256 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



tinues growing and may branch from time to time. Soon 

 plants, in consequence, show forked nodules sometimes 

 resembling masses of coral. Certain well-differentiated 

 areas apparently are always set apart for the growth of the 

 microorganisms. The nuclei of the invaded cells eventually 

 undergo degeneration. Stained preparations made from 

 nodules show that in the older cells at least, most of the 

 bacteria have assumed involution shapes resembling in 

 many respects those found in culture media. These were 

 termed bacteroids by the earlier investigators before their 

 true bacterial relationship was understood. The morphology 

 of the organism seems to vary somewhat with the age of the 

 nodule and the species of legume. 



Comparative studies of legumes grown in soils which 

 have been sterilized and freed from Rhizobium legumino- 

 sarum, consequently having no nodules upon the roots, with 

 plants having nodules prove conclusively that the organism 

 fixes atmospheric nitrogen and in some manner passes this 

 on in combined form to the plant, that is, to the legume. 

 Legumes are, therefore, not dependent merely upon soil 

 nitrogen, but by the aid of the bacteria can utilize the 

 atmospheric nitrogen for their development. There seems 

 to be no reasonable doubt but that the energy required for 

 fixation of the nitrogen by the bacteria is furnished by the 

 plant through carbohydrates, probably sugars, which are 

 supplied. In just what manner the legume secures the 

 nitrogen through the help of the bacteria, is somewhat less 

 certain. It has been assumed by some writers that the 

 bacteria fix the nitrogen and use it in building up their own 

 protoplasm, and that only after the death of the bacterial 

 cell and its digestion, either as a result of autolysis or as a 

 result of the secretion of enzymes in the nodules by the 

 plant, the digested products may be absorbed by the 

 legume. Other investigators have concluded that a part, at 

 least, of the nitrogen fixed is excreted by the bacteria in the 



