SPECTRA OF PHOTOSYNTHETIC PIGMENTS 349 



plants affects their distribution in nature, (2) to determine the efficiency 

 of light utilization in photosynthesis, (3) to determine which pigments 

 are present in various organisms, (4) to attempt to characterize the physi- 

 cal and chemical state of the pigments in living organisms, (5) to measure 

 the concentrations of pigments, (6) to make comparisons with action 

 spectra in order to find which pigments utihze the light they absorb for 

 photobiological processes, (7) to provide information for the art of camou- 

 flage. We will emphasize particularly those absorption spectra which 

 relate to the interpretation of action spectra. Inasmuch as the absorp- 

 tion spectra of the individual components in vivo are only very crudely 

 measurable, the absorption spectra of the pigments in organic solvents 

 are of great value for this purpose. 



One of the earlier reasons for interest in the absorption spectra of 

 leaves and of algae was primarily ecological. The questions from this 

 point of view are: How efficient are leaves as light traps? Does the 

 efficiency of light absorption by a leaf account for the death or survival 

 of a plant in nature under light-limiting conditions? For ecological pur- 

 poses the absorption of light by leaves can be summarized, at least to 

 the satisfaction of nonecologists, by the statement that ordinary green 

 leaves absorb 75-90 per cent of the light in the red or blue part of the 

 spectrum and that only very pale leaves absorb less than 50 per cent of 

 the green light, where the absorption is least. Very dark leaves, such as 

 those of Ficus, show 90-95 per cent absorption throughout the visible 

 spectrum. Differences of this magnitude between various leaves seem 

 to have little ecological significance. Since the rate of photosynthesis in 

 nature may more often be dependent on carbon dioxide availability than 

 on absorption of an adequate amount of light, it appears that the pri- 

 mary problem in the design of an efficient photosynthetic plant would 

 probably be to provide for efficient carbon dioxide absorption from large 

 volumes of air rather than for covering a large area with light-absorbing 

 material. 



Although the positions of the peaks of the absorption spectra of some 

 pigments in leaves can be measured with fair precision, the same does 

 not, however, apply to determinations of the true absorption as a func- 

 tion of wave length for the individual pigments in their native state. 

 The difficulties are due to several causes. In the first place, there is a 

 good deal of general absorption throughout the spectrum in leaves by 

 what are often most unfortunately called "colorless components." This 

 term would perhaps better be replaced by something more realistic, such 

 as "residual brown matter." This probably includes well-known sub- 

 stances, such as flavones, absorbing primarily in the blue, as well as other 

 colored materials. Another serious difficulty in the quantitative esti- 

 mation of absorption spectra of individual pigments in leaves is the fact 

 that the total pigment content is generally very high. This leads to 



