614 LIGHT AND LIFE 



column is the time the vessels were shaken in the bath in the dark 

 prior to addition of the grana from the side-arm. The illumination 

 began with the tipping in of the grana. The amount of ferricyanide 

 actually reduced was determined by spectrophotometric examination 

 of the trichloroacetic acid filtrates. In the case of the reaction carried 

 out in the absence of COo, there was reasonably good agreement be- 

 tween the amounts of oxygen evolved and ferricyanide reduced, 

 at least for the samples not subjected to a prolonged preincubation. 

 When CO2 was present, the manometric changes reflected not only 

 the oxygen evolution but also the pressure increase due to the CO2 

 released by the acid formed during ferricyanide reduction. The 

 figures have been omitted because the conditions were not sufficiently 

 well-controlled to enable us to calculate Oo evolution accurately. 



Note that the magnitude of the COo effect, if calculated as per 

 cent stimulation, increased with the time of preincubation, although 

 the overall rate decreased with time. The presence of dye ^vas neces- 

 sary at least to get effects of the magnitude shown in Table 5. Though 

 the reaction mixtures containing CO2 had a somewhat lower pH 

 than those without, the pH difference was not large, and equivalent 

 lowering of the pH was shown in separate experiments to have little 

 effect on the photoreduction rate. We therefore regard the effects 

 shown as specific for CO2 or bicarbonate. 



Stimulation ranging from 20 to 100 per cent has been obtained in 

 fifteen out of fifteen separate experiments conducted approximately in 

 the manner here described. We feel reasonably confident, therefore, 

 that this effect is reproducible, and that it can be obtained in other 

 laboratories. We would not venture to extrapolate from these data to 

 conclusions as far-reaching as Warburg's, but we feel encouraged to 

 continue to explore the possibility that his conclusions may be well- 

 foiuided. 



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