THE ACQUISITION OF ATMOSPHERIC NITROGEN. 121 



gall shows three somewhat distinct stages. First there appears 

 a branching filament which grows among the cells of the root 

 and which soon stimulates an active growth of the root cells. A 

 little later, after the tubercle is formed, the central cells become 

 filled with the bodies called bacteroids. Lastly the bacteroids 

 of the central cells are absorbed by the plant and the tubercle 

 becomes empty. These facts are agreed upon by all. 



In regard to the significance of these facts there are three dis- 

 tinct opinions. The first is that of Prazmowski, who calls the 

 organism which produces the infection a bacterium, and claims 

 that the branching filaments are simply colonies of bacteria 

 inclosed in a membrane of their own manufacture, for their pro- 

 tection against the injurious action of the plant tissue. The 

 filaments swell with the multiplication of the bacteria until they 

 burst. The bacteria then coming into contact with the plant 

 tissue and no longer being able to gTow, owing to an injurious 

 influence of the plant plasma upon them, degenerate into the 

 bacteroids. They are subsequently absorbed by the plant and 

 incorporated into the substance, serving therefore as food. 



"The view held by Frank differs from this essentially in its 

 explanation of the filaments and bacteroids. The filaments are 

 said to be a mixture of plant protoplasm and bacteria. They 

 are produced by the plant and serve to conduct the infectious 

 matter into the midst of the root. The bacteroids are also pro- 

 ducts of the plant plasma and not distinct organisms. Their 

 absorption does not, therefore, especially help the plant. 



"The third view, that of Ward and Laurent, regards the 

 infecting organism not as a bacterium, but as a low fungus, 

 somewhat closely related to the yeasts. The filament is really a 

 mycelial growth of the organism, and the bacteroids arise from 

 it by budding. The bacteroids are thus distinct organisms — not 

 degenerate forms, but normal growths. 



"None of these views would regard the tubercle organisms as 

 true parasites on the plant, since the plant is not injured by them, 

 but is probably directly benefited. The association is rather to 

 be regarded as an instance of symbiosis, an association of two 

 organisms together in such a way that each receives benefit 

 from the other. The plant is probably benefited in gaining 

 nitrogen, and the infecting organism is benefited in gaining a 

 brood pouch for its development." 



