226 Bacteria in Relation to Country Life 



soils for alfalfa, has taught us to beware of blight, and also 

 of dodder, which is frequently a greater scourge than 

 blight. Thus the ground was prepared for the abandon- 

 ing of legume-earth as inoculating material in favor of 

 another method, that of pure cultures. 



Pure cultures. — A pure culture is a growth of any one 

 kind of bacteria (or of other microorganisms), uncon- 

 taminated by the presence of other kinds. A pure cul- 

 ture of Bacillus radicicola, the organism of the legume- 

 nodules, is the uncontaminated growth of these bac- 

 teria on any material (medium), whether it be a solution 

 of sugar and mineral salts, gelatin, or potato. Since the 

 nodules of legumes are formed in each case by the pene- 

 tration of bacteria into the root-hairs, and since the 

 many thousands of bacteria found in any particular 

 nodule are the offspring of the single organism that 

 entered the root-hair, it follows that every nodule con- 

 tains a pure culture of Bacillus radicicola. To be sure, 

 other microorganisms may find their way into the 

 nodules after the latter are already formed. Neverthe- 

 less, it still remains true that pure cultures of the nodule 

 organism may be secured without difficulty from legume 

 tubercles. 



After the latter are removed from the roots, they are 

 carefully washed in sterile water to remove the particles 

 of soil adhering to them and are thus freed from most 

 of the soil bacteria on their outside. The nodules are 

 then washed in a solution of corrosive sublimate, rinsed 

 again in sterile water, cut open with a sterile knife, 

 and the bacteria within them transferred to a nutrient 

 solution in which they could multiply. 



