102 THE STORY OF GERM LIFE. 



into simpler and simpler compounds, and the 

 final result is a very thorough destruction of the 

 animal body or the material excreted by animal 

 life, and its reduction into forms simple enough 

 for plants to use again as foods. Thus the bac- 

 teria come in as a necessary link to connect the 

 animal body, or the excretion from the animal 

 body, with the soil again, and therefore with that 

 part of the circle in which the material can once 

 more serve as plant food. 



But in the decomposition that thus occurs 

 through the agency of the putrefactive bacteria 

 it very commonly happens that some of the food 

 material is broken down into compounds too sim- 

 ple for use as plant food. As will be seen by a 

 glance at the diagram (Fig. 25 D), a portion of the 

 cleavage products resulting from the destruction 

 of these animal foods takes the form of carbonic- 

 acid gas and water. These ingredients are at 

 once in condition for plant life, as shown by the 

 dotted lines. They pass off into the air, and the 

 green leaves of vegetation everywhere again 

 seize them, assimilate them, and use them as 

 food. Thus it is that the carbon and the oxygen 

 have completed the cycle, and have come back 

 again to the position in the circle where they 

 started. In regard to the nitrogen portion of the 

 food, however, it very commonly happens that the 

 products which arise as the result of the decom- 

 position processes are not yet in proper condition 

 for plant food. They are reduced into a condition 

 actually too simple for the use of plants. As a 

 result of these putrefactive changes, the nitrogen 

 products of animal life are broken frequently 

 into compounds as simple as ammonia (NH 3 ), or 

 into compounds which the chemists speak of as 



