56 THE CORNELL READING-COURSES 



Bacteria 



Bacteria are carried on particles of dust, in liquids, and on the surface 

 of fruits and vegetables as well as other articles of food exposed in the 

 market. They may possibly find their way into the house by means 

 of drains, and they are carried by insects. Normally, they are found 

 in the air, in the soil, in water, in food, in the mouth and the digestive 

 tract, on the skin, under the nails, in the hair, in the clothing. 



Bacteria are reproduced by a process of division known as " fission," 

 some of the different forms of which are shown in Fig. 27. The rapidity 

 of reproduction depends on warmth, moisture, and food supply. Some 

 species produce a new generation every half-hour; thus a single bacterium, 

 if its growth were totally unchecked, might become in twelve hours an 

 ancestor of sixteen million descendants. In two days the descendants 



''' 



FIG. 27. Various forms of bacteria, or germs, showing different 

 methods of fission. Greatly magnified 



would fill a pint measure. This rapidity of reproduction does not occur, 

 because there are countless checks to the life of every species of bacteria. 

 We may form some idea of the minuteness of bacteria when we con- 

 sider that the length of a single bacterium of some species is 1/25,000 of 

 an inch. Many thousands of them may be packed into the space that a 

 grain of sugar would occupy. If one falls into a minute wrinkle of the 

 hand, it is as though it had fallen into a deep ditch. 



Molds 



Molds also are micro-organisms. A colony of mold organisms growing 

 on some substance forms a velvety pile having a dark center. We often 

 see long threads budding and branching to form a network over food. 

 Each head produces thousands of dust-like spores. Some molds grow 

 with less moisture than is required for bacteria, and some flourish in the 

 light. They are frequently found in bread, on meat, on leather, and on 

 sugary liquids. They increase very rapidly after rainstorms, and wind 

 affects them less than it does bacteria. 



