C. S. IRENCH 471 



(iO. ^Vitt. H. T., Moraw, R., Miiller, A., and Rumberg. B.. Z. physik. Chem. 



N. F. 23, 133-138 (1960). 

 (il. Vociim. C. S.. ami Blinks. 1.. R.. ./. i.cn. I'hysioL. 41. 11131117 (I9.')8). 



DISC:USSI()N 



Dr. Frinch: I woukl like to say that it is nice to measure the action 

 spectrum lor the other one of tlic pair of tlie mutually enhancing pigments 

 by doing it the other way around, set on 670 m^ and then vary the long 

 wavelength beam. 



Dr. RABiNowrrcii: 1 liiink you are too modest when you say don't know 

 which yield is affected. On the one side you have a wavelength which by 

 itself gives normal high yield, on the other side a wavelength which gives 

 a subnormal yield, with a quantinn requirement of 20 or more. I don't 

 think there is any douljt that what is happening upon combination of the 

 two wavelengths is improxement of activity in the far red. 



Dr. French: I still like to look at it as a balanced affair. There is no 

 real reason for this opinion. 



Dr. Franck: The new results on the Emerson effect presented by Drs. 

 French and Rabinowitch are very interesting. So far as I can tell without 

 more careful studies, they are in perfect agreement with an interpretation 

 of the Emerson effect given by me in a paper (Proc. Nad. Acad. Sci. U.S. -^-t, 

 941, 1958). It astonished me that no attempt was reported of comparing 

 these results with the theory mentioned. In case our explanation is unac- 

 ceptable to the speakers, they should have mentioned their objections. The 

 whole purpose of this meeting is airing of differences of opinions and at- 

 tempts to straighten them out. Since I have neither the intention, nor the 

 time to discuss the theory in any detail, I mention only its basis and refer 

 those who are interested to the published paper and to a supplement I might 

 write to include new results on chlorophyll in vitro and vivo. A comparison 

 of the spectroscopic results on chlorophyll in vitro with those in vivo, is the 

 fundament of our interpretation of the Emerson effect. Chlorophyll a pos- 

 •sesses two sets of electronic transitions which lie close together: the so-called 

 IT ^ TT* transitions and n -> n* transitions. The relative energetic position of 

 the two sets of levels depends on presence or absence of water, as absorption 

 spectra, fluorescence observations, etc., reveal. There can be no doubt that 

 both types of electronic transitions appear in chlorophyll a in vivo, too. The 

 structure of the chloroplasls is responsible for certain differences. One of 

 them is that in vivo the bulk of chlorophyll a, sandwiched between lipid and 

 protein layers, is protected against fluid water and its solutes, and has, de- 

 pendent upon the cell material, to a limited and varying degree access to 

 single water molecules. Only a small percentage is exposed to liquid water 

 and its solutes. These exposed chlorophylls receive their excitation energy 

 from the protected ones by resonance transfer, or by transfer from excited ac- 

 cessory pigments. It depends upon the wavelengths of the irradiated light, 



