USEFUL BACTERIA 



187 



They are useful because they feed upon dead bodies of plants and 

 animals which otherwise would soon cover the surface of the earth 

 to the exclusion of everything else. Bacteria may be considered 

 scavengers. They oxidize organic materials, changing them to 

 compounds that can be absorbed by plants and used in building 

 protoplasm. Without bacteria it would be impossible for life to 

 exist on the earth, for green plants would be unable to get the raw 

 food materials in forms that they could use in making food and 

 living matter. 



Relation of bacteria to fermentation. Bacteria continue the 

 process of fermentation begun by the yeasts. In making vinegar 

 the yeasts first make alcohol which the bacteria change to acetic 

 acid. The lactic-acid bacteria, which sour milk by changing the 

 milk sugar to an acid, are useful when they sour the milk for the 

 cheese maker. 



Other useful bacteria. Certain bacteria give flavor to cheese 

 and butter, others give flavor to sauerkraut, while still other 

 bacteria aid in the 



" curing " of to- /. . V- — ^ --/ . _\ 



bacco, in the prep- 

 aration of the 

 dye indigo, in the 

 ".retting " or fer- 

 mentation of cer- 

 tain fibers of plants 

 for the market, as 

 hemp, flax, and 

 ramie, in the rot- 

 ting of animal mat- 

 ter from the skel- 

 etons of sponges, 

 and in the process 

 of tanning hides 

 to make leather. 



Relation of bac- 

 teria to free nitrogen. It has been known since the time of the 

 Romans that the growth of clover, peas, beans, and other legumes 



[ plcc nts 1^ fccni metis J 



/ of decccW fcoeteri 



lxxe thrice 



Explain from the text and diagram what is meant by the nitrogen 

 cycle. What is the value of nitrogen-fixing bacteria ? 



