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 



