IMPORTANCE OF PLANTS TO MAN 245 



all other organisms. Some bacteria can live even with- 

 out free oxygen. They multiply very rapidly by simple 

 division. An adult individual may divide into two, each 

 of these into two, and so on. The rapidity with which 

 they multiply is almost inconceivable. It has been 

 estimated that a single individual will produce about 

 17,000,000 offspring in twenty-four hours. 



Conditions for growth of bacteria. Like other living 

 things, bacteria will not grow at freezing temperature 

 or below. They will grow at nearly all tem- 

 peratures above freezing, some species even growing 

 at 140 F. Most bacteria are killed by excessive heat, 

 and usually a temperature of from 149 F. to 160 F., if 

 continued for an hour, is sufficient to destroy them. 

 When the bacteria in a substance are destroyed^ the 

 substance is said to be sterilized. A low temperature 

 may not kill them, but it does prevent their rapid 

 development. Most bacteria need plenty of air and 

 plenty of water. Fortunately direct sunlight kills 

 them, and this fact suggests a possible advantage in 

 having plenty of sunshine in our houses. 



Bacteria and disease. Many diseases of plants and 

 animals are produced by bacteria. By far the most 

 important of the disease-producing bacteria are those 

 which may be parasitic in the human body and cause 

 such diseases as tuberculosis, pneumonia, cholera, 

 typhoid fever, diphtheria, leprosy, lockjaw, and many 

 others. These bacteria are frequently transferred from 

 one person to another. A disease which can be spread 



