BACTERIA 91 



minute in size and the observable differences in structure are so 

 slight, any classification grounded on morphological characters, 

 such as that of Migula (1897), meets with many difficulties, and 

 would seem at present to be premature. Because of the great prac- 

 tical importance of physiological qualities, bacteriologists have 

 come to lay great stress upon bacterial functions, and considera- 

 tions of convenience have often led to groups being established on 

 physiological characteristics. The practice of dealing with bacteria 

 in related groups is growing. For the identification of specific and 

 group characters the Report of the Committee of the Society of 

 American Bacteriologists on Method of Identification of Bacterial 

 Species should be consulted. 



The forms of bacteria are very simple. The complex and elabo- 

 rate structures found among certain other groups of unicellular 

 organisms (diatoms, desmids, radiolaria) do not occur among bac- 

 teria. Three principal type forms are recognized: the sphere 

 (coccus or micrococcus), the rod (bacillus), and the spiral (spirillum 



FIG. 30. Forms of Bacteria. 



and spirochaete) (Fig. 30). Closely resembling these are certain 

 filamentous organisms known as Trichomycetes, which connect the 

 bacteria with the higher fungi or moulds. 



The minute size of bacteria renders the study of their firier 

 structure somewhat difficult, but a few features have been clearly 

 determined. Most species, perhaps all, are provided with a cap- 

 sule or outer layer of gelatinous substance originating from the 

 cell-membrane and seen in stained preparations surrounding the 

 cell like a halo. The capsule is much more prominently developed 

 in some species than in others. The cell-membrane is chiefly re- 

 markable for its chemical composition, differing as it does from the 

 cell-membrane of the higher plants in not being composed of cel- 

 lulose. The nature of the cell-substance of bacteria has been the 

 object of much discussion from the standpoint of its relation to the 



