1074 



BIOLOGICAL EFFECTS OF RADIATION 



Blaauw (5) the curvature of a plant resulting from unilateral illumination 

 is caused by the light-growth responses of the opposite sides which are 

 differently illuminated. The minimum amount of radiation required to 

 produce phototropism is 20 meter-candle-seconds. It also appears that 



the product of light intensity and 

 time of exposure is a constant.^ 



80 M.C. 

 2,400M.C. 



LIGHT-GROWTH RESPONSE 



Blaauw pointed out the fact that 

 plant organs which show phototropic 

 curvatures also show typical light- 

 growth responses and where no re- 

 sponse is observed neither is there 

 phototropic curvature . D i 1 1 e w i j n 

 (17) reasoned that, "If the theory of 

 Blaauw is correct, it must be possible 

 to deduce the phototropical curva- 

 tures from the light-growth response 

 of proximal and distal sides." Using 

 Avena saliva, he determined the light- 

 growth response to several different 

 quantities of light and also to one- 

 FiG. 1.— Graphs showing growth of thirtieth of thcse quantities which 



Avena saliva. The abscissa represents , „ i i i- i • 



time in hours and the ordinate rate of he figured to be the light mtensity 



growth in microns per minute. The in- received on the distal sidc in com- 



tensity of hght is indicated in meter- . • i i • i 



candles. The light lines represent the parison With that received on the 



growthresponseforJ.^0 the light intensity proximal side. This value is greater 



of the response represented by the heavy . . <• rr 7 • 7 1 • 



lines. The arrow indicates the moment lOr Avena than ior Helianthus and IS 



of illumination with the duration marked y^ry largely due to the light absorbed 



above. The predicted phototropic re- . 1 /• -\r v 1 



sponse is indicated by plus and minus by the primary leaf. More light 

 signs. {From Diiiewijn, 17.) passes through the upper 0.5 mm., 



but here the light distribution is complicated by the shape of the tip. 



He assumes the 30:1 ratio for the whole coleoptile. 



Diiiewijn made use of the auxanometer of Koningsberger (26) for his 



growth measurements. The light source was above the plant and was 



reflected upon the seedling by three mirrors. Directly over the plant a 



circular cardboard disk was placed for protection against the direct rays. 



The results of his experiments on growth are reproduced in the four pairs 



of curves of Fig. 1. The abscissa represents time in hours and the 



ordinate the rate of growth (microns per min.). The light intensity is 



' Parr (33) discusses briefly the data of many of the early phototropic experiments 

 under the four general theories of intensity, ray direction, wave-length, and energy. 

 A more extensive treatment covering the historical review of phototropism in general 

 with its development and a critical discussion of the early theories has been published 

 by Mast (Mast, S. O. Light and the behavior of organisms. New York, 1911). 



