160 Introduction to Botany. 



show the importance of bacteria in the economy of nature, 

 a brief statement will be made of some of the things which 

 are known to be accomplished by them. 



156. Bacteria of Economic Importance. There are forms 

 existing in the soil, known as nitrifying bacteria, which 

 bring about the oxidation of certain nitrogen compounds 

 brought down by rains or resulting from the decay of plant 

 and animal bodies, nitric acid being produced, a sub- 

 stance absolutely necessary to the nutrition of plants. Its 

 production by the bacteria is of great importance, since it 

 exists in the soil in relatively minute quantities and is easily 

 carried away by percolating water. 



Bacteria of another kind inhabit the root tubercles of 

 leguminous plants, and, taking the free nitrogen of the air, 

 build it into the constitution of their own bodies. Finally 

 the bacteria appear to be digested and appropriated as 

 food by the leguminous plants (Fig. 13). In this way the 

 green plant is able to obtain indirectly the free nitrogen of 

 the atmosphere, which otherwise would be inaccessible to 

 it. A knowledge of these facts is of utility to agriculture, 

 for land naturally poor in nitrogen can be sown to alfalfa 

 or plants of similar character, which, when plowed under, 

 leave the land much richer in nitrogen than it was before. 

 It has been found practicable to inoculate with liquid cultures 

 of the bacteria soils which do not naturally contain them. 

 The importance of these bacteria to leguminous plants grow- 

 ing in soils poor in compounds of nitrogen is shown by the 

 experiment illustrated in Fig. 135. 



Other forms are instrumental in the production of vinegar 

 from alcohol, in the proper ripening of cheese, and others 

 add to the agreeable flavor of butter, as has been efficiently 

 demonstrated in the dairies of Finland and Denmark. 



157. Disease-producing Bacteria. There are other forms 



