Bacteria 141 



ammonia to nitrites; a second kind oxidizes the 

 nitrites to nitrates. By these successive operations the 

 stores of nitrogen that are gathered together within the 

 living bodies of plants and animals are again released 

 for further use. The simple nitrates are proper food 

 for the green alga?, with whose growth the cycle begins 

 again. And those bacteria which promote the pro- 

 cesses of putrefaction, are thus the world's chief agen- 

 cies for maintaining undiminished growth in perpetual 

 succession. 



Bacteria are among the smallest of organisms. Little 

 of bodily structure is discoverable in them even with 

 high powers of the microscope, and consequently they 

 are studied almost entirely in specially prepared cul- 

 tures, made by methods that require the technical 

 training of the bacteriological laboratory for their 

 mastery. Any one can find bacteria in the w T ater, but 

 only a trained specialist can tell what sort of bacteria 

 he has found; whether pathogenic species like the 

 typhoid bacillus, or the cholera spirillum; or whether 

 harmless species, normal to pure water. 



The higher bacteria Allied to those bacilli that grow 

 in filaments are some forms of larger growth, known as 

 Trichobacteria, whose filaments sometimes grow 

 attached in colonies, and in some are free and motile. 

 A few of those that are of interest and importance in 

 fresh-water will be briefly mentioned and illustrated 

 here. 



Leptothrix* (Fig. 56^7 , b and c] grows in tufts of slender, 

 hairlike filaments composed of cylindric cells sur- 

 rounded by a thin gelatinous sheath. In reproduction 

 the cells are transformed directly into spores (gonidia ) 

 which escape from the end of the sheath and, finding 

 favoring conditions, grow up into new filaments. 



*Known also as Streptothrix and Chlamydothnx. 



