106 



James Franck and J. L. Rosenberg 



b) The fluorescence of the protected chlorophyll, if the 

 absorbed energy is forced to remain inside the unit, should thus 

 also be higher than of the exposed chlorophyll. If;, for instance 

 by photo-oxidation of the exposed chlorophylls during strong 

 illumination or by other methods, the energy is prevented from 

 migrating outvard in the unit, fluorescence would be observed 

 from the protected pigment molecules with up to a four-fold 

 increase in yield. Indeed such observations have been made with 

 a variety of external treatments (°^ 9 ) some of them destructive 

 for the chloroplastsv^'-'). 



c) Our scheme forces us to the conclusion that half of the 

 excited exposed chlorophyll molecules emit their filLl share of 

 fluorescence, independent of the level or even absence of photo- 

 synthesis, while the other half, involved in Step (2), use 

 singlet excitation immediately dioring good photosynthesis without 

 any fluorescence. The consequence, that the lifetime of excita- 

 tion of those emitting the fluorescence, is independent of photo- 

 synthesis, has been proved by several types of observation ' '. 



EMERSON EFFECT AUD RE VERSIBLE BLEACH ING AT TOO my. 

 Green Plants 



We first discuss our interpretation of the far-red absorp- 

 tions. After being forced by a number of new observations (to 

 be described in the forthcoming extensive discussion) to abandon 

 oirr previous idea that nit transitions were responsible for the 

 long wavelength absorption. We based oiur picture on a proposal 

 originally made by Brody that chlorophyll exists within the 

 photosynthetic apparatus partly in monomeric form and partly in 

 aggregates(l2) , This idea received strong support from the 

 discovery by Olson et al. that the absorption at 700 to.\s. and the 

 fluorescence above TOO mjj. are highly polarized in the planes of 

 the lamellae, while the major components observed in both 

 absorption and luminescence are not polarized, ^'l^^l^j Specif- 

 ically, we visualize most of the pigment molecules as being in a 

 partly disordered, amorphous state within their \inits, while in 

 some small fraction of the units there are regions of two- 

 dimensional crystallinity bordering on the reaction center. As 

 are all absorption peaks of the crystalline patches the red 

 absorption is shifted slightly toward the inra-red and is split 

 into two components . This is indeed the type of behavior 

 observed by chlorophyll microcrystallites in vitro by 

 Rabinowitch et al.^ 5/ Fig, 1 shows the red shift and splitting 



