350 LIGHT AND LIFE 



required in order to produce the unpaired electron, and that this may 

 occur either by resonance transfer of exciton energy amongst the 

 chlorophyll molecules until it comes to molecules so situated that 

 this state may be excited, or by direct excitation of this state by 

 absorption, as presumably we have done when we examined the ac- 

 tion spectrum for the production of unpaired spins. Failure to ob- 

 serve a distinct absorption peak at this point would have to be 

 accounted for. What the nature of this emitting state is remains 

 to be seen. It could be, of course, that this state is one from which 

 an electron transfer to a certain low-lying acceptor occurs. Energy 

 absorbed in the 6800 A state might be degraded to this emitting state 

 to produce the same electron transfer, or might be used directly from 

 the higher energy state to transfer an electron to a somewhat higher- 

 lying acceptor. 



It should be noted that this would, in fact, correspond to two 

 different types of primary quantum conversion processes. Such an 

 idea has already appeared in the work of Emerson (25) , which has 

 since been explored further by French and Myers (9, 57) . Emerson 

 had observed that the apparent long-wave limit for photosynthesis 

 was shifted to still longer wavelengths (somewhat beyond 7000 A) 

 if light of shorter wavelength (around 6500 A) was also present. A 

 further examination of this effect by Myers and French seemed to 

 confirm the suggestion that the quantum yield of an increment of 

 7000 A light is greater when light of shorter wavelength (around 

 6500 A) is also impinging than when it is not. In addition to this, 

 Myers, following Blinks (9) , observed a number of transients in 

 changing from one wavelength of light to another, transients best 

 interpreted in terms of the requirement for the collaboration of two 

 different products resulting from two different quantum conversions, 

 one in the region of 6500 A and another in the region of 7000 A. 



It is tempting to suggest, following the analogy of the phthalo- 

 cyanine model experiments described earlier, that corresponding to 

 the two model pigments, we have present in the chloroplast both 

 chlorophyll and plastoquinone (8, 20, 48) . The plastoquinone in 

 this case would not have a sufficiently low-lying orbital to act as 

 acceptor to the chlorophyll in its ground state, but could accept an 

 electron from chlorophyll brought to an excited state by illumination, 

 corresponding to the 7200 A emission. The transfer of a second elec- 

 tron to the quinone negative ion radical thus jModuced would re- 

 quire the excitation of the chlorophyll to a somewhat higher state, 

 which could result from absorption at 6800 A or shorter wavelength. 



