FIXATION OF FREE NITROGEN 461 



this term should be reserved exclusively for those microorganisms 

 which are able to set nitrogen free from nitrites or nitrates. Many 

 bacteria, particularly in the absence of free oxygen, under anaerobic 

 conditions, can obtain their oxygen supply by a reduction of nitrates 

 into nitrites, and among these is the Bacillus coli communis, the 

 typhoid bacillus, and the Bacillus ramosus, or root bacillus. The 

 bacilli which in the more strict sense are dentrifying bacteria, i. e., 

 those which can carry the reduction far enough along to the setting 

 free of nitrogen, have been designated as Bacillus denitrificans a and /? 

 (of Gayon and Dupetit). Aberson's Bacillus denitrificans is a par- 

 ticularly energetic denitrifier and can reduce nitrates contained in pure 

 cultures to complete liberation of all of the nitrogen. In addition 

 to these three species, others of the group have been described by 

 various investigators. These bacteria and other denitrifiers may 

 cause great loss to the plant-available nitrates in soil under faulty 

 methods of fertilization, as, for example, particularly when nitrates 

 and fresh manure (especially horse manure) are distributed simul- 

 taneously to the soil. 



FIXATION OF FREE NITROGEN. 



The enormous quantity of free nitrogen contained in the atmosphere 

 was formerly believed to be entirely useless in so far as assimilation 

 by living organisms was concerned. Nitrogen is a comparatively 

 inert gaseous elementary body, unlike oxygen, which easily enters 

 into combination with a variety of other chemical elements and 

 compounds. It has, however, been known for some time that certain 

 plants can indirectly derive their nitrogen supply from the atmosphere 

 through the intervention of bacteria. Most plants, if brought into a 

 nitrogen-free soil, wither and perish after a short time; the leguminosce, 

 to which clovers, peas, beans, lentils, and similar plants belong, how- 

 ever, can thrive even in a nitrogen-free soil. They develop nodules 

 from the size of a pea to that of a hazelnut on their roots. These 

 root nodules of the leguminosse contain innumerable bacteria which 

 have united with the higher plant in a symbiotic community and 

 which assimilate free nitrogen, transforming it in such a manner that 

 it becomes soluble and available to the leguminous plant for the 

 preparation of the required vegetable proteids. The exact details of 

 this chemical transformation, however, are not fully known, but the 

 nitrogen fixation by the bacteria of the root nodules of the leguminosse 

 is an established fact. Wronin was the first investigator who observed 

 that the root nodules contained cells filled with bacteria. The micro- 

 scopic examination of the nodule shows an outer, colorless cortical zone 

 and an inner, pale red, later greenish-gray, medullary zone, which has 

 rather irregular outlines and in shape somewhat resembles a mulberry. 

 This inner zone is composed of the cells which contain the bacteria. 

 Beyerinck, in 1888, first isolated such bacteria in pure cultures, and 

 he named the organism isolated Bacillus radicicola. 



