238 L. HORWITZ AND F. L. ALLEN 



oxyhydrogen reaction running concurrently. On the other hand, 

 under two of the hmited number of circumstances explored in this 

 and preceding papers, it is possible to demonstrate a true photoreduc- 

 tion. Gaffron had previously shown that in the presence of certain 

 poisons, a true photoreduction occurs with oxygen production com- 

 pletely excluded. We were able to demonstrate a true photoreduction 

 under the conditions obtaining in Warburg manometers, as discussed in 

 the preceding section. These facts, of necessity, lead to the conclusion 

 that the nature of the photochemical uptake of hydrogen and carbon 

 dioxide by adapted Scenedesmus is variable, and depends on several 

 factors which apparently control a sensitive balance between photo- 

 reduction and photosynthesis. Depending on the circumstances, there 

 can be almost any proportion of photosynthesis and photoreduction 

 in adapted algae, ranging from exclusive photoreduction to combina- 

 tions of photoreduction, photosynthesis, and the oxyhydrogen reac- 

 tion, to photosynthesis and the oxyhydrogen reaction running con- 

 currently with photoreduction practically excluded. 



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