38 BACTERIOLOGY 



that the nodules found on the roots of the leguminous 

 plants (clover, peas, beans, etc.), might properly be re- 

 garded as communities of bacteria which were beneficently 

 co-operating with the plants in the performance of their 

 fundamental life processes, i. e., they were in "symbiotic" 

 relationship. The result of this co-operation they showed 

 to be the power of the legumens to fix and store the free 

 atmospheric nitrogen. When one realizes how inexhaustible 

 is the supply of free atmospheric nitrogen it is difficult 

 to exaggerate the importance of this function. It also sheds 

 interesting light upon certain practices of the agriculturalist 

 that have been in empirical operation since the cultivation 

 of the soil began. It has always been known that the 

 rotation of crops is essential to the successful tillage of the 

 soil and we find that in such rotation one or another of the 

 legumens was always used. The reason is evident, they do 

 not impoverish, but though their ability to fix nitrogen 

 through the aid of the "nodule bacteria" on their roots, 

 they actually enrich the soil. 



From the foregoing it is obvious that the expression 

 "nature's laboratory" is properly applied to the soil. It 

 is here that all the saprophytes are sooner or later found. 

 Among her various analytic and synthetic performances 

 nature concerns herself with none so important to life as 

 those having to do with the several transformations of nitro- 

 gen to which allusion has just been made. 



SPECIFIC FUNCTIONS OF THE PARASITIC BACTERIA. 



As already intimated the parasitic bacteria are not charac- 

 terized by such beneficent activities as are possessed by 

 the saprophytic group; they exist at the expense of living 



