130 THE BIOLOGY OF FLOWERING PLANTS 



This is of great interest in relation to the kind of light 

 which is at the disposal of the plant. Stahl (1909), in a 

 stimulating book, has treated of the question of the relation 

 of the colour of the pigment to the quality of light 

 available. 



The green colour is very constant, the quality of light is 

 not. Every one who has taken photographs knows the 

 difference in the quaHty of sunlight at noon and in the 

 evening, and the much longer exposure which is required 

 later in the day. As the sun nears the horizon its rays 

 pass through a much thicker layer of atmosphere, through 

 gas molecules, minute globules of water, and flying dust. 

 These interfere with the short wave-length blue rays much 

 more than with the longer wave-length red rays. The 

 blue rays are more extensively scattered, and the red rays 

 pass on in greater proportion. The thicker the atmospheric 

 layer the greater is the loss of blue light, and so, in the 

 evening, the sunlight is relatively much less active on a 

 photographic plate. The extensive scattering of blue rays 

 in the atmosphere is responsible for the blue colour of the 

 sky and of the diffuse light received from it, for the blue 

 of hazy weather, and for the blue of distant hills, seen 

 through the atmosphere in which blue light is scattered. 

 Thus, with the sun at its zenith, the intensity of radia- 

 tion at the A line in the red is 1*28 times that at the 

 F line in the blue ; with the sun at 1 1 degrees above the 

 horizon the total light intensity has fallen to one-half, and 

 the intensity at the A line is 3*5 times that at the F line. 

 In diffuse light from the sky the intensity at the F line is 

 6 times that at the A line, 4 times that at the B line, and 

 3" 5 times that at the C Hne. 



The relative intensities of rays of different wave lengths 

 which the plant receives from the sun thus change con- 

 tinually throughout the day. But, as we have insisted, the 

 plant receives not only sunlight ; it often depends mainly 

 on diffuse light. A Virginia creeper on the north wall of 

 a house may, in the brightest weather, receive only light 

 from the sky. This light, as well as diffuse light in the 



