450 LIGHT AND LIFE 



tion, or in the dark, the peak position of the newly formed C,,682 

 shifts to 670 m^. Mature green plants and algae usually contain both 

 C,,682 and C,,670 in roughly equal amounts (11), giving an absorption 

 peak at 675-680 m^. 



The chemical nature of the different forms is not known, but 

 their simultaneous presence can be recognized from detailed analyses 

 of the shape of the red absorption band which, in live plants, is the 

 sum of several overlapping components. The forms of chlorophyll 

 a at present recognizable in the plant or in the water extracts of 

 chloroplast particles have their red absorption peak at about 670, 

 682, and 695 ni/x (3, 10, 11), but all these substances revert to 

 ordinary chlorophyll a on extraction with organic solvents. These vari- 

 ous forms of the natural holochrome are therefore different kinds of a 

 complex containing a single extractable chlorophyll. The diversity 

 of the spectra of live plants is due to the simultaneous presence of 

 these forms in differing proportions in vivo. The exact wavelength 

 of these forms may vary a few m^; a fine distinction of the peak posi- 

 tions of overlapping components better than ± 2 m^ is not yet 

 possible. 



Because these absorption bands overlap extensively, their presence 

 is hard to detect in absorption spectra as ordinarily measured. We 

 therefore measure the first derivative of absorbance with respect to 

 wavelength. These derivative spectra show the presence of the in- 

 dividual components by shoulders or extra peaks on the derivative 

 curves, as in Figs. 2 and 3. To get an approximate idea of the peak 

 positions and relative quantities of the components represented by 

 derivative spectra, we have attempted to match the observed spectra 

 by adding together derivatives of normal probability curves, as illus- 

 trated in Fig. 4. 



The forms Ca684 and C.,673 are those whose presence has long been 

 suspected, and which have been frequently used to explain various 

 phenomena (37, 41, 42, 43, 60). Their separate identity is clearly 

 shown by the characteristic shoulder near the peak absorption wave- 

 length in the derivative spectra of ChloreUa and other green algae. Fig. 

 2 (25, 26) . C,,684 is probably the major functional entity classically 

 known as chlorophyll a in vivo. C„673 may do the same job as 

 C„684 and also is suspected to function in part as does C,,650, as 

 we will see later. There is also evidence for less effective light utiliza- 

 tion by C;.684 than by Q,673 in red algae (30, 33, 61) . 



The C„695 form found in weak-light cultures of Euglenn (25) also 

 appears in small amounts in the detailed analysis of many plant 



