II.] NITROGEN FIXED BY BACTERIA 35 



host plant, provided they are supph'ed with some carbo- 

 hydrate, by the oxidation of which they derive the 

 energy necessary to bring the nitrogen into combination. 

 Of these bacteria the best known and probably the 

 most effective is a large organism, discovered by 

 Beijerinck in Holland, and called by him Azotobactcr 

 chroococcum. It is widely distributed in cultivated soils 

 both in Europe and America, and although the author 

 failed to detect it in the arid soils from the high veldt 

 or the Karoo in South Africa similar though perhaps 

 slightly varying bacteria were obtained from cultivated 

 soils from tropical East Africa, Egypt, India, Russia, 

 Western America and Canada, Sarawak, and Monte 

 Video. It appears to be only active when there is some 

 calcium carbonate in the soil, possibly because in its 

 oxidising reaction certain acids are produced which must 

 be neutralised before its activity will continue. Roughly 

 speaking, its action is to oxidise carbohydrates to 

 carbon dioxide and water, forming as bye-products 

 certain organic acids, and some dark brown humus 

 (whence the name " chroococcum "), and incidentally 

 bringing a certain amount of nitrogen into combination, 

 not more, however, under the most favourable laboratory 

 conditions than i to 2 per cent, of the carbohydrate 

 consumed. It is, however, extremely probable that we 

 may look to this organism and its allies as the origin 

 of the continued accumulation of nitrogen in such rich 

 virgin soils as the black soils of the Russian Steppes or 

 of Manitoba. As long as these lands were uncultivated, 

 the annual fall of the leaf and dying down of the summer 

 vegetation furnished the conditions necessary for the 

 activity of the Azotobactcr. The carbohydrate-containing 

 material thus returned to the soil provided the organism 

 with its necessary food supply, by the oxidation of which 

 it gained energy to fix the atmospheric nitrogen. In 



