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 (CO 2 ), which is taken in by green 
plants and recombined into the complex carbohydrates 
by the process of photosynthesis (page 77). 
.;-; Nitrite';- :.- 
::.':' Bacteria '/. 
':( Hi tr o s omonaa ) 
. Hitrate... 
' ."Bacteria'. -.'.. 
(Nitrobaeter) 
Nitrogenl 
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 (NH 3 ), or still further ^dis- 
integrated until free nitrogen results. Were it not for 
