Highly Organized Flora of Running Streams. 13. 



nature of the material and the conditions in which it is 

 placed and the micro-organisms present. One or other of 

 the bacteria concerned will take precedence, when circum- 

 stances favor its growth. Thus, the aerobic bacteria can- 

 not grow unless putrefying material is freely exposed to 

 atmospheric oxygen. The anaerobic bacteria require its 

 exclusion. Some saprophytic bacteria grow at a low tem- 

 perature ; others take precedence when the temperature is 

 high ; some, no doubt, thrive only in the presence of pro- 

 ducts evolved by other species and are consequently asso- 

 ciated with and dependent upon these species. Some are 

 restrained in their growth sooner than others, by products 

 evolved as the result of their own vital activity, or by that 

 of associated organisms. Some grow in the presence of 

 acids, and give rise to fermentation, which wholly prevents 

 the development of their species. 



The malodorous volatile products of putrefaction are to 

 a considerable extent produced by the anaerobic species. 

 For this reason these odors are more pronounced when 

 masses of albuminous material undergo putrefaction in 

 situations where the oxygen of the air has not access, or 

 where it is displaced by carbon dioxide. The gases pro- 

 duced in the interior of a putrefying mass are mainly CH 4 , 

 H 2 S and H. Many of the bacteria of putrefaction are 

 facultative anaerobics ; they are able to multiply either in 

 the presence or absence of oxygen. The products formed 

 by these differ, no doubt, according to whether they are or 

 are not supplied with atmospheric oxygen. The decom- 

 position due to aerobic bacteria is not attended with the 

 same putrefaction odors as in the case of the anaerobic 

 organisms ; the products evolved being of a simple chemi- 

 cal composition, CO 2 , NH 3 . 



The most common organisms in running and stagnant 

 water are the leptotricheae and the cladotricheae. The 

 former includes the four genera : Crenothrix, Beggiatoa, 

 Phragmidiothrix, and Leptothrix, the latter a single 



