478 LIGHT AND LIFE 



is possible. One would then expect the yield either to become con- 

 stant, or to decline to zero, depending on whether the light absorbed 

 only by chlorophyll a (or, more precisely, the form of chlorophyll a 

 responsible for the absorption in this region) does or does not produce 

 by itself a finite yield of photosynthesis. The curve in Fig. 1 suggests 

 that the latter is the case. 



However, according to a suggestion made by Dr. E. I. Rabinowitch, 

 the situation may be more complex. The measurements by Brody and 

 Emerson (2) (and confirmed by Govindjee) suggest that c^ remains 

 measurable in the narrow spectral bands centered at 695 m^ and even 

 700 vUfx.. Why, then, should there be no photosynthesis at all in the 

 "5% band" which extends from 680 m^ to longer waves? Perhaps 

 there exists in photosynthesis, as in many other photobiological 

 phenomena, a "counter-effect" caused by extreme red light, which 

 deactivates some of the intermediates in photosynthesis. The amount 

 of this light in the "5% band" may be sufficient to reduce the photo- 

 synthesis to zero. Further experiments are in progress to check this 

 tentative suggestion. 



Summary 



The relation between quantum yield and white light from which 

 the wavelengths predominantly absorbed by both phycobilins were 

 removed was established for the red alga Porphyridiujn cruentum. 

 It proved possible to obtain incident light of such a spectral composi- 

 tion that no photosynthesis could be measured in it at all, although 

 a significant part of it was absorbed, and while the Emerson effect still 

 occurred. 



REFERENCES 



1. Brody, iM., and Emerson, R., Am. J. Botany, 46, 433-440 (19r)9). 



2. Brody, M., and Emerson, R., /. Geu. Physiol., 43. 251-264 (19.")9). 



3. Emerson, R., Ann. Rev. Plant Physiol., 9. 1-24 (1958). 



4. Emerson, R., and Chalmers, R., Plant Phy.uol.. 30, 504-529 (1955). 



5. Emcrs(m. R.. and (.lialmers, R. V., Phycol. Soc. Am. News Hull.. 11. 51-56 (1958). 



6. Emerson, R., Clhalmers, R., and (ledcrstrand, C., Proc. \at. Acad. Sri. U. S.. 



43, 113-143 (1957). 



7. Emerson, R., Chalmers, R., Cederstrand, C, and Brody, M., Science, 123, 



673 (1956). 



8. Emerson. R., and Lewis, C. M., Am. J. liolnnv. .30. 165-178 (1943). 



9. Haxo, v., and Blinks, L. R., ./. Gen. Ph\.siol.,3S. 389-422 (1950). 



