178 



OUTLINES OF BACTEEIOLOGY 



were regarded as pathogenic until, in 1866, Woronin called attention 

 to their cellular structure. Then they were more carefully studied, 

 with the result that, in 1886, Hellriegel proved that there was a 

 direct connection between the presence of these nodules and the 

 power of nitrogen-assimilation from the atmosphere. From that time 



Fig. 109. — Nodules on root of 

 Lupin. (After A. Mayer.) 



Fig. 110. — Nodules on root of Robinia 

 pseudacacia. (After F. Nobbe.) 



onward great strides have been made. It was ascertained that the 

 nodules did not appear if the soil on which the leguminous plants were 

 grown, was sterilised ; so that the cause of the production of nodules 

 had to be sought for among the organisms living in the soil. Finally, 

 we owe to Beijerinck the credit of being the first, not only to identify 

 this important organism, but also to isolate and cultivate it on artificial 

 media. The organism turned out to be a member of the genus Bacillus, 

 and the name Bacillus radicicola was henceforth assigned to it. The 

 artificial medium used by Beijerinck was made up as follows : 



Concoction of leaves of leguminous plants, 100 c.c. 



Gelatine, 7% 



Asparagin, i% 



Cane sugar, J% 



The medium is finally made slightly acid. 



The pure culture can be made in the following manner : A small 

 portion of the contents of the bacteroid tissue is placed in a cubic centi- 



