GROWTH SUBSTANCES FOR PHOTOTROPISM 149 



criticism of the matter will be mentioned later. Cholodny 

 (193 Ir/, 19326, 1933a) has pointed out additional discrepancies 

 between the light-growth reaction and phototropism. He 

 demonstrated that coleoptiles, immersed in water, can curve 

 phototropically with very brief periods of illumination, without 

 showing any kind of light-growth reaction. 



In an entirely different way, Boysen Jensen (1928) showed the 

 inadequacy of Blaauw's theory for phototropism. The coleop- 

 tile was split lengthwise, and a rectangular platinum plate was 

 inserted in such a way as to divide the organ into two halves, so 

 that each could be illuminated by itself. If Blaauw's theory were 

 applicable, a phototropic curvature should have been produced by 

 suitably decreasing both light sources. The front half was 

 illuminated with 51, the back half with 0.9, 1.6, and 2.6 meter- 

 candles, so that the light decrease from the front to the back was 

 about 51:1, 32:1, and 20:1. Only minimal phototropic curva- 

 tures were obtained. In other experiments, a glass plate was 

 inserted instead of a platinum one, and the tip was illuminated 

 unilaterally at right angles to the plate. Here, again, only a 

 minimal phototropic curvature resulted, although the distribution 

 of light was the same as in intact plants. These experiments 

 show plainly that the phototropic curvature does not come about 

 through separate reactions of the single parts of the tip. 



To test the validity of Blaauw's theory, Li (1934) conducted 

 experiments with decapitated Avena coleoptiles which were 

 exposed to different amounts of unilateral light. It was found 

 that immediately following decapitation, short exposures of 

 10 minutes even at 28,800 meter-candle seconds were incapable 

 of inducing bending, while in exposures of 30 minutes curvature 

 was caused by 3,600 meter-candle seconds. When the exposure 

 was extended to 3 hours, even an intensity of 100 meter-candle 

 seconds could elicit a response. It is quite clear that Blaauw's 

 theory does not apply to decapitated coleoptiles. 



In many cases, a rather far-reaching parallel can be found 

 between the light-growth reaction and the course of phototropic 

 curvature. By a method of compensation whereby the coleop- 

 tile is bilaterally illuminated, the stimulus values of different 

 regions of the spectrum were obtained (Bergann, 1930). It 

 was found "that illumination from all regions of the spectrum 

 (except red and infrared), when applied in corresponding intensi- 



