C. S. FRENCH 467 



physics have been used to explain tlifTcrcnt aspects of the photo- 

 synthetic jjrocess. So lai we ha\'e no universally acceptable concept 

 as to how the photochemical part ol the reaction takes place, but 

 we have no lack at all of proposed theories. 



The basic difference between well-known photochemical reactions 

 and photosynthesis is that what seems to be a single step — the splitting 

 of a water molecule — takes more energy than is contained in one 

 quantum. Ordinary photochemical reactions use one quantum at a 

 time to do a job within their capacity; photosynthesis seems to go 

 by means of some mechanism for collecting the energy of several 

 quanta for use at one time. The present discussion will avoid this 

 more basic problem and will be limited to the question of how 

 the accessory pigments may come into the picture. 



The phenomena presimiably attributable to the action of accessory 

 pigments are: chromatic transients, enhancement, the wavelength de- 

 pendence of the time course, and the action spectrum at saturating 

 light intensity. All of these effects appear to be related, and are 

 probably most easily discussed as enhancement, which may perhaps 

 give rise to the other phenomena. That the 670 m^^ form of chloro- 

 phyll a appears to act as an accessory pigment, and that the activation 

 of the accessory pigments makes 0^695 more effective, may refer the 

 whole accessory pigment question back to that of imderstanding the 

 mode of action of the various forms of chlorophyll a. 



Without facing the fundamental problem of collecting the energy 

 of several quanta, we may nevertheless speculate about the means by 

 which some extra photosynthesis is produced by the action of accessory 

 pigments to give the phenomena discussed above. 



Possible theories to explain enhancement fall into two classes: (a) 

 theories that account for the interaction within the pigment system 

 itself; and (b) theories that have various pigments produce different 

 photochemical products and therefore locate the interaction in the 

 products rather than in the pigment system. 



The latter concept is favored by the possible separation in time 

 of the two light beams giving enhancement and by the long time 

 constants involved (see Fig. 10) . It is supported even more strongly 

 by the different time course of respiration after illimiination by 650 

 myn and by 700 m^. How can cells remember the color of light they 

 have previously been exposed to, except by storing different chemical 

 products? 



Theories of interaction within the pigment system can no doubt 

 be made in terms of solid state physics and may even account for the 



