1 8 THE NATURE OF BACTERIA. 



the nineteenth century these plants .were not recognized as 

 forming a logical group, and not until about twenty-five 

 years ago were they grouped together under the name Bac- 

 teria. To-day, while we regard them as a distinct group, 

 we recognize that they are closely related to yeasts and only 

 a little more distantly related to molds. In the following 

 pages both yeasts and molds will be occasionally referred to, 

 but the part they play in dairy products is far less important 

 than that of bacteria. 



In order to understand bacteria and their allies a brief 

 account of their structure, classification and general func- 

 tions is necessary : 



I. CLASSIFICATION OF FUNGI. 



Bacteria belong to the group of colorless plants called 

 Fungi. All of the group of Fungi are of great significance 

 to agriculture, performing important functions based upon 

 their power of decomposing organic substances and using 

 them as food. Unlike the ordinary green plants, they are 

 commonly unable to live upon mineral foods and are in con- 

 sequence extremely important agents in disposing of the 

 great quantities of dead organic matter in nature. These 

 Fungi may be for our purpose conveniently divided into 

 three divisions: 



i. Higher Fungi. Under this head we may group to- 

 gether a large variety of colorless plants comprising several 

 large classes, including such forms as molds, mushrooms, 

 etc. We are only incidentally concerned with them in this 

 work, although many are of great importance in .agriculture. 

 They are generally characterized by the development of long 

 slender threads (mycelium), which grow like delicate roots 

 through the substance of the material upon which they are 

 nourished (Fig. i). These threads make it possible for 

 them to force their way into hard substances, like wood, and 

 to effect their decomposition. 



