FIXATION OF NITROGEN 37 



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 l the 

 bacteroids give rise to a growth of a motile bacillus 

 known as Pseudomonas raciicicola ; this " fixes " the 

 atmospheric nitrogen. The organisms penetrate the 

 young roots through the root-hairs, multiply and form a 

 filamentous zoogloea, 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 compounds which can be assimilated by the 

 plant. Leguminous plants grown from sterile seeds in a 

 sterile soil dwindle and die, but if inoculated with the 

 organisms derived from another plant of the same species 

 growth becomes vigorous ; if inoculated with those 

 derived from another species growth still takes place, 

 but not nearly to the same extent. The Leguminosae thus 



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. 



