732 



LIGHT ABSORPTION BY PIGMENTS IN VIVO 



CHAP. 22 



the light intensity can drop to as low as 0.02 cal./cm.'- min., or even less, 

 i. e., to only 1% of full sunshine. At the same time, the spectral composi- 

 tion changes in the way characterized by figure 22.52, taken from a paper 

 by Taylor and Kerr (1941); the light intensity becomes almost uniform 

 throughout the visible spectrum, and the common designation of such 

 days as "gray" proves to be correct. 



400 440 480 520 560 600 640 680 

 WAVE LENGTH, m/i 



Fig. 22.52. Average energy distribution 

 curves for daylight (after Taylor and Kerr 

 1941). (A) Zenith sky, color temperature 

 13,700° K.; (B) north sky on 45° plane, 

 color temperature 10,000° K.; (C) totally 

 overcast sky, color temperature 6,500° K. ; 

 (D) sun plus sky on horizontal plane, color 

 temperature 6000 ° K. ; (E) direct sunlight, 

 color temperature 5335° K. 



400 500 600 700 



WAVE LENGTH, m/i 



Fig. 22.53. Energy distribu- 

 tion in the shade (after Seybold 

 1936). (G) Edge of wood (black 

 square = 2.5 X 10 "^ cal. per 

 (cm.2 min.); (1), (2), (3) three 

 shade habitats of Oxalis (blaf;k 

 square = 0.025 X 10 "^ cal. per 

 (cm.^ min.). 



When the plant is in the shadow of a rock, house or mountain, and re- 

 ceives light mainly from the blue sky ("blue shade"), the spectral composi- 

 tion of its light field is entirely different from that to which it is exposed in 

 direct sunlight, as shown by figure 22.52. The intensity of radiation from 

 a clear blue sky is of the order of 20% of that of full sunlight (i. e., about 

 0.1 cal./cm.2 jj^jn ) at sea level. It decreases mth increasing altitude, 

 as the scattering air layer above becomes thinner and the color of the sky 



a deeper blue. 



The plants that live in the shadow of other plants, e. g., the floor vege- 

 tation in the forest, receive their light filtered through the chlorophyll layers 



