50 NATURE’S FOOD SUPPLY. THE CARBON CYCLE 
but when it is acid, as in sour soils, bacteria cannot grow. Under 
these conditions certain of the molds in the soil may ferment the 
cellulose. The chemical nature of the fermentation need not 
concern us, only so far as to notice that, as a result, the carbon is 
set free, either in the form of carbonic dioxid or marsh gas (CH,), 
the latter gas becoming readily converted later into carbon dioxid. 
The total result is the restoration of the carbon to its original con- 
dition in the air, where it can be utilized by the next generation of 
plants. Certain mineral matters are also set free from the cellu- 
lose in the form of ash, which adds to the fertility of the soil. 
A fermentation of cellulose is believed to occur also in the intes- 
tines of herbivorous animals. These animals utilize, to a certain 
extent, cellulose materials as a food; these undergo a fermentation 
in the intestines resulting in the formation of certain substances 
that are assimilated by the animals asfood. Cellulose-fermenting 
bacteria are found in the intestines of such animals in considerable 
abundance, and it is thought that they play an important part in 
the ordinary digestion of celluloses. Whether the animal might 
not be able to digest them without the aid of bacteria has not yet 
been proved, but it is almost certain that the bacteria do, under 
ordinary conditions, play an important part in the process. The 
fermentation begun in the intestines is finally completed in the 
manure heap, and thus, after a time, the cellulose is completely de- 
composed and its carbon restored to the atmosphere. 
Wood.—Another product of plant life somewhat closely 
related to cellulose is woody tissue. The fermentation and de- 
struction of wood is certainly a matter of necessity, if the carbon 
supply is to be kept constant. That there is such a fermentation is 
evident to anyone who has walked through a forest and noticed the 
condition of the fallen trunks and branches. <A fallen tree will 
remain for a time upon the surface of the ground, apparently 
unaltered. But presently it becomes softened by some agency, not 
manifest at first, and the hard, woody mass is slowly but surely 
converted into a soft friable substance, which eventually crumbles 
