BENT LEY GLASS 8G9 



systematically \aiicil in wavelength. The preceding explanation of 

 the action ol the accessory pigments leaves luiexplained the parallel- 

 ism bet^\•een the action spectra of photosynthesis and the fluores- 

 cence of chlorophyll a. Rabinowitch and Govindjee, like French 

 and his colleagues, have foiuul clear-cut evidence of a form of chloro- 

 phyll a absorbing maximally at 670 ni/^, not only in Clilorella but 

 also in the diatom Naxncula, which possesses fucoxanthol (peak ab- 

 sorj)tion, 535 ni/x) and chlorophyll c (peak at 630 ni/x), but which lacks 

 chlorophyll b. In the blue-green alga Auacystis, Emerson found no 

 (]„670; in fact, a negative region was present at this point in the 

 action spectrum when monochromatic light at 670 m^ was combined 

 with light at 685-700 ni/x. In other words, not only is there no 0^670 

 to activate the eliect of photosynthesis at 695 n\fx, but the main 

 chlorophyll component in Anacystis, Co680, not only fails to enhance 

 the photociiemical action of C269O but actually counteracts it. This 

 may be a saturation effect. Owing to the lower saturation levels in 

 the far-red region, the combination of two monochromatic beams 

 may throw the process into the saturated region and consequently 

 looser the cjuantum yield to a level below that of either beam alone. 

 Further evidence of the several forms of chlorophyll is to be seen 

 in the differential reversible bleaching of Ca670, Ca680, and 0^690 by 

 strong illumination. Ca680 is considerably more sensitive, in Clilorella 

 cells, than the t\vo other forms. 



Rabinowitch suggests, as an alternative to the scheme proposed by 

 French, that the primary photochemical process may consist of two 

 steps, only one of which may l^e effected by excited Ca690, whereas 

 C.,670 can sensitize botli of them. This would fit well with Franck's 

 suggestion that Ca690 leads almost exclusively to the triplet state, 

 ^\"hile Ca670 leads to an excited singlet state with a long enough life 

 to emit fluorescence. Photosynthesis would then require coopera- 

 tion of a molecule in an excited singlet state with one in a triplet 

 state, though why that should be so remains an enigma. 



The quantum yield of chlorophyll a in the far-red region of the 

 spectrum has been measured by J. B. Thomas and Govindjee in the 

 red alga Porphyridium cruentum. These algae lack chlorophyll b, and 

 the phycocyanin absorption at 625 m^ and phycoerythrin absorption 

 at 500-565 ni/x was eliminated by passing white light through a series 

 of filters containing extracts of the phycobilin pigments. The quantum 

 yield of photosynthesis did not change until absorption by the phy- 

 cobilin filter surpassed 20%; then it decreased linearly until absorp- 

 tion reached 85 % ; finally, after an even sharper drop, photosynthesis 



