282 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



maximum with medium light intensities, and becomes less when the light in- 

 tensity is still further increased. It is due to unequal illumination of the two 

 sides of the sensitive region of the bending organ, and the difference in illumina- 

 tion between the two sides is of course generally greatest with medium intensities 

 of the light impinging on them. When the light upon one side of an organ is 

 very strong the tissues are penetrated and the cells on the opposite side receive 

 nearly as much illumination as do those on the directly illuminated side. It is 

 for this reason that phototropic bending is not frequent in plants growing in 

 intense sunshine, and this explains the retardation of the phototropic movement 

 in Tragopogon when exposed to intense lights 



The various wave-lengths of sunlight do not all have the same phototropic 

 influence upon plants, as is shown by the graphs of Fig. 132. In this figure the 

 letters at the base represent the positions of the Fraunhofer lines in the solar 

 spectrum, A, B, C, etc. The curve XY represents the comparative growth 



Fig. 132. — Graphs representing rates of growth and phototropic sensitiveness of plants in 



various wave-lengths of sunlight. 



rates of sunflower seedlings in the different regions of the spectrum, this rate being 

 highest at X and lowest at Y. Curve I represents the phototropic sensitiveness 

 of vetch seedlings, curve II that of cress seedlings, and curve III that of etiolated 

 willow shoots. 



In yellow light, about the D-line, no phototropic response is apparent. 

 With longer or shorter wave-lengths phototropism becomes evident, and the 

 sensitiveness of the plants becomes greater as the wave-length increases or di- 

 minishes. The rays of the shorter wave-lengths, in the right half of the spec- 

 trum, are the most effective to produce bending in all three cases, and etiolated 

 willow shoots fail to show any response to the long wave-lengths of the red 

 region. Thus, of the visible spectrum, violet light is most effective to produce 

 phototropic bending. 



Light retards plant elongation, as is clear from the daily periodicity of 



growth, but this retardation differs in amount with different wave-lengths. 



h Also, with strong illumination the light received by reflection from the sky, from sur- 

 rounding objects, etc., is comparatively intense. — Ed. 



