THE BIOLOGICAL ACTIVITIES OF BACTERIA 63 



plants upon nitrogen-free soil sand and prevented the formation 

 of root tubercles in some, by sterilization of the sand, while in others 

 they encouraged tubercle formation by inoculation. An example of 

 their results may be given as follows : 25 



Lupiiius luteus was cultivated upon sterilized sand. Some of the 

 pots were inoculated with B. radicicola, others were kept sterile. 

 Comparative analyses were made of the plants grown in the different 

 pots with the following striking result : 



Root tubercles present. 

 No root tubercles. . 



The great importance of this process in agriculture is demon- 

 strated, furthermore, by a comparison made by the same observers 

 between a legume, the pea, and one of the common nitrogen-con- 

 suming crops, oats. 26 Exactly what the process is by which the 

 bacteria supply nitrogen to the plant is as yet uncertain. Although 

 the degenerating bacteroids in old nodules are bodily absorbed by 

 the plant, this can not be conceived as the only method of supply, 

 since the total nitrogen gain many times exceeds the total weight 

 of bacteria in the nodules. It is probable that the microorganisms 

 during life take up atmospheric nitrogen and secrete a nitrogenous 

 substance which is absorbed by the plant cells. 



Although formerly the relationship between plant and bacterium 

 was regarded as one of symbiosis and of mutual benefit, the opinions 

 as to this subject show wide divergence. While, according to some 

 authors, the entrance of the bacteria into the plants is regarded 

 as a true infection against which the plant offers at first a determined 

 opposition as evidenced by tissue reactions, other observers, notably 



~*Pfeffer, "Planzenphysiologie," Leipzig, 1897. 



2 Hellriegel und Wilfarth, Zeit. d. Ver. f. d. Rubenzucker Industrie, 1888. 

 Quoted from Fischer, "Vorles. iiber die Bakt.," Jena, 1903. 



Nitrogen contents Nitrogen contents 



of seed and soil. of crop. Gain or loss. 



Oats 0.027 gram 0.007 gram .020 



Peas 0.038 " 0.459 " +.421 



