/8 AGRICULTURAL BACTERIOLOGY. 



pieces and reduced to much simpler compounds. It is, how- 

 ever, not decomposed sufficiently to bring the nitrogen back 

 within the reach of plant life. The carbon in this proteid 

 is in part removed from it and combined with oxygen to be 

 exhaled as CO 2 . This causes the molecule to fall to pieces 

 and various by-products to arise, but eventually it practically 

 all assumes the form of urea (CON 2 H 4 ), a nitrogen molecule 

 far simpler than proteid. Urea, or a closely allied compound, is 

 the form in which nearly all of the nitrogenous material result- 

 ing from proteid metabolism in the animal body is excreted 

 by the animal. Urea thus represents one stage in the destruc- 

 tion of proteid compounds, and to this stage the proteids are 

 brought as the result of the metabolism in the life processes of 

 animals. This urea is secreted by the kidneys and must be 

 looked upon as the nitrogen which is no longer of any use to 

 the animal world. But, although much simpler than proteids, 

 this urea is not a plant food for it is still too complex for 

 the use of plants. Common green plants are not able to utilize 

 these nitrogenous secretions of animal bodies. It is estimated 

 that some 38,000 tons of urea are excreted daily by the human 

 race. To this quantity must be added the far greater amount 

 excreted by other animals, and the total is thus enormous. 

 What becomes of it all ? 



3. The second part of the animal's food is not metabolized 

 during the animal's life, but becomes incorporated into the 

 body and remains at the close of its life as dead animal tissue, 

 still existing in as highly complex a form as ever. At the 

 animal's death its body contains sugars, fats and proteids, as 

 well as other nitrogenous substances. The sugars are readily 

 disposed of by fermentation as already noted, but the de- 

 struction and final utilization of the fats and the nitrogenous 

 compounds demand some further agencies than any we have 

 yet noticed. 



Thus the nitrogenous food consumed by animals has reached 



