PUTREFACTION OF PROTEINS 97 



and vegetable matter of all kinds which but for this action 

 would accumulate to such an extent that all life, both on 

 land and in the water, must cease. It is further to be noted 

 that not only is all this dead organic matter removed, but 

 it is converted into forms which are again available for 

 plant growth. Carbon dioxide forms the source of the car- 

 bon in all green plants, hence in all animals; the sulphates 

 and phosphates are likewise taken up by green plants and 

 built up again into protein compounds; the ammonia is not 

 directly available to green plants to any large extent but 

 is converted by the nitrifying bacteria (Chapter XI) into 



Dead animal tyrotein Decompqsiiion bacteria 



Animals Dead ulcCht HgVS 



4- jpr6tein \ 



Flark protein Sulphir bacteria 



Greerb,i>lants Sulpmr bacteria 



Sulphates in the soil 



Fig. 70. — Diagram to illustrate the circulation of sulphur through the 

 agency of bacteria. 



nitrates which is the form in which nitrogen is assimilated 

 by these higher types. Even the free nitrogen of the air is 

 taken up by several kinds of bacteria, the symbiotic "root- 

 tubercle bacteria" of leguminous and other plants, and some 

 free-living forms, and made available. Hence bacteria are 

 indispensable in nature, especially in keeping up the circu- 

 lation of nitrogen. They are also of great service in the 

 circulation of carbon, sulphur, and phosphorus. Though 

 some few kinds cause disease in man and animals, if it were 

 not for the saprophytic bacteria above outlined, there could 

 be no animals and higher plants to acquire these diseases. 



