692 



the org^anism. lie fnrtlier asserts that the baeteroidg found in the 

 tissue of tlie tubercles arise, not by a degeneration, but by a normal 

 process of budding from the hyphae. The hyphae themselves he looks 

 upon as filamentous growths of the organism, and not as pouches filled 

 with bacteria nor as products of the root cells. Now, since bacteria 

 always multiply by division and never by budding, it is plain that if 

 these observations of Laurent are correct, the organisms in question 

 can not be called bacteria. Laurent, therefore, like Ward and other 

 earlier investigators, affirms that the organism is really a low fungus, 

 that the hypha is a sort of mycelium of some low fungoid plant, and 

 that the bacteroids are buds from it. It seems to be related to the 

 yeasts in its method of growth, and Laurent regards it as intermediate 

 between the yeasts and the filamentous fungi. He accepts the name 

 formerly suggested by Frank, BMzohiiim leguminosarum. 



Comparison of views. — Summary.— Tho, three views thus outlined give 

 in substance our present knowledge of the origin and structure of these 

 tubercles. It may seem strange that there should be such a difl'erenceof 

 opinion on mere matters of fact, but the differences are explained by the 

 difficulties of observation. The tubercles grow naturally under ground, 

 Laurent alone having had much success with water culture. They are 

 opaque, and can therefore only be studied by tearing them to pieces or 

 by cutting sections of them. The organisms which produce changes are 

 microscopic, and it is therefore impossible to watch their action on the 

 root cells. The only method of observation is by examining a large 

 number of tubercles in different stages of growth, and in this way 

 important points are sure to be missed. Differences in results of 

 observation as wide as above sketched are, therefore, not surprising. 



Taking all of these observations together we may conclude that our 

 present knowledge of the nature of these tubercles is somewhat as fol- 

 lows : They are not normal products of the plant, but are in all cases 

 produced by infection from some organisms which exist in the soil and 

 attach themselves to the young root. Their presence in the root tissue 

 stimulates the root cells to active growth and a mass of new tissue is 

 formed around the growing organisms. This tissue forms the tubercle 

 and confines the infectious action within narrow limits. The tubercle 

 is thus a sort of gall. The study of the development of this 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 

 ])lant and the tubercle becomes empty. These facts are agreed upon 

 by all. 



In regard to the significance of these facts there are three distinct 

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

 which produces the infection a bacterium, and claims that the branch- 



