178 HOW PLANTS BENEFIT AND HARM MANKIND 



Fahrenheit for five to twenty minutes for the purpose of killing 

 the bacteria) will not sour readily, if kept in a cool place. Why ? 



These same lactic acid bac- 

 teria may be useful when 

 they sour the milk for the 

 cheese-maker. Certain 

 other bacteria give flavor 

 to cheese and butter, while 

 still others are used by the 

 tanner. 



Nitrogen-fixing Bacteria. 

 Still other bacteria, as 

 we have seen before, ' ' change 

 over " nitrogen in organic 



A pasteurizing apparatus. 



material in the soil and even the free nitrogen of the air so that it 



can be used by plants in the form of a compound of nitrogen. The 



bacteria living in tubercles 



on the roots of clover, beans, 



peas, etc., have the power 



of thus " fixing " the free 



nitrogen in the air found 



between particles of soil. 



This fact is made use of 



by farmers who rotate their 



crops, growing first a crop 



of clover or alfalfa, which 



produce the bacteria, then 



plowing these up and plant- 

 ing another crop, as wheat 



or corn, on the same area. 



The latter plants, making 



use of the nitrogen com- 

 pounds there, produce a 



larger crop than when grown 



in ground containing less 



nitrogenous material. 



Bacteria cause Disease. Nodules contain the nitrogen fixing bacteria on 

 - The most harmful bac- the roots of clover. 



