LIGHT ABSORPTION 131 



shade and from clouds, is relatively richer in blue rays. 

 Not only is this the case, but the plant growing in the open 

 receives, as well as sunlight, light from the sky. As Black- 

 man and Matthaei (1905) have shown, the " sky " light is a 

 very important fraction of the whole. In a horizontal leaf 

 illuminated by the sun and by a cloudless sky, the ratio of 

 sunlight to " sky " light is, with the sun at 60 degrees 

 elevation, 1-58, with the sun at 45 degrees, 0-83, and at 

 15 degrees, 0-24. The plant growing in the open, even 

 more than the shade plant, is subject to illumination 

 varying constantly not only in strength but in quality, and 

 like the shade plant it depends very largely on diffuse 

 light. 



Now, as Stahl points out, the green leaf pigment is able 

 to utilise both blue and red light, as it absorbs both very 

 markedly. It is unnecessary to make the carotin and 

 xanthophyll responsible for the absorption of the blue rays, 

 as Stahl does, for the chlorophyll itself shows, as we have 

 seen, strong absorption of the shorter wave lengths, as well 

 as very strong absorption of the orange-red. The question 

 might be asked, Would not a grey or black leaf absorbing all 

 light be even more efficient ? Stahl denies this, for the 

 transmission of the extreme red and ultra red, and of the 

 yellow and green, is advantageous, since the danger of over- 

 heating which would occur if these rays, so strong in direct 

 sunlight, were absorbed, is partly obviated. This is a good 

 general explanation ; though the most strongly heating 

 red rays are in fact absorbed, the transmission of a part of 

 the radiant energy must lessen the danger. 



Wiesner (1907) has criticised Stahl 's views. He argues 

 that, if chlorophyll is so closely related to the colour of the 

 light, we should find variations in its tone corresponding to 

 the different qualities of light falling on plants in such 

 different situations as an exposed mountain top, a steppe, a 

 meadow, or a woodland, and that such differences are not 

 in general to be observed. But this criticism is not well 

 founded. For the point is just that chlorophyll represents a 

 stable compromise which can make the best of all sorts of 



