CHAPTER XYI. 



UNICELLULAR PLANTS (Continued). 



C. Bacteria. 



{Schizomycetes.) 



The smallest, and the most numerous, of all living things are 

 the bacteria. Bacteria occur almost everywhere : they are lifted 

 into the atmosphere as dust particles, in it they float and with its 

 currents they are driven about; water — both fresh and salt — 

 often contains large numbers of them ; and the upj^er layers of 

 the soil teem with them. But they are most abundant in liquids 

 containing dissolved organic matters, especially such as have stood 

 for a time — for example, stale milk and sewage, these fluids 

 often containing millions of individual bacteria in a single cubic 

 centimetre. 



In respect to their abundance in the surface layers of the 

 earth (one gram of fertile soil often containing a million or more), 

 and the work which they do there in producing the oxidation of 

 organic matters and changes in the composition of the soil, bac- 

 teria may well be compared with earthworms (cf . p. 42). They 

 are also of much general interest because some are what are 

 known as ' ' disease-germs. ' ' Most bacteria, however, are not 

 jparasitie^ hvX saprophytic^ i.e., live upon dead organic matters, 

 and therefore are not merely harmless, but positive]y usef-ul in 

 rendering back to the inorganic world useless organic matters. 

 Some species such as the vinegar bacteria are commercially 

 important. 



In systematic botany bacteria constitute a well-defined group, 

 the Schizomycetes {fissio7i-fnngi)^ their near allies being the 

 Cyanoj)hyce(B or " blue -green algae." 



Morphology. Under the microscope bacteria appear as 

 minute rods {Bacilli) (Fig. 98), balls {Cocci) (Fig. 100), or spirals 

 {Spirilla) (Fig. 104), sometimes at rest, but often, at least in 

 the case of the rods and spirals, in active motion. Little or no 



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