THE BIOLOGY OF BACTERIA 



27 



action of micro-organisms, it is desirable, in the first place, 

 to remember the two great functional divisions of saprophyte 

 and parasite. A saprophyte is an organism that obtains its 

 nutrition from dead organic matter. Its services, of what- 

 ever nature, lie outside the tissues of living animals. Its life 

 is spent apart from a *' host." K parasite, on the other hand, 

 lives always at the expense of some other organism which is 

 its host, in which it lives and upon which it lives. There is a 

 third or intermediate group, known as " facultative," owing 



Method of Producing Hydrogen by Kipp's Apparatus for 

 Cultivation of Anaerobes (See page 139) 



to their ability to act as parasites or saprophytes, as the 

 exigencies of their life-history may demand. 



The saprophytic organisms are, generally speaking, those 

 which contribute most to the benefit of man, and the parasitic 

 the reverse, though this statement is only approximately true. 

 In their relation to the processes of fermentation, decompos- 

 ition, nitrification, etc., we shall see how great and invaluable 

 is the work which saprophytic microbes perform. Their re- 

 sult depends, in nearly all cases, upon the organic chemical 

 constitution of the substances upon which they are exerting 



