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. As a 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 



