THE NATURE OF PHOTOSYNTHESIS 169 



for from 1 to 15 minutes, the rate of photosynthesis was always depressed 

 when the plants were returned to the temperature of their previous environ- 

 ment, 16°. The depression is greater the higher the temperature and 

 the longer the exposure to the higher temperature. Similar effects were 

 produced with glycerine, which are taken to show that the photosynthetic 

 apparatus is more delicate than that of respiration. Warburg ^^^ has also 

 shown that the photosynthetic rate is reduced by hydrocyanic acid and 

 urethanes of extreme dilutions in which the respiratory activity is not 

 affected or in certain cases is even stimulated. The effect of narcotics on 

 photosynthesis is discussed in Section i. of this chapter. 



Warburg ^^*' found that, while respiration is not influenced by different 

 partial pressures of oxygen (probably above a certain minimum) photo- 

 synthesis is less at higher pressures. This decrease in the photosynthetic 

 rate is noticeable only at high illumination intensities. A change in the 

 partial pressure of oxygen from %o to 1 atmosphere reduces the photo- 

 synthetic rate by about one-third. From this it would appear that the 

 reaction 



nCOa + nHoO -^ (CH20)n + nOa, 



can be checked by a higher concentration of oxygen. This may be of 

 considerable significance in determining the kinetics of the reaction and 

 should be investigated more fully. 



In the discussion of the temperature coefficient of photosynthesis at- 

 tention has already been called to the fact that the photosynthetic process 

 is composed of two reactions. One of these is a photochemical reaction 

 with a low Qio, the other an ordinary chemical reaction with a Qio of 

 about 2, in approximate agreement with the van't Hoff rule. There is a 

 variety of evidence which is in harmony with this conception. It is the 

 ordinary chemical reaction which is associated with the protoplasmic ac- 

 tivity of the plant. It is this reaction which is affected by changes in 

 temperature and is sensitive to narcotics. The nature of this reaction 

 is not known, but has been the subject of considerable speculation. War- 

 burg ^^' has called this reaction the "Blackman Reaction" and advanced 

 a theory that the reaction resulted in the formation of a photochemical 

 acceptor, a compound of carbon dioxide sensitive to light. Willstatter and 

 Stoll proposed the formation of a peroxide in the reduction of carbonic 

 acid, and that the splitting off of oxygen from the peroxide constitutes the 

 "Blackman Reaction." Later Warburg considered that his original theory 

 was not tenable and on the basis of measurements of the inhibiting action 

 of certain narcotics on the decomposition of hydrogen peroxide by 

 Chlorella supported Willstatter and Stoll 's theory. This phase of the 



"= Warburg, Biochem. Zeit., 100, 264 (1919) : 103, 196 (1920). 



^Warburg, /. c, 103, 193 (1920). 



"nVarburg, Biochem. Zeit., 100, 230 (1919) ; 103, 188 (1920) ; 146, 486 (1924). 

 Willstatter and Stoll, "Untersuchungen iiber die Kohlensaure Assimilation," 1918, 

 p. 395. 



