232 



THE FUNGI 



later into alcohols and organic acids. The last stages result in 

 the formation of the gas carbon dioxide (C0 2 ) and sometimes 

 marsh gas (CH 4 ), which, when mixed with hydrogen phosphide, 

 becomes the " will-o'-the-wisp " of swamps. 



FIG. 207. Diagram illustrating the circulation of nitrogen 



Nitrogen is taken by green plants from the nitrates, and through energy derived 

 from the sunlight the proteids are formed. Animals carry the process of 

 proteid manufacture somewhat farther. The nitrogen of the proteids is then 

 returned, through the decay of waste products (urea, etc.) and dead tissues, to 

 simpler substances, and finally to ammonia, which is worked over into nitrates 

 by the nitrifying bacteria. Free nitrogen is brought into the circle by the 

 nitrogen-fixing symbiotic bacteria 



Some important forms of fermentation have no connection 

 with living organisms, but are due to special substances called 

 unorganized ferments, or enzymes (Sec. 10). Such a ferment is 

 diastase, which converts starch to sugar. 



