380 LIGHT AND LIFE 



is in itself insufficient to produce photosynthesis, and needs supplemen- 

 tation by the aljsorption of quanta by one of the accessory pigments. 

 This picture, too, seems consistent and plausible — but it fails to 

 explain the above-mentioned parallelism of the action spectra of 

 photosynthesis and of chlorophyll a fluorescence. Why should ex- 

 citation of chlorophyll a need supplementation by excitation of an 

 accessory pigment, if the latter excitation is transferred practically 

 quantitatively to chlorophyll «? 



Prior to Emerson's experiments on the red drop, Duysens (8) sug- 

 gested, as explanation of the apparent low photochemical efficiency 

 of chlorophyll a in red algae, the presence in these organisms of two 

 forms of chlorophyll a, one of which is preferentially coupled to the 

 accessory pigments. This form is fluorescent and photochemically ac- 

 tive, while the rest of chlorophyll a is non-fluorescent and photo- 

 chemically inactive. To account for the "red drop," it could be 

 further assumed that the two forms of chlorophyll have somewhat 

 displaced absorption bands. Specifically, it could be suggested that 

 the main red bands of both Chlorella and Porphyridium consist of 

 two superimposed component bands — one with a peak at about 670 

 n\fi ("Chi a 670"), which belongs to the photochemically active 

 form of chlorophyll a, and another with a peak at 690-700 m^ ("Chi 

 a 690") , which belongs to the photochemically inactive form of the 

 same pigment. To explain the action spectrum of chlorophyll fluores- 

 cence, one would naturally suggest that the Chi a 670 form is fluores- 

 cent and the Chi a 690 form, non-fluorescent. (However, there is 

 evidence (2) of a fluorescent band at 710-715 mfx probably associated 

 with the 695 xWfx absorption band.) It was suggested by Brody (2, 3) 

 that the difference in the location of the red drop in Chlorella (at 680 

 m/x) and in Porphyridium (at 650 m^) can be due simply to a much 

 larger proportion of the inactive form, Chi a 690, in the latter. How- 

 ever, this interpretation probably is insufficient. The shape of the 

 band is, in every case, suggestive of the presence of a third, and per- 

 haps most abundant component, with a peak near 680 m^x — "Chi a 

 680." This is particularly clear in French's "derivative spectra" (4, 

 16) . (He assigns positions at 673, 683, and 695 mfx to the three com- 

 ponent bands in Chlorella) . The question of the nature, the photo- 

 chemical role, and the fluorescence of Chi a 680 naturally poses it- 

 self and the answer to it is far from clear. We will return to this 

 subject in discussing the new experimental results presented in the 

 following section. 



