62 



ELEMENTARY BIOLOGY 



to cause certain diseases. Some of these germs, when supplied 

 with carbohydrates, are capable of fixing the nitrogen of the 

 atmosphere by combining it into proteins and other nitrogen 

 compounds. 



91. Nitrogen-fixing bacteria. On the roots of bean plants, 

 peas, clover, alfalfa, and other plants of this family there are 

 tiny swellings, or tubercles. Some of the nitrogen-fixing bacteria 



are found in all 

 of these tubercles. 

 The bacteria can 

 make much more 

 protein than they 

 can use, just as 

 most green plants 

 can manufacture 

 much more sugars 

 or starches than 

 they can use. The 

 plants of the leg- 

 ume family con- 

 tain, as a result, 

 a much larger pro- 

 portion of nitroge- 

 nous compounds 

 than any other 

 plants (Fig. 21). 



92. Rotation of crops. If we grow several crops of grain 

 on a farm, and find that the size of the crop tends to diminish 

 through lack of nitrogen, we do not have to abandon the farm, 

 nor do we have to import expensive nitrogen fertilizer. We 

 have only to plant a crop of peas or clover, and to see to it 

 that there is a plentiful supply of the special kinds of bacteria 

 that form the tubercles on the roots of our plants. It is now 

 possible to buy cultures of the species of bacteria that are 

 known to thrive best on any particular legume plant. 



FIG. 21. Tubercles on the roots of red clover 



