56 BIOLOGY AND TECHNIQUE 



The classical experiments of Hellriegel and Wilfarth conclusively 

 demonstrated the important relation of these tubercle-bacteria to nitro- 

 gen assimilation by the leguminosse. 



These observers cultivated various members of this group of 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: ' 



Lupinus luteus was cultivated upon sterilized sand. Some of the 

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

 parative 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 demonstrated, 

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

 legume, the pea, and one of the common nitrogen-consuming crops, oats.^ 



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 con- 

 ceived 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 hfe 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 in- 

 fection against which the plant offers at first a determined opposition as 

 evidenced by tissue reactions, other observers, notably A. Fischer, regard 



' P/e^er, " Planzenphysiologie," Leipzig, 1897. 



' Hellriegel und Wilfarth, Zeit. d. Ver. f. d. Riibenzucker Industrie, 1888. Quoted 

 from Fischer, " Vorles. Tiber die Bakt.," J^na, 1903. 



