THE ACQUISITION OF ATMOSPHERIC NITROGEN. 119 



attacked by the putrefactive bacteria in the soil and becomes 

 decomposed." 



Frank has also published an extended series of observations 

 upon the same subject.* While he differs from Prazmowski in 

 some important particulars, his later results, on the whole, con- 

 firm those of the latter writer. He finds the tubercles produced 

 as the result of infection by some organism in the soil, and he 

 describes the organism as a micrococcus or short rod, — very 

 probably the same as that studied by Prazmowski. His explan- 

 ation of the hyphae and the bacteroids is different from the one 

 just noticed. The hyphae he finds filled with bacteria, as does 

 Prazmowski, but he regards the membrane that surrounds them 

 as a product of the root cells rather than of the bacteria. He 

 thinks that the root cells produce these peculiar threadlike forms 

 in which the bacteria multiply, and that by means of the threads 

 the bacteria are conducted into the inner cells of the root to 

 produce the infections there. He therefore calls them "infec- 

 tion threads." 



The essential point in which Frank's theory differs from that 

 of Prazmowski is, in regarding the filaments as products of the 

 root cells instead of the bacteria. He thinks that in some cases 

 the infection occurs without the development of the filaments. 

 After the infection, the cells of the roots are stimulated into 

 growth to form the tubercle, as already described, and bacte- 

 roids appear in the central cells. Frank, however, regards the 

 bacteroids as peculiar formations of the plant tissue and not as 

 distinct organisms or degenerate bacteria. According to him 

 the presence of the bacteria produces abnormal changes in the 

 plasma of the root cells, causing it to become separated into 

 numerous irregular masses which contain the bacteria inside of 

 them. These masses are the bacteroids which fill the central 

 cells. They are subsequently absorbed by the plant in the 

 manner described by Prazmowski. 



In a series of experiments performed at the Pasteur Institute, 

 Paris, Laurentf reaches a different conclusion. In his studies 

 of pure cultures of the tubercle organism, he finds that in 

 gelatin the organisms spontaneously assume, by a sort of bud- 



*Landw. Jahr., Bd. 17, (1888), pp. 421-552, and 19, (1890), pp. 523-640. 

 fAnn. i.l. L'Institute Pasteur, 1S91, No. 2. 



