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 bef ore, " 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. Nodu i es C0 ntain the nitrogen fixing bacteria on 



- The most harmful bac- the roots of clover. 



