THE TRANSFORMATION OF CARBON. 43 
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 2 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. Some of them act 
in the absence of oxygen (anaerobic), while others act only in its 
presence (aerobic). The former can carry on their activity in 
the midst of a manure heap so tightly packed as to exclude air, 
while the latter will occur in moistened masses of vegetable tissue 
exposed to the air. These cellulose-fermenting bacteria are abun- 
dant 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; but when it is acid, as in sour soils, bacteria 
cannot grow. Under these conditions certain of the molds in the 
soil may ferment the cellulose. The chemical nature of the fermen- 
tation need not concern us, only so far as to notice that, as a result, 
the carbon is set free, either in the form of carbonic dioxid or 
marsh gas (CH 4 ), the latter gas becoming readily converted later 
into carbon dioxid. The total result is the restoration of the 
carbon to its original condition in the air, where it can be utilized 
