THE BIOLOGICAL ACTIVITIES OF BACTERIA 55 



Agriculturally of even greater importance than the free nitrogen- 

 fixing bacteria of the soil are the bacteria found in the root tubercles of 

 a class of plants known as "leguminosae." It has long been known that 

 this class of plants, including clover, peas, beans, vetch, etc. , not only does 

 not withdraw nitrogen from the soil, but rather tends to enrich it. Upon 

 this knowledge has depended the well-known method of alternation of 

 crops employed by farmers the world over. The actual reason for the 

 beneficial influence of the leguminosse, however, was not known until 

 1887, when Hellriegel and Wilfarth ' succeeded in demonstrating that 

 the nitrogen-accumulation was directly related to the root tubercles of 

 the plants, and to the bacteria contained within them. 



These tubercles, which are extremely numerous — as many as a 

 thousand sometimes occurring upon one and the same plant — are formed 

 by the infection of the roots with bacteria which probably enter through 

 the delicate root-hairs. They vary in size, are usually situated near the 

 main root-stem, and, in appearance, are not unlike fungus growths. 

 Their development is in many respects comparable to the develop- 

 ment of inflammatory granulations in animals after infection, inas- 

 much as the formation of the tubercle is largely due to a reactionary 

 hyperplasia of the plant tissues themselves. They appear upon the 

 seedlings within the first few weeks of their growth as small pink 

 nodules, and enlarge rapidly as the plant grows. At the same time, 

 later in the season, when the plants bear fruit, the root tubercles begin 

 to shrink and crack. When the crops are harvested, the tubercles with 

 the root remain, rot in the ground, and re-infect the soil. 



Histologically the tubercles are seen to consist of large root cells 

 which are densely crowded with microorganisms. 



The microorganism itself, "Bacillus radicicola," was first observed 

 within the tubercles by Woronin ' in 1866. The bacilli are large, slender, 

 and actively motile during the early development of the tubercles, but 

 in the later stages assume a number of characteristic involution forms, 

 commonly spoken of as "bacteroids." They become swollen, T and Y 

 shaped, or branching and threadlike. Their isolation from the root 

 tubercles usually presents little difficulty, since they grow readily upon 

 gelatin and agar under strictly aerobic conditions. On the artificial 

 media the bacillary form is usually well retained, involution forms 

 appearing only upon old cultures. 



1 Hellriegel und Wilfarth, Cent. f. Bakt., 18S7. 



2 Woronin, Bot. Zeit., xxiv, 1866. 



