44 NATURE'S FOOD-SUPPLY. THE CARBON CYCLE. 
by the next generation of plants. Certain mineral matters are 
also set free from the cellulose 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 as food. 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 destruc- 
tion 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 
into a brownish powder and is incorporated into the soil, contributing 
to the formation of the humus. This destruction of woody tissue 
is also brought about by microorganisms, but in this case it is not 
bacteria that are at first concerned. 
The first phenomenon that occurs in such a decaying tree trunk 
is the growth of larger fungi. Various forms of mushrooms and 
tree fungi start their growth on its surface and send delicate mycelium 
threads into the substance of the wood. These threads grow first 
underneath the bark and in the superficial layers of wood; but 
