GENERAL FUNCTIONS OF MICROBES 21 



freed from nitrogen compounds ; he kept them in an atmosphere 

 similarly freed from all compounds of nitrogen ; when the young 

 plants had overcome these rather unfavourable conditions they 

 contained at the end of the experiment more nitrogen than 

 was present in the seeds. 



Malpighi observed long ago in 1687 little nodules 

 on the roots of Leguminosae whose function has only in 

 modern times been discovered thanks to Pasteur. These little 

 nodules are in fact crammed with bacteria the function of which 

 is to fix the atmospheric nitrogen. The plant suffers from a 

 malady which is actually beneficent (Hellriegel and Wilfarth). 



Grown in a sterile soil and protected from the germs of the 



FIG. 7. Nodules on the roots of Leguminosae ( Viciafaba}. 



air and of the soil, the roots of Leguminosae never present 

 these nodosities. If this soil is watered with a suspension of the 

 nodules or of earth in which Leguminosae have grown, or is 

 simply sprinkled with garden soil, the Leguminosae grow in 

 it possessing nodules, but if these suspensions are boiled for a 

 sufficient time they lose this property. 



It is possible to inoculate the plant with the infection by 

 simply pricking with a needle first a nodule on the root of a 

 normal pea, then the root of a pea which has been grown 

 aseptically without nodules ; under such conditions the roots 

 which were free from nodules develop them. 



The bacteria of the nodules have an irregular and peculiar 

 shape, and have been given the name of bacteroidia. The 

 best known is the B. radicicola of Beijerinck. Two groups 



