32 A MANUAL OF BACTERIOLOGY 



medium is not so suitable as the silica jelly. A 2 per cent, aqueous 

 agar is prepared and poured into Petri dishes ; the film is then sown 

 with Proteus, and allowed to grow for seven to ten days. It is 

 then thoroughly washed, collected, melted, and mixed with the salts 

 mentioned above. The object of growing the Proteus upon it as a 

 preliminary is to eliminate the organic matter admixed with the agar. 



Nitrification in the soil is thus brought about by two 

 groups of organisms. The first oxidises ammonia into 

 nitrous acid, and is isolated by successive cultivation in 

 solutions of ammonium carbonate. The second group 

 oxidises nitrous acid into nitric acid, and may be separated 

 by successive cultivations in a solution of potassium 

 nitrite containing a little sodium bicarbonate. In the 

 soil the nitric and nitrous organisms are equally active. 



Besides the derivation of nitrogen from nitrogenous 

 compounds, the free atmospheric nitrogen is also " fixed " 

 through the agency of certain micro-organisms and 

 rendered available for plant life. 



Thus, the Leguminosae are able to obtain their nitrogen 

 directly from the nitrogen of the air. If the roots of a 

 pea, bean, or vetch be examined, numerous little nodules 

 will be found upon them ; these contain minute irregular 

 and Y-shaped bodies, which have been termed " bac- 

 teroids," and seem to be of the nature of involution 

 forms. On inoculation into suitable culture media 1 the 

 bacteroids give rise to a growth of a motile bacillus known 

 as Pseudomonas radicicola ; this " fixes " the atmospheric 

 nitrogen. The organisms penetrate the young roots 

 through, the root-hairs, multiply and form a filamentous 

 zooglcea, which grows into the tissue of the root and 

 penetrates the cells. Large amounts of nitrogen are taken 

 up by the bacteroids, and are converted into nitrogenous 



1 Such as wood-ashes maltose agar. Boil 8 grm. of wood-ashes 

 with 500 c.c. of water for one minute ; filter. To 400 c.c. of this extract 

 add 4 grm. maltose and 4 grm. agar. Boil until dissolved ; filter, tube, 

 and sterilise. 



