BACTERIA. 
15 
soil unless the supply was continually replenished. 
Now, the source of carbon and nitrogen is the excre¬ 
tions and secretions of animals, which contain these 
elements in combination with other elements. By the 
action of bacteria the complex animal matter is de¬ 
composed into the chemical elements that compose it. 
In this way the plants derive their carbon and nitro¬ 
gen from the soil. Within the body the bacteria carry 
on much the same activities. The digestion and ab¬ 
sorption in, the intestine is dependent to a large extent 
on the breaking-down action of bacteria. We cannot 
absorb meat and vegetable as such, and it is only after 
our food has been separated into simple compounds 
and elements that it is absorbed to nourish the body. 
In this process the bacteria play no small part. But 
bacteria are not only agents capable of breaking down 
complex substances; they also build up substances 
from chemical elements. Some plants take their nitro¬ 
gen from the air, but they would not be able to do so 
were it not for the presence of certain bacteria grow¬ 
ing in the roots. 
The maintenance of life in the world is often 
described as a cycle; first, the chemical elements are 
built up into plants, the plants nourish the animals, 
then the animal tissue is consumed and excreted to be 
broken down into elements. In each step the bacteria 
play a most important part. 
These activities of bacteria and their enzymes are 
made use of commercially; the fermenting action on 
sugars converting them into alcohol is used in making 
Commer¬ 
cial use 
of bac¬ 
teria 
