APPORTIONMENT OF ENERGY TO CHLOROPHYLLS 719 



Even after the average absorption curve of the pigment mixture has been 

 determined, this will not give us the desired knowledge of the absorption 

 curves of the individual pigments; but it will be a step in the right direc- 

 tion : Some sections of this average absorption curve will be due to a single 

 pigment or a small group of related pigments (e. g., the part above 550 mM 

 in green plants, to chlorophylls a and h, and in brown algae, to chlorophylls 

 a and c). The changes in shapes of the absorption bands, found in these 

 regions, may be considered, by analogy, as valid also for the bands of the 

 same pigments in the regions of composite absorption. (However, the dif- 

 ferent polarizabilities of a molecule in different electronic states, and the 

 possibility of resonance effects between molecules with overlapping absorp- 

 tion bands call for caution in the use of such analogies.) 



By constructing true absorption curves of cells or plastids with var>ang 

 contents of the individual pigments, one can hope to assemble material 

 whose analysis will permit derivation of the absorption curves of the indi- 

 vidual components. (Here, too, caution will be needed because resonance 

 phenomena may destroy simple additivity of absorption coefficients.) 



In the light of these considerations, all attempts undertaken so far to 

 apportion the light energy absorbed by plants, among individual pigments, 

 are but first crude approximations. In some of these studies, the analysis 

 was made entirely on the basis of comparison of the spectra of extracts 

 with those of the solutions of separated pigments. In others, a certain 

 improvement was achieved by assuming that all bands were shifted in vivo 

 by the same amount, without change of shape. In a third group of inves- 

 tigations, the analysis was further improved by assuming individual values 

 for the shifts of the bands of different pigments; but still no attempt was 

 made to take into consideration the possible changes in shapes of the bands. 



3. Absorption by Chlorophylls a, b, c and d 



All absorption of light by chlorophyllous plants or plant organs at 

 wave lengths longer than 550 m/x can be attributed to the chlorophylls, 

 except in red and blue algae, where phycobilins may absorb light up to 650 

 or 700 mfji {cf. figs. 21.39 and 21.40), and purple bacteria, which contain 

 carotenoids with absorption maxima at 550-570 m/x {cj. fig. 22.27 and Table 

 21.VIII). 



No serious attempts have been made to apportion the absorption be- 

 tween chlorophylls a and h. A crude idea of this distribution in an ex- 

 tract from a green alga is given by INIontfort's figure (fig. 22.39). (It will 

 be noted that the abscissae in this and other figures of Montfort and Sey- 

 bold are absolute values of absorption in each separate solution, not per- 

 centages of the total absorption by the mixture, and thus do not add to 



