Minnesota Plant Diseases. 



195 



up of compounds accompanied by the production of gases 

 just as is effected by yeast plants in bread- and beer-making 

 processes. These fermentation processes are of many kinds. 

 Butter becomes rancid and milk may be broken up and 

 soured by the action of these bacteria. Upon the action of 

 milk-fermenting bacteria depend other processes in certain 

 methods of curing hay and ensilage. Again, fermenting bac- 

 teria are the agents of fermentation in the production of vin- 

 egar. 



Nitrifying bacteria. Of great importance in agriculture 

 are those bacteria which live in the soil and by their action 

 prepare crude materials for leaf-green plants. The latter re- 

 quire a certain gas known as nitrogen which must be fur- 

 nished, however, in a particu- 

 lar kind of compound known 

 as a nitrate. Leaf-green 

 plants are unable to utilize 

 nitrogen gas in the free state 

 and this is the condition in 

 which it exists in the atmos- 

 phere. Now the nitrifying 

 bacteria are capable of using 

 compounds unavailable to the 

 leaf-green plants and by the 

 united action of several bac- 

 teria finally build up the ni- 

 trates desired by the leaf-green 

 plants. 



Nodule bacteria. Certain 

 plants such as clover and 

 many other plants of the pea 

 family form small nodules on 

 their roots. In these nodules 

 dwell bacteria, which are capa- 

 ble of using free nitrogen 

 from the air. They then pass 

 the manufactured nitrates on to the clover plant. These nod- 

 ules are therefore special habitations for nitrogen-fixing bacte- 

 ria, w T hich are thus protected and fostered by the clover plant. 



FIG. 98. Bacterial nodules on root of com- 

 mon bean. In these swollen portions of 

 the roots are found bacteria which as- 

 sist the plant in obtaining nitrogenous 

 food material. Original. 



