The Smallest of Living Things 237 



do not belong to the animal kingdom but to the vegetable 



kingdom. Each individual is a tiny plant, without 



roots, leaves, or stem (Fig. 



190). Unlike ordinary plants, ^/ a ^i/ & ^ J^ T Pf/y^- 



most bacteria are not able to 



prepare their supply of 



food from raw materials in 



Soil and air. None of them FIG- 190. Bacteria of different 



, shapes; some are spherical, some 



are able to Convert Carbon are rodlike, and some are spiral. 



from the air into plant food, 



though the nitrogen-fixing bacteria are able to use 

 nitrogen from the air in manufacturing their supply of 

 protein. Practically all of the bacteria must have for 

 food some plant or animal matter. 



For the most part, bacteria increase in the simplest How they 

 and speediest manner by constantly dividing (Fig. 191). ^^ber " 

 A single bacterium, which may be in the shape of a 

 ball or a rod, grows smaller and smaller at one point 

 until the parts separate and each becomes a new in- 

 dividual. Bacteria may double their numbers every 

 half hour if conditions are favorable. But supposing 

 that divisions into two took place every hour, a single 

 germ would in twenty-four hours increase to the enormous 

 number of 16,777,216, as can very easily be figured. 



The increase in the number of bacteria cannot go on Why they 

 indefinitely, for it is limited by the supply of food within multiply 

 their reach, and also by substances that they themselves without 

 produce, which, in large enough quantities, are poisonous 

 to them. An example of this is seen in the case of 

 vinegar, which is produced by bacteria that work in the 

 fermented juice of apples or other fruit. The bacteria 



