62 PHOTOSYNTHESIS 



the amount of B substance. This interpretation, suggested 

 by Briggs (1935) and in a rather different form by Franck 

 and Herzfeld (1941), shows the possibiHty of measuring the 

 effective amount of an 'internal' factor in terms of some 

 known concentration such as that of chlorophyll. From this 

 reasoning it follows that the hypothetical substance B must 

 be present in an amount of i molecule per 2,000 chlorophyll 

 molecules, provided we assume that each molecule of photo- 

 chemical product produces one molecule of oxygen. If more 

 than one molecule of photochemical product is required 

 the concentration of substance B must be correspondingly 

 greater. 



We have already seen that cyanide probably inhibits 

 photosynthesis by slowing down a dark process. Emerson 

 and Arnold investigated its effect on photosynthesis in inter- 

 mittent light and found that the maximal yield per flash is 

 unchanged but the dark time between flashes necessary to 

 obtain a given fraction of the maximum yield is increased. 

 This is to be expected if cyanide does not alter the amount 

 of B substance present; it might however lengthen the time 

 of reaction of the photochemical product with the product 

 of process i by reducing the concentration of carbon 

 dioxide complex. 



The experiments of Emerson and Arnold differed from 

 those of Warburg in that the latter used a rotating sector 

 to produce intermittent illumination and consequently the 

 flashes used by Warburg were of longer duration. Briggs 

 pointed out that the yield per flash, expressed on some 

 standard basis such as in terms of the rate of photosynthesis 

 in continuous light, was much greater in the experiments of 

 Warburg than in those of Emerson and Arnold. Experi- 

 ments using flashes intermediate in duration between those 

 used by Warburg and those used by Emerson and Arnold 

 gave a value for the yield per flash intermediate between the 

 earlier values. With very long flashes and yet longer dark 

 times a maximum yield per flash of one oxygen molecule 

 per chlorophyll molecule was obtained and the dark time 

 was then of the order of 15 seconds. Briggs (1941) therefore 

 postulated the existence of a second dark process possibly 



