678 



LIGHT ABSORPTION BY PIGMENTS IN VIVO 



CHAP. 22 



ture, the over-all transmission may be much greater. Selenium barrier 

 layer photocells ("photronic cells"), because of their low sensitivity in the 

 infrared and in the far red, indicate, in direct sunlight, a transmission of 

 only 5 to 10%, depending on the thickness of the leaf (c/. Seybold 1932^, 

 and Egle 1937). 



The light absorption by a leaf depends on its thickness and the concen- 

 tration of the pigments. As mentioned above, fully green leaves transmit 

 and reflect only 10 or 15% of incident visible light, and absorb as much as 

 85 or 90%; on the other hand, green onion skins, one cell thick, transmit 

 85%, reflect 10% and absorb less than 5% (Seybold 19322). In chapter 15 



70 - 



I 80 



OC 

 O 

 </) 



5 90 



400 



500 600 



WAVE LENGTH, m/i 



700 



Fig. 22.3. Absorption spectra of a shade leaf and a sun leaf of Fagus sylvatica 

 (the shade leaf contains 50% more chlorophyll and 80% more carotenoids) (after 

 Seybold and Weissweiler 1943). 



(Vol. I) we described the adaptation of plants to the intensity and compo- 

 sition of the incident light. We found there that typical "shade plants" 

 contain two or three times more pigment than typical "sun plants." The 

 probable purpose of this "intensity adaptation" is to ensure an adequate 

 supply of energy to plants that grow in the shade, or to algae that live deep 

 under the sea, and, inversely, to prevent light injury to plants or algae ex- 

 posed to direct sunlight. 



There is no doubt that the presence of fucoxanthol or of the phycobilins 

 in brown, red and blue algae has considerable influence on the amount of 

 light adsorbed; this will be brought out in detail in part C. The effects of 

 variations in the concentration of chlorophyll are much smaller. Even 

 "light-green" plants absorb so large a proportion of incident light that a 

 doubling of their chlorophyll content can increase the absorption only com- 

 paratively little. Three examples can be found in our illustrations: Fig- 

 ure 22.3 shows the very slight enhancing effect that a 50% excess of chloro- 

 phyll and 80% excess of carotenoids have on the absorption of light by a 



