422 LIGHT AND LIFE 



the one at 555 iii/x) show up in fhishing as well as in weak continuous 

 illumination. Also an interpretation of the effects on the JKisis of induction 

 phenomena, as Dr. Franck suggests, I cannot cjuite see. The fast effect with 

 its very short lifetime we measured only by integration over many cycles. 

 Time response of the instrument was of the order of a second. Only the 

 positive effect may show induction and overshoot but both effects reach 

 a steady state in the light. 



Dr. C:ommo.\er: I would like to make a very brief extension of the earlier 

 remarks on electron spin resonance. Dr. Franck's remark may be very 

 pertinent. From our data, it would appear that the signal at g = 2.005 

 saturates at very low light intensities, whereas the signal at g = 2.002 saturates 

 hardly at all. It is probable if any signal is associated with chlorophyll, it 

 will turn out to be the signal at g = 2.002. If so. Dr. Franck may be right 

 in concluding that chlorophyll is converted, photochemically, into a free 

 radical fomi. The chlorophyll free radical, which in ChloreUa is observed 

 only at high light intensities, may have little to do with the photosynthetic 

 process which is already saturated with respect to light intensity. Considera- 

 bly more kinetic data are needed before this question can be resolved. 



Dr. Rabinowitch: 1 am not inclined to agree that this effect has nothing 



to do with photosynthesis. 1 believe it is related to the light saturation of 



photosynthesis, and is not something extraneous like photooxidation. It is 



instantaneously reversible. 1 cpiite agree with Dr. Kok that these experiments 



should be done with monochromatic actinic light; we are trying to do it now. 



Our first experiments with colored glass filters did not show much variation 



in the difference spectrum, whether obtained with red or green or blue 



light. However, it should be done with only monochromatic light. To return 



to results obtained on white light: now that we have a much improved 



instrument, we get the following picture. At a given wavelength of the 



monitor iieam, there is a steady signal in the dark; when actinic light is 



admitted, there is a practically instantaneous effect (in less than one second.) 



If the light is left on for one minute, a second effect appears, a "slow" effect, 



which can have the same sign as the fast effect, or an opposite sign. In the 



latter case, the total effect may go through zero and reverse its sign. Strehler 



has ol)served this behavior. If after a minute you switch the light off, you 



get an instantaneous reversal of the instantaneous effect; then the slow 



effect is also reversed, but slowlv-in a couple of minutes. One thus sees 



the induction phenomena— hrst the instantaneous induction (two seconds 



induction) , then the slow induction (two minutes induction) —both well 



known from fluorescence measurement; then the reversal of the instantaneous 



induction, and the reversal of the slow induction, somediing you cannot 



obser\e in liuorescence. Sometimes the cycle ends on a somewhat different 



level, indicating an irreversible effect. From the induction phenomena of 



the liuorescence, we know that induction involves chlorophyll; but in 



fluorescence, vou only see the dark to-light part: in absorption spectra you 



