38 FUNGI 



5. Putrefactive Bacteria. Putrefactive bacteria break up 

 the complex nitrogeneous and other compounds, occurring 

 in lifeless material of both plants and animals, into nauseating 

 gases and simple compounds which are utilized by plants as 

 food. If it were not for these bacteria the earth would be 

 an immense graveyard piled mountains high with dead plants 

 and animals. 



Nitrifying, denitrifying and putrefactive bacteria make 

 it possible, therefore, for present generations to live upon the 

 food furnished by past generations. In like manner present 

 generations of plants and animals will furnish food for future 

 generations. 



III. Parasitic Bacteria, derive their food from living 

 organisms. There are two classes of parasitic bacteria: 



1. Plant parasites, or those living on plants. 



2. Animal parasites, or those living on animals. 



1. Plant Parasites. (a) Useful Parasites. The parasitic 

 nitrogen-fixing bacteria (Pseudomonas radicicola), appear 

 as swellings or nodules on the roots of beans, peas, clovers 

 and other leguminous plants and form nitrates from atmos- 

 pheric nitrogen and the inorganic salts of cell sap. It is now 

 a common practice for farmers to mix cultures of nitrogen- 

 fixing bacteria with leguminous seeds in order to insure the 

 formation of nodules and large crops. After the soil has been 

 inoculated it is not necessary to mix bacteria with seed before 

 planting. The association of the nitrogen-fixing bacteria 

 with leguminous plants is mutually helpful and is known as 

 symbiosis. 



(b) Harmful Parasites. Disease-producing bacteria derive 

 their food from their host. They are all harmful and fre- 

 quently fatal to the plants upon which they live. Some of 

 the destructive plant diseases caused by bacteria are black rot 

 of cabbage, wilt of sweet corn, and wilt or blight of the bean. 



