FRIENDS AND ENEMIES OF PLANTS 169 



one apple to another, and hence the necessity of removing at once 

 all apples affected with rot. If we apply some mold with a 

 match or nail to a clean, damp piece of bread and in lines or rows 

 across the bread, we find that in a little while the mold begins to 

 appear in these rows. 



Bacteria are very minute plants so small that more than 1500 of 

 them would be required to reach across an ordinary pin head. 

 Some of them are harmful, some produce decay and disease, while 

 others are beneficial. Those which cause diseases are often called 

 microbes or germs. Many bacteria, however, are of great value 

 to us and assist in the digestion of our food. Millions of bacteria 

 are present in the air we breathe, in the water we drink, and in the 

 food we eat. They also inhabit the bodies of both plants and 

 animals. 



With reference to structure bacteria may be divided into three 

 classes or forms: (1) spherical bacteria, which are one-celled organ- 

 isms grouped in various ways; (2) rod-shaped bacteria, in which 

 the cells lie end to end, either attached or unattached; (3) elon- 

 gated bacteria, in which each bacterium is an elongated cell and is 

 curved somewhat like a comma or a spiral. 



Bacteria reproduce themselves by cell division in fifteen to 

 forty-five minutes, and may repeat this operation continuously. 

 Many of them are able to withstand long-continued dry weather 

 or extreme cold. It is said that the bacterium of typhoid fever 

 may become quite active again after being frozen in ice for several 

 months. The only safeguards against such bacteria are to boil all 

 the water we drink and to cook well all the food we eat. 



With reference to their relation to animals and plants, bacteria 

 may be grouped under the following heads : (1) agents of fermenta- 

 tion, (2) agents of disease, (3) agents of nitrification. 



Agents of Fermentation. These are the bacteria that aid in the 

 manufacture of vinegar and cause the souring of milk and various 

 fruit juices. They also cause decomposition of organic matter in 

 the soil and are very important in relation to soil fertility. 



Agents of Disease. Among animals these bacteria produce 

 diphtheria, typhoid fever, cholera, pneumonia, consumption or 

 tuberculosis, and other kindred diseases. Among plants they 

 produce many diseases, such as pear blight, apple blight, crown gall 



