32 AGRICULTURAL BACTERIOLOGY. 



almost unlimited power which is constantly curbed by the lack 

 of proper conditions. Bacteria may thus be looked upon as 

 possessing this wonderful possibility of reproduction, a force of 

 inconceivable magnitude, held more or less in check by adverse 

 conditions, but ever ready to exert its influence when the con- 

 ditions are favorable. It is this reserve force, possessed in 

 greater or less degree by all bacteria, which makes them such 

 wonderful and powerful agents in producing the great changes 

 in nature which we are now forced to attribute to them. 



GENERAL PROPERTIES OF BACTERIA. 



Relation to Food. A few general facts concerning the 

 conditions of life of bacteria must be mentioned as necessary 

 to an understanding of their activities. They are colorless or- 

 ganisms and consequently are commonly unable to make use 

 of the energy of sunlight, but must, like animals, depend upon 

 the decomposition of organic foods as a source of life energy. 

 Hence organic material is commonly needed for food. Some 

 of them obtain this organic food from the dead bodies of ani- 

 mals and plants existing in abundance throughout nature. 

 Such bacteria are called saprophytes, and they find their sus- 

 tinence in the lifeless organic matter in water and soil. Other 

 species have the habit of feeding upon living organic matter, 

 a habit which makes them parasites. These live upon the 

 living tissues of either animals or plants. It must be noted 

 here that there are a few bacteria which are exceptions to this 

 need for organic food and which can subsist upon mineral 

 food. These will be referred to in a proper place. 



Relation to Oxygen. Most bacteria demand oxygen to 

 enable them to carry on their life processes, thus agreeing with 

 animals and plants in general. Hut there are some species, 

 cjuitc numerous and abundant as we now know, that can live 

 uithout a supply of oxygen, and some indeed, that can only 

 grow and multiply when in an atmosphere devoid of oxygen. 



