24 BACTERIA: FORMS AND REPRODUCTION 



The former class usually contains chlorophyll or an 

 allied colouring material, and its representatives are very 

 commonly distributed in water or on moist soil, wood, 

 rock, and stones, where they form patches, usually of a 

 green or bluish-green colour. Schizomycetes or bacteria 

 resemble the Schizophyceae in the simplicity of their 

 organization, but contain no chlorophyll, except possibly 

 in a few instances. Many species, however, have the 

 power of forming purple, pink, yellow, and other colour- 

 ing material, and are not uncommon on decaying meat, 

 bread, and other organic substances, where they form 

 red, purple, or yellow spots. 



The grouping of bacteria into families, genera, and 

 species is a matter of great difficulty. Various methods 

 have been proposed, based on the forms of the cells, the 

 presence and absence of spores, the possession or want 

 of flagella, and the power of producing disease and set- 

 ting up chemical changes. Unfortunately, a coccus may 

 lengthen into a bacillus form, or a bacillus may develop 

 in the form of a coccus under certain altered conditions 

 of growth and nutrition, so that the shape of the cell 

 cannot be used as a perfect basis of classification ; nor 

 are the presence or absence of cilia and the possession of 

 spores very satisfactory for the purpose, since the same 

 organism may be unable to form spores or develop cilia 

 at one time, and yet may do so when other external con- 

 ditions prevail. Many of the chief generic names of 

 bacteria are based on the form which the organisms most 

 commonly assume. We thus have the genera, Micrococcus, 

 Streptococcus, Sarcina, Bacterium, Bacillus, and Spirillum, 

 characterized by the cell-forms described on pages 1 1, 

 1 2, and 1 9. There is a tendency among some authorities 

 to restrict the generic term Bacillus to rod-like bacteria 



