THE TRANSFORMATION OF CARBON 49 
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 organ- 
isms bring about the gradual destruction of the different types of 
carbonaceous 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 
fermentation which goes on in a decaying apple is identical 
with that which occurs in the brewer’s vat. The result is the 
formation of CO: and alcohol, the carbon dioxid passing into 
the atmosphere to contribute to the store of this important food. 
The alcohol, under normal conditions, also passes into the air and 
is eventually further oxidized into carbonic acid and water. Thus, 
the carbon of the sugars, by the agency of yeasts and molds, is 
restored to the air. Starches have nearly the same history, since 
they are readily converted into sugars by enzymes secreted by the 
plants, and are then fermented. To a certain extent bacteria 
also ferment sugars, producing a series of acids. 
Cellulose.— Cellulose is a material closely related to starch, and 
is found in the cell walls of all plants. Wood and straw contain it 
in considerable quantity, while cotton and wood fibers are almost 
pure cellulose. Swedish filter-paper is one of the purest forms. 
The material is quite resistant to ordinary forms of decay and is 
seldom affected by common plant decay. But certain bacteria 
are able to act upon it so as to ferment it and set free its carbon. 
Several of these have been isolated and studied. These cellulose- 
fermenting bacteria are abundant everywhere and are constantly 
at work in the soil, fermenting the hard cellulose parts of the great 
variety of plant roots, stems, and leaves that accumulate in the 
soil or in the waters of streams and swamps. When the mass is 
alkaline in reaction the cellulose may be fermented by bacteria; 
4 
