160 ELEVENTH ANNUAL KEPORT 



the microscope. Their size, though variable, is always very 

 minute and even the highest powers of the microscope tell us 

 but little in regard to their internal structure; therefore in 

 studying bacteria and attempting to classify them, more atten- 

 tion is directed to what tliey do than to their form and struc- 

 ture; in other words, their physiological characteristics are of 

 more importance than their anatomical ones. 



The following are some of the more important facts in the 

 life-history of the bacteria: 



Size. They are so small that from ten to twenty-five thou- 

 sand micrococci would be required to form a chain an inch 

 long; from three to twenty thousand bacilli placed end to end 

 Avould only aggregate the same length. In a mass no larger 

 than the head of a pin there would be several millions of bac- 

 teria. 



Distrihntion. Bacteria may be carried from place to place 

 in various ways, but their chief means of distribution is by 

 particles of dust floating in the air. They are found in the 

 air, in water, in the soil, on all surfaces exposed to the air, 

 and especially wherever decay is going on, being themselves 

 the active agents in producing it. 



Food. The food of bacteria consists of organic matter, that 

 is, of substances that form a part or that have formed a part of 

 some plant or animal. 



Moisture. Bacteria can thrive only where there is moisture; 

 while drying does not necessarily kill them, it completely 

 checks their growth. 



Oxygen. Some bacteria will grow only in the presence of 

 oxygen; others only in its absence; while still others tolerate 

 either condition. 



Temperature. There is a certain temperature at which bac- 

 teria grow best ; this varies in the different species, but for most 

 lies somewhere between sixty and one hundred degrees Fahren- 

 heit. As the temperature approaches the freezing point it 

 becomes more and more unfavorable for them, and at or near 

 that point all or nearly all cease to be active, though they may 

 not be killed and may resume their activity Avhen a higher 



