H. GAFFRON 



reaction which yields again hydrogen donors capable of being utilized 

 in the light (20). 



A second difference between plants and purple bacteria lies 

 in the relation of cell multiplication to the assimilation of carbon. 

 Green plants can, as a rule, grow normally in air on a heterotrophic 

 diet without photosynthesis. Spoehr (17) succeeded in obtaining 

 growth of hereditary chlorophyll-free albino corn plants by feeding 

 them with only one organic compound, sucrose. On the other hand, 

 we have not observed true growth of algae, during a photochemical or 

 thermal reduction of carbon dioxide, under anaerobic conditions, 

 that is, in the absence of respiration. 



By contrast, some strains of purple bacteria multiply exclusively 

 under anaerobic conditions and only during periods of illumination, 

 and never in the dark. They are not capable of linking a synthetic 

 process to either oxidative reactions (which do occur in the presence 

 of oxygen) or fermentations (which seem not to occur at all). They 

 depend for synthesis and growth upon photoreduction with special 

 hydrogen donors like fatty acids, hydrogen sulfide, or molecular 

 hydrogen. A few species among the nonsulfur purple bacteria 

 {Athiorhodaceae) appear to grow in ways similar to that of the green 

 plants in that they do not depend upon the photochemical reaction 

 alone. As van Niel has shown, they can also grow aerobically in the 

 dark by oxidizing the same substrates which they use as hydrogen 

 donors in the light. But the fact that no carbohydrates are attacked 

 points to a deviation from the metabolism of the green plants. 



The third interesting difference between plants and purple 

 bacteria concerns a direct interrelation between the respiratory and 

 the assimilatory systems. In the ordinary green plant, respiration 

 and photosynthesis can go on simultaneously. Metabolic measure- 

 ments appear most consistent if we assume that both reactions run 

 independently of one another and that any conspicuous increase of 

 the rate of respiration in the light is caused in an indirect way. Photo- 

 synthesis appears to provide only reserve material, while the synthetic 

 reactions leading to cell multiplication are coupled exclusively with 

 respiration (and perhaps with fermentation). In those purple bacteria 

 capable of growing at the expense of oxidation reactions, the utiliza- 

 tion of oxygen must compete with the utilization of carbon dioxide 

 plus light for the same hydrogen donors. The reactions do not occur 



46 



