TRUE ABSORPTION SPECTRUM 709 



the cyanobilin-proteid becomes dissociated from the pigment-protein- 

 Upide complex present in the Hve cell. It was stated in chapter 15 (Vol. 

 I, page 399) that, of all plastid pigments, only the phycobilin chromopro- 

 teids pass into true colloidal solution upon extraction with water. 



To sum up, it is certain that all the pigments found in extracts from 

 plants retain their spectroscopic identity in live cells, despite their prob- 

 able close association in a common complex. The association causes, 

 however, considerable band shifts, and probably also changes the shape 

 of the bands, particularly those of accessory pigments, such as fucoxanthol. 



4. True Absorption Spectrum of the Pigment Mixture in the Living Cell 



In the two preceding sections, we tried to derive as much information as 

 possible from the positions of the absorption maxima in the empirical plant 

 spectra described and reproduced in section 1. It was mentioned re- 

 peatedly that the difference between the "plant spectra" and the spectra of 

 extracted pigments is not limited to band shifts, but includes also changes 

 in the height, width and shape of the individual bands. However, as 

 stated on page G97, the latter changes are, to a large extent, the product of 

 scattering and other geometrical-optical phenomena. In the present sec- 

 tion, we will deal first with a more detailed description of the appearance of 

 the absorption bands in living plant cells, and then with the possibility of 

 deriving from the empirical plant spectra the true absorption curves of the 

 pigment mixtures contained in them. 



General diffuseness was stated on page 697 to be the most striking char- 

 acteristic of plant spectra as compared to the absorption spectra of the 

 pigment extracts. The ratios of the "optical densities," log (I/S) or log 

 (To/T) in the absorption "peaks" and "valleys" can serve to illustrate this 

 statement. 



Table 22.Vn shows that the ratio of the optical densities in the "green 

 minimum" and the "red maximum," which, according to Table 21. IB is 

 less than 0.01 in pure chlorophyll a, and about 0.05 in an ether extract 

 from barley leaves (which contains all the chloroplast pigments), is as high 

 as 0.3 in live algae and may reach 0.6 in green leaves. 



The ratio (violet peak: red peak) also is changed: it declines from 1.75 

 in ethereal extract, to 1.6 in colloidal aqueous extracts, 1.4 to 1.5 in hve 

 algae, and 0.9 to 1.2 in green leaves. 



It ^^•ill be noted that the ratios derived from true absorption spectra, 

 log (I/S), are not very different from those derived from the transmission 



