PUTREFACTION OF PROTEINS 95 



mineralize them, as is frequently said. By so doing the 

 bacteria act as the great scavengers of Nature removing the 

 dead animal 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 fur- 

 ther 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 carbon 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 



Dead animal protein Decompqsiiion bacteria 



Freb S 



Animals Dead pja:ht 



-f jjr&iein 



Plan^ protein Sulphur bacteria 



vreje ," 



Greeris.plants Sulpltur bacteria 



Sulphates in the soil 



Fig. 65.— Diagram to illustrate the circulation of sulphur through the agency 



of bacteria. 



large extent but is converted by the nitrifying bacteria 

 (Chapter XI) into nitrates, which is the form in which nitro- 

 gen 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 avail- 

 able. Hence bacteria are indispensable in Nature, especially 

 in keeping up the circulation 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 outHned there could be no animals and higher plants 

 to acquire these diseases. 



