172 PHYTOHORMONES 



of light on growth. We can imagine Hght having an effect 

 on (1) auxin production, (2) auxin transport (already con- 

 sidered), (3) auxin destruction, or (4) on the reactivity of 

 the plant to the auxin in it. Effects of light on permeability 

 or on the protoplasm directly would appear, in most cases, 

 under the last heading. As we have seen, the effect of light 

 on lateral transport can only take place in an intact system 

 or structure in which there is correlation between the dif- 

 ferent parts. In other words, the Cholodny-Went effect 

 depends essentially on the behavior of the organ as a whole. 

 This is shown very clearly by the experiments of Boysen 

 Jensen and Nielsen (1926) and Boysen Jensen (1928), which 

 show that direct contact between the light and dark sides 

 of the coleoptile tip must be maintained in order to obtain 

 good phototropic curvature. Insertion of platinum foil 

 through the middle of the tip, perpendicular to the plane 

 of the beam of light, practically prevents curvature. 



The other effects listed above, however, can take place 

 at a single point, and each cell may react independently, its 

 reaction being determined by the amount of light it receives. 

 These reactions may be grouped together as light-growth 

 reactions, and as such have been studied intensively by 

 Blaauw (1914, 1915, 1918) and successors (Vogt, 1915; 

 Sierp, 1918, 1921; Koningsberger, 1922; Van Dillewijn, 1927), 

 and in Phycomyces by Castle (1930). In general, the origi- 

 nally constant growth rate of dark-adapted or dark-grown 

 plants changes after illumination. This light-growth reaction 

 in higher plants consists principally of a decrease in growth 

 rate, with a maximal retardation M to 13^ hours after il- 

 lumination, followed by an acceleration. When plants are 

 unilaterally illuminated it is evident that the sides towards 

 and away from the light will receive different amounts of 

 light, and hence will give different light-growth reactions; 

 this will in itself cause phototropic curvatures. However, 

 the relative parts played in phototropic curvature by these 

 light-growth reactions and by the auxin redistribution re- 

 mains a point of controversy. 



