60 BACTERIOLOGY. 



of the air. Owing- to the absence of chlorophyll the 

 majority of bacteria cannot utilize this source of carbon 

 They are, as a rule, dependent upon organic compounds. 

 That is to say, the more or less complex carbon compounds 

 elaborated by higher plant and animal life constitute the 

 food out of which the bacteria can appropriate the carbon 

 and build up their own protoplasm. To a very large ex- 

 tent, therefore, bacteria depend for their food upon dead 

 animal and vegetable matter. In this respect they resemble 

 animal life. As is well known the animal cannot utilize 

 the carbonic acid of the air but obtains its carbon from the 

 preformed carbon compounds in the animal or vegetable 

 food. The proteins, carbohydrates and fats supply the 

 necessary carbon to the growing animal and also to the 

 vegetating bacteria. The latter may also obtain their 

 carbon from more simple organic compounds such as 

 glycerin, or lactic, or tartaric acids. It may be interesting 

 to note that bacteria, like animals, instead of assimilating 

 actually give off carbonic acid. 



The above is true, undoubtedly, for the majority of bac- 

 teria. There are, however, certain bacteria which can live 

 on wholly inorganic matter. The interesting group of nitro- 

 bacteria, although they do not contain chlorophyll, are nev- 

 ertheless able to assimilate carbonic acid even in the absence 

 of sun-light. Organisms of this type may well be consid- 

 ered as belonging 1 to the earliest inhabitants of the globe. 



Nitrogen. -^Carbon is a characteristic and essential con- 

 stituent of protoplasm. There are other elements equally 

 important, and among these nitrogen deserves especial at- 

 tention. Although the air contains nearly 80 per cent, of 

 free nitrogen, the higher plant cannot obtain this element 

 from this source. All the nitrogen which enters into the 

 composition of the higher plants, with certain exceptions 

 presently to be mentioned, is derived from various nitrogen 

 compounds present in the soil. The ammonia, nitrous and 



