BACTERIA IN NATURAL PROCESSES. IOI 



body. But urea is not a plant food; for ordinary 

 plants are entirely unable to make use of it. 

 Part of the nitrogen eaten by the animal is stored 

 up in its body, and thus the body of the animal, 

 after it has died, contains these nitrogen com- 

 pounds of high complexity. But plants are not 

 able to use these compounds. A plant can not be 

 fed upon muscle tissue, nor upon fats, nor bones, 

 for these are compounds so complex that the sim- 

 ple plant is unable to use them at all. So far, 

 then, in the food cycle the compounds taken from 

 the soil have been built up into compounds of 

 greater and greater complexity ; they have reached 

 the top of this circle, and no part of them, except 

 part of the carbon and oxygen, has become re- 

 duced again to plant food. In order that this 

 material should again become capable of enter- 

 ing into the life of plants so as to go over the 

 circle again, it is necessary for it to be once 

 more reduced from its highly complex condition 

 into a simpler one. 



Now come into play these decomposition 

 agencies which we have been studying under the 

 head of scavengers. It will be noticed that the 

 next step in the food cycle is taken by the de- 

 composition bacteria. These organisms, exist- 

 ing, as we have already seen, in the air, in the 

 soil, in the water, and always ready to seize hold 

 of any organic substance that may furnish them 

 with food, feed upon the products of animal life, 

 whether they are such products as muscle tissue, 

 or fat, or sugar, or whether they are the excreted 

 products of animal life, such as urea, and produce 

 therein the chemical decomposition changes al- 

 ready noticed. As a result of this chemical 

 decomposition, the complex bodies are broken 



