H. GAFFRON 



to the plants after their adaptation to photoreduction it does not 

 interfere with a reduction of carbon dioxide. The photochemical 

 reaction proceeds with a normal quotient of two. Two volumes of 

 hydrogen are absorbed together with one volume of carbon dioxide. 

 The only change of importance concerns the quantum yield. It is 

 exactly one-half of that found with the unpoisoned algae. Figure 3 

 shows how, with increasing concentrations of phthiocol at 560 lux, 



. 30 



25 



20 - 



LlI 



2 



o 15 



:d 

 o 



LU 



a: 



g 10 

 O 



X 

 Q. 



o 5 



Ixl 



< 



2 4 6 8 10 12 



CONCENTRATION OF PHTHIOCOL X 10"^ M 

 Fig. 3. — The inhibition and stabilization of photo- 

 reduction in Scenedesmus by increasing concentrations of 

 phthiocol: • — •, rates at 560 lux; A — A, same a day 

 later; X — X, rates at 400 lux; O — O, same a day later. 



the rate of the reaction falls from 26 to 13 mm. per 10 min. (or at 400 

 lux from 18 to 9 mm. per 10 min.) and then stays constant. What this 

 means is quite obscure. A possible solution of the riddle may be 

 found in the following considerations. The probable occurrence was 

 mentioned above of back reactions between the reduced and oxidized 

 products whenever photosynthesis is artificially inhibited. Now we 

 postulate that, in the poisoned algae, all intermediates react back, and 

 thereby activate a reduction of carbon dioxide with hydrogen in a 

 manner similar to that brought about by the "Knallgas" reaction in 

 the dark. Apart from any specific explanation we can state that, if 



44 



