GROWTH SUBSTANCES FOR PHOTOTROPISM 147 



effect of light upon the rate of growth in the coleoptile and con- 

 chided that the Blaauw theory is an adequate explanation of the 

 observed light-growth response. Brauner (1922) also found a 

 general agreement between the course of phototropic curvature 

 and the light-growth reaction for small amounts of light, thus 

 supporting the validity of the Blaauw theory (also see Brauner, 

 1927a). In these experiments, however, the distribution of light 

 in the coleoptile was not taken into consideration; therefore, 

 these attempts to derive the laws of phototropic curvature from 

 the light-growth reaction of the front side alone appear to be 

 illogical. Evidence that might favor the Blaauw theory is to be 

 found in the work of Bergann (1930). He observed that the 

 order of phototropic effectiveness dechnes from the blue to the 

 orange regions of the spectrum in the same way as does the light- 

 growth response, which consists of a depression of the growth 

 rate. Van Dillewijn (1927a) made quantitative determinations 

 of the course of the light -growth reaction on the front and back 

 sides of the coleoptile and attempted to describe the course of the 

 phototropic curvature. In van Dillewijn's computations, the 

 light-growth reactions for different amounts of light were deter- 

 mined on the assumption that the illumination of the dark side 

 was about one-thirtieth of that of the light side. Computations 

 of growth curvatures for various methods of illumination yielded 

 good qualitative agreement between the experimental results and 

 what might have been expected on the basis of Blaauw's theory. 



The weakness in van Dillewijn's reasoning is the fact that he 

 did not actually observe the phototropic curvatures computed 

 from the light-growth reactions but compared the latter with the 

 curvatures obtained by Arisz. When an accurate quantitative 

 comparison is made between the phototropic curvatures which 

 are to be expected from light-growth reactions and those actually 

 appearing, no agreement between light-growth reaction and 

 curvature can be demonstrated. 



Evidence Opposing the Blaauw Theory. — Evidence in opposition 

 to the Blaauw theory has been obtained by Lundegardh (1922), 

 von Guttcnberg (1922), Pisek (1926, 1928), Beyer (1927c, 19286), 

 Went (1928a), Boysen Jensen (1928), du Buy (1933), and others. 

 Pisek determined the difference in the growth reactions of coleop- 

 tiles subjected to intensities of 2.5 and 80 meter-candle seconds 

 and at the same time measured the difference in length between 



