FUNGI 



375 



other plants and animals, and it has been a question whether 

 to consider them plants or animals, but it is customary now 

 to class them among plants as a special group of the fungi. 



Bacteria are the direct causes of most of the diseases of 

 men and of other animals. The study of bacteria is a 

 special subject called bacteriology, and it is by means of 

 learning the habits of bacteria that our most effective meth- 

 ods of preventing and curing disease have been discovered. 

 They are the smallest known organisms, and their study 

 requires microscopes of very high power of magnification. 

 Doubtless there are some which are too small to be seen even 

 by microscopes of the highest power. The existence of such 

 bacteria has been indicated by the effects they produce. 



As to structure, bacteria are of three distinct types. 

 They are spherical, rodlike, or spiral. (See Figure 173.) 

 They occur either singly 

 or in filaments. Some- 

 times they have cilia and 

 swim actively. Under 

 favorable conditions they 

 multiply very rapidly by 

 division like the division 

 of Pleurococcus. They 

 appear to be present 

 everywhere, ready to re- 

 produce rapidly under 

 any condition favorable 

 to their growth. The 

 most important principle of modern sanitation is the pre- 

 vention of conditions which are favorable to the growth 

 and spread of disease-producing bacteria. Many kinds of 

 bacteria are capable of enduring conditions of temperature, 



Fig. 173. 



Various forms of bacteria, very 

 highly magnified. 



