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RADIATION BIOLOGY 



20 sec. As Darwin (1880) had already noticed, the zone of curvature 

 does not have to be illuminated, since it is primarily the tip of the cole- 

 optile which perceives the light stimulus. Therefore perception and 

 reaction in phototropism are separated both in space and in time; the 

 intervening processes of transmission of the stimulus account for this. 



The quantitative aspects of phototropism were simultaneously eluci- 

 dated by Blaauw (1909) and Froschel (1908), both of whom found that 

 minimal curvature is produced by a definite amount of light energy. 

 Blaauw showed this amount of light to be about 20 m-c-sec, irrespective 

 of the light intensity. This is shown in Table 9-1. Thus the product 

 law of phototropism was established, which states that the response to 



Table 9-1. The Relation between Light Intensity and Duration of 

 Exposure Necessary to Give a Just-visible Curvature in 



Avena Coleoptile 

 (Blaauw, 1909.) 



unilateral light is a function of the product of light intensity and duration 

 of illumination. Further work, especially by Arisz (1915), showed that 

 curvature was a periodic function of the light energy. Between 4 and 

 400 m-c-sec the phototropic curvature increases with increasing amount 

 of illumination, and beyond that the curvature decreases with increasing 

 Hght until a total light energy of 2000-4000 m-c-sec is reached. Between 

 2000-4000 and 20,000 40,000 m-c-sec negative curvatures may be pro- 

 duced. At still greater amounts of light a so-called "second positive 

 curvature" occurs. Du Buy and Nuernbergk (1932) showed that at 

 around 1,000,000 m-c-sec there was another region of light energy where 

 the plants responded only slightly or not at all, but beyond that higher 

 energies produced a third positive curvature. 



In stimulation reactions it was found that the law of Weber-Fechner 

 held. This law states that, when an organism is subjected to two stimuli 

 of different intensity, the response is not due to the absolute difference 

 in intensity but is proportional to the ratio between the two stimuli. 

 Thus, when an Avena coleoptile is illuminated from opposite sides by 



