42 NATURE'S FOOD-SUPPLY. THE CARBON CYCLE. 
found in the starch alone; while, by combining them with some 
nitrogen, various proteid bodies are produced. When once carbon 
has assumed these forms it is no longer within the reach of another 
generation of plants. It is locked up and cannot again be utilized 
until it has once more been reduced to a condition of carbon dioxid. 
The compounds thus built up have different destinies. Some of 
these are eaten by the animal kindgom and, after serving the needs 
of the animals, are exhaled as CO 2 to join the atmosphere again. A 
large part is not appropriated by animals, but begins at once to 
undergo destructive changes which bring their ingredients back again 
to their starting-point. Some of the processes of chemical destruction 
are comparatively simple. The starches, sugars, and fats are sub- 
ject to chemical changes which take place under the direct in- 
fluence of chemical forces, since they may be directly oxidized. All 
forms of active combustion in fires produce such oxidation, the result 
of which is that the carbon in the compounds burned is united with 
oxygen and liberated in the form of CO 2 , the hydrogen being 
liberated in the form of water. These join the atmosphere, while 
the minerals remain behind as ash. Thus, all forms of combustion 
in carbonaceous material restore some of the carbon to the atmos- 
phere in the form of CO 2 , and upon this the plants again feed. 
But although direct oxidation may form a considerable part of 
this process of food reduction, another very large factor is due to the 
agency of microorganisms. Fires rarely occur in nature, unless 
started by man, and there must be some other means of oxidation. 
A slow oxidation of carbonaceous material occurs in nature at all 
times, and ordinarily it has been attributed to direct chemical 
processes. It is quite doubtful, however, if this slow oxidation 
would occur were it not for the agency of microorganisms. At all 
events a considerable part of the so-called slow oxidizing processes 
is the direct result of their growth. The various kinds of organisms 
bring about the gradual destruction of the different types of car- 
bonaceous materials. 
Sugars. These are contained in fruits and some vegetables, 
and as they decay, the sugar commonly undergoes an alcoholic 
fermentation, produced by the action of yeasts and molds. The 
