NITROGEN-FIXING BACTERIA 



267 



to the amount of organic material decomposed. Another of the 

 nitrogen -fixing Bacteria, Clostridium pasteurianum (Fig. 145, e), is 

 anaerobic, the principal product of its activity, apart from nitro- 

 genous compounds, being butyric acid. It is the organisms 

 mentioned that are largely responsible for the gradual increase 

 in the nitrogen-content of unmanured grassland. 



Even in the time of the Romans the inclusion of Leguminous 

 plants in a rotation of crops was recognised as beneficial. Ex- 

 perience has shown that cultivation of Clovers, Sainfoin, Lucerne, 

 etc., materially increases the nitrogen- 

 content of the soil, especially if the crop 

 is subsequently ploughed into the field (so- 

 called green manuring). This phenomenon 

 remained unexplained until it was dis- 

 covered that the swellings upon the roots 

 of Leguminous plants are inhabited by 

 Bacteria (Bacillus radicicola, Fig. 145, /), 

 capable of fixing free nitrogen, and present 

 in every healthy soil. Infection of the 

 root takes place through the root-hairs, 

 probably when the organism is in the 

 motile phase ; having penetrated the root- 

 hairs, the Bacteria pass into the adjoining 

 cells, which are thereby caused to divide, 

 so that a gall-like structure arises. Within 

 the cells of this nodule rapid multiplication 

 of the Bacteria ensues, probably at the 

 expense of carbohydrates furnished by the 

 Leguminous plant. At the same time, 

 however, the latter profits by the nitro- 

 genous material formed by the Bacteria, the removal of which is 

 indeed necessary for their continued activity. In the mature 

 condition large numbers of the Bacteria, within the cells of the 

 nodules, assume an irregular form, and become digested by the 

 action of the host. A limited number persist unaltered, and 

 return to the soil as the roots decay away. 



The relation between the nodules and these nitrogen-fixing 

 Bacteria is shown by the fact that Leguminous plants, grown 

 from seed in soil which has been thoroughly sterilised by heating, 



FIG. 145. a-c, spore- 

 formation in Bac- 

 teria, a, Bacillus 

 tetani ; b, Bacillus 

 of malignant cede- 

 ma ; c, Bacillus 

 cedamitis. d-f, ni- 

 trogen-fixing Bac- 

 teria. d.Azotobacter; 

 e, Clostridium pas- 

 teurianum sp. ; /, 

 Bacillus radicicola. 

 (After Engler and 

 Prantl, and Ellis.) 



