256 TEXTBOOK OF PLANT PHYSIOLOGY 



Winogradsky's classical investigations show that these oxida- 

 tions are achieved by two kinds of bacteria; the first, called Nit- 

 rosomonas, having the form of small mobile cocci oxidize ammonia 

 to nitrous acid, according to the equation, 2NH 3 + 30 2 = 2HN0 2 

 + 2H 2 + 158 cal. ; the others, called Nitrobacter, and of the form 

 of immobile short rods (Fig. 103), oxidize nitrous acid, produced by 

 the first group, into nitric acid, according to the equation, 

 2HN0 2 + 2 = 2HNO3 + 38 cal. 



One will note that the second process yields considerably less 

 energy (calories). This may be collated with the fact that Nitro- 



somonas needs on the average for 



I } *£'H. tne assimilation of 1 atom of carbon 



* A \ ' *•*>*?!», the oxidation of 35 molecules of 



J i < nmmrmifl whilp Nitrobacter utilizes 



ammonia, while Nitrobacter utilizes 

 for the same quantity of carbon 135 

 molecules of nitrous acid. 

 Fig. 103.— Nitrobacter. Availing themselves of the oxi- 



dation of nitrogen compounds as 

 a source of energy and securing carbon from carbon dioxide of the 

 air, the nitrifying bacteria not only require no organic compounds 

 but are incapable of transforming them. Furthermore, organic 

 substances, among them the sugars, which are necessary to all 

 other organisms, are poisonous to them. Yet, in spite of all the 

 peculiarities of their metabolism, these bacteria are nevertheless 

 made of materials similar to those of other plants. Protein, in 

 particular, may be found in these organisms, though the question, 

 whether carbohydrates also take part in the building up of their 

 bodies, remains as yet unsettled. 



The nitrifying bacteria have a very important role in the 

 general cycle of nitrogen in nature. They are present in all soils 

 save marshy ones. Moreover, all the nitrates existing under nat- 

 ural conditions are exclusively the result of their activity. A very 

 peculiar feature of these organisms is the strict harmony in the 

 work of both groups. The nitrite- and nitrate-forming bacteria 

 are so closely united, that Winogradsky found it difficult to sepa- 

 rate them. 



The physiology of another group of organisms utilizing inor- 

 ganic substances during respiration, viz., the sulphur bacteria, has 

 also been investigated through the work of Winogradsky. These 

 organisms were referred to in Art. 25, when the sulphur cycle 



