CHAPTER XVI. 



UNICELLULAR PLANTS (Contimted). 



C. Bacteria. 



(Schizomycetet.) 



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 upper 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-genns. " Most bacteria, however, are not 

 parasitic, but saprophytic, i.e., live upon dead organic matters, 

 and therefore are not merely harmless, but positively useful 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 (fission-fungi}, their near allies being the 

 Cyanophycew or "blue-green algae." 



Morphology. Under the microscope bacteria appear as 

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

 (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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