THALLOPHYTA 307 
elements to the soil in a form in which they are available to 
higher plants the bacteria perform a very useful function. If it 
were not for the action of organisms which cause decay it is 
reasonable to suppose that the earth would be deeply covered 
by the products of the life activities of the higher organisms. 
When bacteria cause the decay of substances such as wood 
or food which man desires to preserve, they are regarded as 
injurious. It should be noted, however, that in causing decay 
bacteria perform a function that is naturally very useful. 
Bacteria sometimes produce substances that are useful to 
man. The souring of milk is due to the formation of lactic 
acid by bacteria. Vinegar is essentially a solution of acetic acid, 
and the so-called natural vinegar is produced by the action of 
bacteria that convert the alcohol of wine into acetic acid. | 
Disease bacteria. Infectious and contagious diseases are caused 
by small organisms that grow inside of their hosts. Some dis- 
eases are due to small animals, but the majority of the infectious 
and contagious diseases of man and other animals are caused by 
bacteria. Bacteria are responsible for such diseases as tubercu- 
losis, typhoid fever, cholera, leprosy, plague, and pneumonia. 
Many diseases of plants are also caused by bacteria. There are 
two general ways of preventing bacterial diseases: the most ob- 
vious is to keep the bacteria from entering the host; the other 
method is to render the host immune to the attack of a given 
kind of bacteria, as by vaccination. 
Nitrogen-fixing bacteria. Green plants obtain nitrogen from 
the soil in the form of compounds. Certain bacteria have the 
property of combining atmospheric nitrogen and fixing it in com- 
pounds that can be absorbed and used by ordinary green plants. 
Some of these bacteria live free in the soil. The energy which 
they use in fixing the nitrogen is obtained by the oxidation 
of carbohydrates. Other bacteria cause the growth of nodules 
on the roots of legumes (Fig. 310). The bacteria live in these 
nodules and combine the nitrogen of the air in such a way that 
it can be absorbed by the legumes. When such legumes die they 
enrich the soil with nitrogen that was fixed by the bacteria. It is 
