42 ANIMAL BIOLOGY 



teria) acts upon the ammonia and transforms it into nitrous acid 

 (HNO2). After certain chemical reactions in the soil, forming, e.g., 

 potassium nitrite or ammonium nitrite, still another type (Nitrate 

 Bacteria) oxidizes the nitrites into nitrates (e.g., KNO3 or 

 NH4NO3), so that again this nitrogen is in a form which is avail- 

 able for green plants. 



But, still confining our attention to the nitrogen, it is obvious 

 that there is a leak from this cycle, since some of the nitrogen in 

 the form of ammonia or free nitrogen escapes to the atmosphere. 

 The greatest loss, however, is brought about by a group of Deni- 

 trifying Bacteria whose activities are largely spent in changing 

 nitrates into gaseous nitrogen which escapes into the air, and so 

 is placed beyond the reach of green plants and animals. But 

 fortunately there are many kinds of Nitrogen-fixing Bacteria 

 which rescue the nitrogen from the atmosphere and return it to 

 the cycle of elements in living nature. These organisms are widely 

 distributed, some living freely in the soil and others in tiny nodules 

 which they produce on the rootlets of higher plants, such as Beans, 

 Clover, and Alfalfa; and this accounts for the fact, long known 

 but not understood, that these plants when plowed under are 

 particularly efficient in enriching the soil. 



In brief, there is a cycle of the elements in nature through green 

 plants and animals and back again to the inorganic world through 

 the Bacteria and other colorless plants. Such is the reciprocal 

 nature of the nutritive processes of living organisms. 



It is hardly necessary to state that the chemical changes pro- 

 duced by the Bacteria are either the direct results of, or are in- 

 cidental to the process of nutrition in these organisms. Therefore 

 the material taken as food by certain groups is relatively complex: 

 for example, by those which bring about the early putrefactive 

 changes in proteins ; while that employed by others is very simple 

 since they find adequate chemical combinations less complex than 

 those needed by green plants. Indeed, certain Bacteria are able to 

 utilize carbon dioxide and water just as do green plants, but instead 

 of obtaining energy for the synthesis from sunlight, these auto- 

 trophic forms derive it from chemical energy liberated by the 

 oxidation of inorganic substances in their environment. Such a 

 process possibly represents the most primitive method of nutri- 

 tion from which all the others have been derived in the evolution 

 of life. 



