In the presence of 1:500,000 erythrosin, the uni- 

 laterally illuminated roots exhibited relatively greater growth 

 rates on the shaded side, so that bending occurred toward the 



