54 LOUIS PASTEUR 



their food requirements. Different forms vary in 

 their reactions to oxygen, some requiring this gas, 

 while others (the anaerobes) will not grow if free 

 oxygen is present. 



Bacteria differ not only as to the substances 

 which they take in but also as to the substances 

 which they give out. Just as our own bodies give 

 off carbon dioxide and other products of excretion, 

 so do the bacteria get rid of various substances 

 characteristic of different varieties. It is chiefly 

 with respect to the materials which the bacteria 

 eliminate that many of the characteristic effects of 

 their growth and activity are brought about. The 

 role of bacteria in ripening cheese, curing tobacco, 

 and many other industrial processes is due to the 

 products of metabolism of certain species employed 

 for these purposes. One very important effect of 

 bacteria in relation to agriculture, depends upon 

 the property possessed by a few species of con- 

 verting the free nitrogen of the air into a form 

 which may be used as food for higher plants. Some 

 of these bacteria live within the roots of legumi- 

 nous plants, such as beans, clover, alfalfa, vetches, 

 etc., and consequently crops of these plants are 

 grown in order to increase the supply of available 

 nitrogen in the soil. 



