ASSIMILATION OF NITROGEN 79 



microorganisms by inoculating a grape-sugar solution with garden soil. In 

 spite of the fact that this solution contained no nitrogenous compounds, a 

 vigorous fermentation began immediately, with the formation of carbon dioxide, 

 much hydrogen, and butyric and acetic acids, the process being accompanied 

 by the fixation of atmospheric nitrogen. The amount of nitrogen combined was 

 related to the amount of sugar used up, as is shown in the following table: 



Experiment number 1 2 3 4 



Grams of sugar consumed 2.0 2.0 4.0 20 . o 



Milligrams of nitrogen fixed 3.9 5.9 9.7 2 8.o 



Addition of ammonium salts in very small amounts acted favorably; larger 

 amounts retarded the fixation of nitrogen and finally stopped it altogether. 

 The fixation of atmospheric nitrogen is possible only in substrata which 

 are either entirely deficient in nitrogenous compounds or contain these only 

 in very small amounts. The bacterium to which this fixation is due was 

 named by its discoverer, Vinogradskii, Clostridium pasteurianum. It is an- 

 aerobic, living without free oxygen. 



More recently Beijerinck has found another nitrogen-fixing bacterium, 

 Azotobacter chroococum. Unlike the forms previously mentioned, this is aerobic 

 and thrives best in the presence of air, where it also exhibits its ability to fix 

 nitrogen. Furthermore, other investigators have found other soil microorgan- 

 isms that possess, to a smaller degree, this power to assimilate free nitrogen. 

 The fixation of atmospheric nitrogen is therefore a process that occurs commonlv 

 in nature. 



§7. Assimilation of Nitrogen Compounds by Lower Plants. — We have seen 

 that nitrates usually furnish the best source of nitrogen for higher plants. Of 

 the lower plants without chlorophyll (moulds, yeasts, bacteria) not nearly 

 all are capable of utilizing nitrates. To be sure, this property is possessed by 

 most of the common moulds (Penicillium, Aspergillus and some species of 

 Mucor) and one group of bacteria is sufficiently specialized to utilize nitrates 

 as a source of nitrogen, at the same time reducing them vigorously, with elimi- 

 nation of free nitrogen (denitrifying bacteria 1 ). Nevertheless, most lower 

 plants require organic nitrogenous substances, or at least ammonium salts. 

 Suitable culture media for such forms have already been referred to, and it 

 has also been mentioned that these organisms are in great variety, as far as 

 their nutrition is concerned. 



Summary 



1. The Nitrogen of the Air.— By volume measurement, about four-fifths of the air 

 is free nitrogen. Air also generally contains very small amounts of nitrogen in the 

 form of ammonia. Free nitrogen cannot be assimilated by ordinary higher plants. 

 Under natural conditions these plants may assimilate minute quantities of nitrogen 

 from the ammonia of the air, but this source of nitrogen is generally quite negligible. 



1 Laurent, E., Recherches sur le polymorphisme du Cladosporium herbarum. Ann. Inst. Pasteur 2 : 

 558-566, 581-603. 1888. Idem, Recherches sur la valeur comparee des nitrates et des sels ammomiacaux 

 comme aliment de la levure de biere et de quelques autres plantes. Ibid. 3 : 362-374. 1889. Ritter.'G., 

 Ammoniak und Nitrate als Stickstoffquelle fur Schimmelpilze. Ber. Deutsch. Bot. Ges. 27: 582-588. 

 1909. 



