SPECTRA OF PHOTOSYNTHETIC PIGMENTS 345 



been sought by investigating the abihty of hght absorbed by these other 

 pigments to excite the fluorescence of chlorophyll. This involves the 

 determination of effectiveness spectra for the excitation of chlorophyll 

 fluorescence in various organisms and the correlation of these spectra 

 with the absorption spectra of the individual pigments. Experiments of 

 this type have indeed shown that energy transfer takes place from acces- 

 sory pigments to chlorophyll. This subject was reviewed recently by 

 Smith (1949), but new publications have appeared since then, permitting 

 a more quantitative treatment of results. Much of the research on action 

 spectra has yielded results which are not sufficiently precise to permit 

 quantitative handling of data and which have consequently been omitted 

 irom, this article. One of the main technical weaknesses has been the 

 use of light that was not sufficiently monochromatic. There are a num- 

 ber of physical and biological sources of errors that may enter into the 

 interpretation of action spectra in living organisms. These have been 

 discussed by Blum (1950). Factors that have hindered the interpre- 

 tation of data on photosynthesis are (1) difficulty in estimating the true 

 absorption, corrected for scattering of light, in living organisms; (2) 

 unavailability of absorption spectra for the individual components in vivo; 

 (3) difficulty in estimating the internal filtering effect of light by non- 

 participating components that absorb selectively with respect to wave 

 length; and (4) inadequate knowledge concerning the dependence of 

 quantum yield upon wave length for the individual pigments. 



The best available data have been interpreted in view of these limi- 

 tations and have been presented here to show the correlations that have 

 been made between action, absorption, and fluorescence spectra in various 

 photosynthetic organisms. The means by which these correlations have 

 been made are presented to show how these procedures may be applied 

 to other biological processes in the study of mechanisms of various photo- 

 biological effects. Owing to the fundamental importance and wide dis- 

 tribution of the chloroplast pigments, the correlation betw^een effective- 

 ness and the absorption spectra of their pigments has been worked out in 

 some detail. 



Since consideration of action spectra must be prefaced by a knowledge 

 of the absorption spectra of living organisms and also of extracted pig- 

 ments, this information is presented first. 



Because the photosynthetic process and the pigments of bacteria are 

 different in some major respects from those of higher plants, the bacteria 

 have been treated in a separate section at the end of the chapter. 

 - Graphical Presentation of Spectroscopic Data. Often for very good 

 reasons the same type of data is presented by different authors in very 

 different ways. Thus measurements of the absorption spectra of solu- 

 tions may be plotted with abscissas of w^ave-length units, wave-number 

 units, frequency units, or electron volts. The ordinates of these curves 



