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THE AMERICAN NATURALIST [Vol. LIT 



every case they require the presence of organic carbon 

 compounds in the substrate in which they grow. These 

 compounds may be of the greatest diversity of types, but 

 none of the bacteria, are capable of manufacturing their 

 own carbon food. It is possible that other types of bac- 

 teria than the prototrophic may not have developed upon 

 the earth until after the evolution of higher plants, such 

 as the alga 1 , upon which they could depend for food. 

 Possibly there may have been some start made, however, 

 in the utilization by one type of organism of the dead 

 bacterial protoplasm of another type. 



How may we detect relationships of modern meta- 

 tropic bacteria to these more primitive types? Possibly 

 by a study of intergrading forms. The genus Hydro- 

 genomonas apparently is either autotrophic or meta- 

 tropic according to the conditions of the environment. 

 Some primitive organism may have acquired properties 

 similar to those of the modern Hydrogenomonas and con- 

 stituted the progenitors of the modern forms. Possibly 

 this type of differentiation may have arisen in several 

 groups. It is conceivable, for example, that some organ- 

 ism having characters such as Nitrosococcus might have 

 given rise to an independent branch, possibly to forms 

 like Micrococcus. This, of course, is pure speculation. 



Among the metatropic bacteria we are probably justi- 

 fied in placing the genus Pseudomonas as most closely 

 related to the forms discussed because of its close morpho- 

 logic resemblance, with rod-shaped cell and polar flagella, 

 to the autotrophic forms ; then too, there is the evidence 

 of the intergrading Hydrogenomonas. Somewhat less 

 diversified in nitrogen metabolism are the related genera 

 Azotabacter and Rhizobium, both usually with polar fla- 

 gella. rod-shaped bodies, primitive nitrogen requirements, 

 and marked capacity to utilize carbohydrates, oxidizing 

 them quite completely to C0 2 and H 2 0. The supply of 

 energy is so abundant to these organisms that in the absence 

 of sufficient combined nitrogen in the substrate they can fix 

 atmospheric nitrogen, and build it into their protoplasm. 



