THE ENVIRONMENT OF BACTERIA. 61 



nitric acids present in the earth are absolutely essential to 

 the growth and development of most of the higher plants. 

 This form of life, therefore, obtains all of its nitrogen from 

 inorganic compounds. The animal organism, on the other 

 hand, cannot utilize these compounds as food. The nitro- 

 gen which is necessary to the building up of protoplasm in 

 the animal cell is derived from preformed, organic, nitrogen, 

 containing substances. Moreover, only certain nitrogen- 

 ous, organic compounds can serve as food. These are the 

 protein or albuminous substances which have been made by 

 the living plant or animal cell. Animal life is therefore 

 dependent upon plant life for its supply of nitrogen. 



Bacteria, like animal organisms, may obtain the nitrogen 

 necessary for their growth from organic, nitrogenous sub- 

 stances. The proteins present in dead animal or vegetable 

 matter constitute, therefore, an important source of nitrogen 

 for these lower forms of plant life. It must not be inferred, 

 however, that the proteins are the only source, for such is 

 by no means the case. Many bacteria can obtain their nec- 

 essary nitrogen from compounds that are vastly more .sim- 

 ple than the proteins. Thus, the amido acids, such as 

 asparagin, constitute an excellent food in this respect for 

 certain bacteria. Furthermore, many bacteria may obtain 

 their nitrogen from wholly inorganic compounds, such as 

 ammonium chloride. There are other species which can 

 thrive better on nitrates than on other forms of nitrogen. 



The nodules or tubercles which are found upon the roots 

 of leguminous plants, such as the pea, lupine, etc., are essen- 

 tially masses of certain bacteria, or bacteroids, which possess 

 the remarkable property of assimilating the free nitrogen 

 of the air and in transmitting this element to the growing 

 plant. A vigorous, healthy growth of such plants is directly 

 dependent upon the presence of these parasitic bacteria. 



It is -evident, therefore, that bacteria can obtain their 

 nitrogen from various sources. This element may be appro- 

 priated from the complex organic protein molecule, or from 



