ASSIMILATION OF NITROGEN 8 1 



5. Fixation of Free Nitrogen by Tubercle Bacteria. — Although ordinary plants are 

 not able to assimilate free nitrogen, there are certain groups of them (especially the 

 legumes) that appear to do so. Hellriegel and Wilfarth showed that the roots of 

 legumes are normally infected with nodule or tubercle bacteria, which remain in the soil 

 from season to season, infecting the new plants each year. The same experimenters 

 showed that these microorganisms carry on nitrogen fixation in the structurally 

 characteristic root-tubercles that result from their invasion of the legume root tissues. 

 The tubercle bacteria apparently derive carbohydrates from the host, secure utiliz- 

 able energy through the oxidation of these substances, and use some of this energy for 

 the formation of nitrates or nitrogenous organic compounds from carbohydrates and 

 free nitrogen. Nitrates, or organic nitrogenous compounds, are given off by the 

 bacteria and these substances are assimilated by the host plant. Legumes may thus 

 grow well in soils with very small supplies of nitrates, or none at all, deriving their 

 nitrogen from the free nitrogen of the soil, through the activities of the tubercle 

 bacteria. In the presence of considerable supplies of soil nitrates this fixation of free 

 nitrogen is slight or does not occur. Different legumes have different nodule bacteria; 

 the right form of the latter must be present in the soil for any given legume. Free 

 soil nitrogen may, therefore, be fixed (as nitrates, etc.) (1) through the action of 

 nitrogen-fixing bacteria in the soil (see 4, above), or (2) through the action of tubercle 

 bacteria in root nodules. Some other groups of higher plants have tubercles with 

 nitrogen-fixing bacteria; for example, Pavetta (Rubiaceae), with leaf tubercles in 

 which atmospheric nitrogen becomes fixed. 



6. [4]. Circulation of Nitrogen in Nature. — Free nitrogen is converted into the 

 nitrogen of nitrates and organic nitrogen compounds by the nitrogen-fixing bacteria 

 of the soil and by tubercle bacteria. Some free nitrogen is converted into ammonia 

 nitrogen by the action of atmospheric electricity, the ammonia finding its way into 

 the soil, where its nitrogen is converted into nitrite nitrogen by the nitrite bacteria. 

 Nitrites are changed to nitrates by nitrate bacteria in the soil. Nitrates (and, to some 

 extent, ammonium compounds and nitrites) are assimilated by higher plants and 

 disappear in the formation of complex nitrogenous organic compounds. Animals 

 secure their nitrogen from these complex plant compounds, or from other animals. 

 When animal and plant tissues or materials decay, ammonia and free nitrogen result. 

 Free atmospheric nitrogen can be combined with other elements artificially, as in 

 the production of calcium, cyanamide (CaCNo). 



7. Assimilation of Nitrogen Compounds by Lower Plants. — Many representatives 

 of the moulds, yeasts, and bacteria are unable to assimilate nitrates, and must be 

 supplied with organic nitrogenous substances, or at least with ammonium salts. 

 Animals require organic nitrogen compounds, which they secure from plants or other 

 animals. 



