THE CHEMISTRY OF PHOTOSYNTHESIS 287 



Another source of support for his theory of "acceptor'' formation 

 Warburg saw in the action of potassium cyanide on the rate of photo- 

 synthesis. In cells that have been treated with potassium cyanide, atmos- 

 pheric carbon dioxide cannot be used in photosynthesis. When the 

 illumination is very low, so that the rate of respiration exceeds that of 

 photosynthesis, the rate of the latter is not depressed by concentra- 

 tions of cyanide of 0.02 M. and less. Under these conditions atmospheric 

 carbon dioxide is not used directly in photosynthesis, but in the light 

 no carbon dioxide of respiration is liberated : it is concluded that this 

 or possibly an intermediate product of respiration is used in photosyn- 

 thesis under these circumstances. If a certain cyanide concentration 

 inhibits the rate of photosynthesis by 50 per cent with high illumination 

 intensity, the same concentration will have no effect with low illumina- 

 tion. This fact was taken to indicate that the secondary reaction, i.e. 

 the reaction between the "photochemical primary product" and the "ac- 

 ceptor" is not affected by the cyanide. Xor is the "photochemical primary 

 reaction" thus affected, ^^'arburg considered that it was only the dark 

 reaction, the Blackman reaction, which is affected by cyanide. 



It was originally \\'arburg"s idea that the Blackman reaction constij 

 tuted the formation of the "photochemical acceptor." W^illstatter and 

 Stoll. on the other hand, made the assumption that the photochemical re- 

 duction of carbonic acid, their photoisomerization reaction, resulted in 

 the formation of a peroxide and that the splitting out of oxygen from 

 this compound was a "dark" reaction and constituted the Blackman 

 reaction. If the theory of Willstatter and Stoll is correct it would be 

 expected that if hydrogen peroxide were introduced into a living green 

 cell it would be decomposed according to the known behavior of the 

 Blackman reaction. From the effect of temperature, cyanide and various 

 urethanes on the formation of oxygen from hydrogen peroxide by 

 Chlnrella, Warburg and Uyesuge concluded that the theory of Willstatter 

 and Stoll is more nearly in accordance with the experimental facts than 

 Warburg's original ideas. The agreement between the behavior of hydro- 

 gen peroxide in CMorella and the Blackman reaction is not very close, 

 though perhaps this is not to be expected in view of the fact that the 

 peroxide of Willstatter is not hydrogen peroxide but supposedly formalde- 

 hyde peroxide. While much in these views may assist considerably in 

 forming a conception of the mechanism of photosynthesis they must very 

 clearly be considered as hypotheses. 



h. Other Theories. 



A number of other tlieories of the mechanism of photosynthesis have 

 been advanced by various writers. Some of these are based upon observa- 

 tions regarding the behavior of the chlorophyllous cell, others are merely 

 si^eculative. The latter will not be discussed here. Wurmser ^^ has again 



"Wurmser, "Recherches sur rassimilation chlorophyllienne," Paris (1921). 



