48 NATURE’S FOOD SUPPLY. THE CARBON CYCLE 
THE TRANSFORMATION OF CARBON 
The green plants seize the carbon dioxid (CO:) from the air by 
means of their leaves and, utilizing the energy of sunlight, build 
this carbon into higher compounds. Starch is formed first, and 
later other substances—cellulose, wood, fats, sugar—are built from 
the elements 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 kingdom and, after serving the 
needs of the animals, are exhaled as CO: 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 subject to chemical changes which take place 
under the direct influence 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 com- 
pounds burned is united with oxygen and liberated in the form of 
COz, 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 atmosphere in the form of COs, 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 
