686 



LIGHT ABSORPTION BY PIGMENTS IiV VIVO 



CHAP. 22 



cells bleached with bromine — does not appear reliable, since bleaching by bromine may 

 not be restricted to plastid pigments. 



It was suggested that the proportion of the total light absorption by leaves due to 

 plastid pigments can be determined by extracting the latter and comparing the absorp- 

 tion of the extract with that of the original leaf. Timiriazev (1885) and Lazarev (1924, 

 1927) based this suggestion on the assumption that the absorption of white light by ex- 

 tracted pigments is about equal to the absorption by the same pigments in the leaf. 

 However, light scattering, "sieve effect" and band shifts can make the two magnitudes 

 quite different. True, some of these factors enhance the absorption, while others 

 weaken it, but it would be an unusual coincidence if the net result were exactly nil. 

 Experimentally, Noddack and Eichhoff (1939) found that the increased absorption by 

 live Chlorella cells in the far red almost exactly compensates for the decreased absorption 

 in the region 520-680 m/x (c/. fig. 22.21); but Seybold and Weissweiler (1942) could not 

 confirm this result, and found, to the contrary, that throughout the visible spectrum the 

 absorption by live cells is more complete than that by the extracts. 



B. Spectral Properties of Plants* 

 1. Empirical Plant Spectra 



It is clear from the preceding discussion that what is usually called a 

 "leaf spectrum," or even a "spectrum of a cell suspension," is not the true 

 spectrum of the pigments contained in these materials (meaning by "true 

 spectrum" the plot of the absorption coefficient of the pigment mixture 



400 



700 



500 600 



WAVE LENGTH, m>i 



Fig. 22.11. Transmission and reflection spectrum of a leaf of Parie- 

 taria officinalis (after Seybold and Weissweiler 1942). 



* Bibliography, page 737. 



