Bacteria and Fungi 



251 



hygiene, are primarily based on our knowledge of the be- 

 havior of this group of plants. 



FIG. 148. Various forms of bacteria. 



Economically the bacteria are of the greatest importance. 

 Together with the fungi they are the principal cause of decay. 

 Bacteria bring about the ripening of milk in butter and 

 cheese making, and they produce both the pleasant flavors 

 in these products and the unpleasant flavors that develop in 

 them with age. The bacteria are also the source of much of 

 the available nitrogen in agricultural soils (page 32). The 

 drying of hay, vegetables, and fruits, the canning and pickling 

 of vegetables, fruits, and meats, and refrigeration and cold 

 storage, are methods of avoiding or making impossible the 

 growth of bacteria. Thus a knowledge of these plants is 

 fundamental to our understanding of thousands of details 

 of our daily life. 



Growth and reproduction of bacteria. Bacteria grow best 

 and reproduce most rapidly in the presence of organic food 

 materials, in the absence of light, in the presence of moisture, 

 and at moderately high temperatures like those of the mid- 



