136 A TEXT-BOOK OF BOTANY 
in various other ways the loss of available nitrogen for 
plants is enormous; and to meet this loss is one of the most 
important problems, for the known sources of suitable 
fertilizers cannot yield them for very many years. 
Since an endless supply of free nitrogen exists in the air, 
it is natural to turn to this source of nitrogen supply for 
plants. This means that the free nitrogen of the air must 
be “fixed”? in some combination that can be used by plants. 
It is just here that bacteria of the soil play a very important 
part. Not only do those bacteria that produce fermenta- 
tion and decay lay hold of plant and animal bodies and 
produce the necessary nitrogen-containing substances in 
the soil, but certain other bacteria of the soil have the 
power of fixing the free nitrogen of the air into compounds, 
and hence they are called “nitrogen-fixing bacteria.” If 
worn-out land lies fallow for a few years there will be a 
slow accumulation of nitrogen salts through the activity of 
these bacteria. They have been cultivated artificially, and 
it is hoped that such cultures will be obtained that it may 
be possible to use them to inoculate impoverished land 
with nitrogen-fixing bacteria and so hasten its restoration. 
A peculiar group of soil bacteria penetrates the roots of 
certain leguminous plants, as clover, alfalfa, peas, beans, 
etc., and produces little wart-like outgrowths known as 
root-tubercles (Fig. 123). The cells of the tubercles swarm 
with bacteria, which are found to have the power of fixing 
the free nitrogen of the air circulating in the soil. Asa con- 
sequence, such plants can live and thrive in a soil exhausted 
of its nitrogen salts, and can be used in restoring the soil. 
After an ordinary crop, such as wheat, has exhausted the 
soil, a crop of clover or of alfalfa plowed under will restore 
such an amount of nitrogen salts to the soil that it can be 
used again for wheat; often with a surprising yield. This 
indicates the significance of what is called rotation of crops. 
It is a very interesting and important fact that these 
