ECONOMIC IMPORTANCE OF FUNGI 



319 



remarkable as the carbon cycle. By respiration and 

 burning, complex compounds of carbon (carbohydrates 

 and hydrocarbons) are broken down and the carbon re- 

 leased, either as pure carbon (C) or as the simple com- 

 pound, carbon dioxide (CO2), which is taken in by green 

 plants and recombined into the complex carbohydrates 

 by the process of photosynthesis (page 77). 



Benitrifying-'. 

 .'.■. Bacteria ■.•,•-' 



; (Hltrogen] 

 Fig. 229. — ^The nitrogen cycle. 



So also, by the action of proteolytic (protein dissolving) 

 enzymes produced by bacteria, certain complex com- 

 pounds of nitrogen, the proteins, are broken down, by 

 putrefaction and similar processes, into simpler com- 

 pounds, such as ammonia (NH3), or still further dis- 

 integrated until free nitrogen results. Were it not for 



