BACTERIA OF THE SOIL 59 



Exercise 86. Cellulose Decomposition with Formation of Hydrogen 



In the foregoing experiments methane (CH^) is the principal 

 gas formed from the decomposition of cellulose. If the culture 

 made by one of the first transfers be heated after inoculation to 

 75° C. for fifteen minutes and then cooled, the production of 

 methane will be replaced by that of hydrogen. The organisms 

 causing the hydrogenic fermentation of cellulose appear to be 

 spore-formers which can withstand the temperature of 75° C. 



SECTION XII 

 BACTERIA OF THE SOIL 



Of all the varied activities of bacteria in nature, none com- 

 pares in importance with the work of the soil bacteria. They 

 not only determine the fertility of the soil, but they serve as the 

 connecting link between the world of the Uviug and the world 

 of the dead. They are the great scavenging agents which tear 

 down the dead bodies of animals and plants and restore the car- 

 bon, nitrogen, sulphur, and other elements of the tissue to the 

 round of nature. The processes of nature are such that the 

 same material is repeatedly used, passing in endless cycle from 

 plant to plant or from plant to animal, and back again to plant, 

 but always with the intervention of bacteria. Without their 

 action dead bodies would accumulate and cover the surface of 

 the earth ; the kingdom of the living would be replaced by the 

 kingdom of the dead; and the world's supply of carbon and 

 nitrogen would be locked up in a form useless to most forms 

 of life. 



The soil may be regarded as the greatest field of bacterial 

 activity we know. The surface layers of the soil are usually 

 inhabited by many thousands of germs per gram. The soil fur- 

 nishes many of the germs found in lakes and streams, most 

 of those found in air, and some of those concerned in animal 

 diseases. Bacteria appear to be most numerous in the surface 

 layers of the soil and to diminish at increasing depths. Below 



