62 BIOLOGY AND TECHNIQUE 



known method of alternation of crops employed by farmers the 

 world over. The actual reason for the beneficial influence of the 

 leguminosae, however, was not known until 1887, when Hellriegel 

 and Wilfarth 23 succeeded in demonstrating that the nitrogen-ac- 

 cumulation 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 development of inflammatory granulations in animals after 

 infection, inasmuch as the formation of the tubercle is largely due 

 to a reactionary hyperplasia ol 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 ob- 

 served within the tubercles by Woronin 24 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 character- 

 istic 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. 



The classical experiments of Hellriegel and Wilfarth conclusively 

 demonstrated the important relation of these tubercle-bacteria to 

 nitrogen assimilation by the leguminosag. 



These observers cultivated various members of this group of 



28 Hellriegel und Wilfarth, Cent. f. Bakt., 1887. 

 24 Woronin, Bot. Zeit., xxiv, 1866. 



