144 PLANT BIOLOGY 



trition of the higher plants. And since plants are con- 

 stantly taking from the soil the food materials which they 

 need, this soil would tend to become less and less fertile 

 were it not for the work of the bacteria that caused de- 

 composition. This is the reason why rotting manure adds 

 to the fertility of soil. 



Again it has been proved that certain kinds of bacteria 

 directly increase the amount of nitrogen compounds that are 



so essential for plant 

 growth. It has long 

 been known that corn 

 and other crops will grow 

 better in soil that has 

 just borne a crop of peas, 

 beans, clover, or other 

 members of the pea 

 family. Within recent 

 years an explanation of 

 this fact has been found. 

 When the roots of those 



FIG. 73.- Bacteria from root-tubercles.- Pd-bearing plants are 



(Duggar.) examined, small swellings 



are seen. These contain 



multitudes of bacteria that are able to take the free nitrogen 

 from the air, where it exists in such abundance, and store it 

 away in the form of nitrates which are very important mineral 

 matters needed by all crops. Since those bacteria can be put 

 into soils that do not have them, it may be possible in the 

 near future to restore much of the fertility which has been lost. 



161. Relation of bacteria to the flavors of food. Again, 

 many of the flavors of food are due to the action of bacteria. 

 Meats, for instance, when freshly killed,. are tough and taste- 

 less. If allowed to stand, however, by the decomposing 



