104 THE STORY OF GERM LIFE. 



means of which the ammonia is united with oxy. 

 gen and built up into a series of products finally 

 resulting in nitrates (Fig. 26). By the action of 

 other species still higher nitrogen compounds, in- 

 cluding the nitrites, are further oxidized and built 

 up into the form of nitrates. Thus these nitrify- 

 ing organisms form the last link in the chain that 

 binds the animal kingdom to the vegetable king- 

 dom (Fig. 25 at 4). For after the nitrifying or- 

 ganisms have oxidized nitrogen cleavage products, 

 the results of the oxidation in the form of nitrates 

 or nitric acid are left in the soil, and may now be 

 seized upon by the roots of plants, and begin once 

 more their journey around the food cycle. In this 

 way it will be seen that while plants, by building 

 up compounds, form the connecting link between 

 the soil and animal life, bacteria in the other half 

 of the cycle, by reducing them again, give us the 

 connecting link between animal life and the soil. 

 The food cycle would be as incomplete without 

 the agency of bacterial life as it would be with- 

 out the agency of plant life. 



But even yet the food cycle is not complete. 

 Some of the processes of decomposition appear 

 to cause a portion of the nitrogen to fly out of 

 the circle at a tangent. In the process of de- 

 composition which is going on through the 

 agency of micro-organisms, a considerable part 

 of the nitrogen is dissipated into the air in the 

 form of free nitrogen. When a bit of meat de- 

 cays, part of the meat is, indeed, converted into 

 ammonia or other nitrogen compounds, but if the 

 putrefaction is allowed to go on, in the end a 

 considerable portion of it will be broken into 

 still simpler forms, and the nitrogen will finally be 

 dissipated into the air in the form of free nitro- 



