CHAPTER II 



BACTERIA 



17. A Hay Infusion. If we place some hay in water and 

 allow the mixture to stand in a warm place, after two or three 

 days we shall find on examination that it contains a great 

 number of small living 

 organisms. Some of 

 these swim actively ; 

 others are quiet except 

 as they are carried about 

 by currents in the liquid. 

 Among the most numer- 

 ous forms are likely to 

 be rod-shaped bodies, 

 some of them single, 

 others joined end to end 



in chain-like rows. Some 



FIG. 6. Bacillus subtilis. A, in the 

 motile condition; B, in the resting con- 

 of the single bodies as dition, the vibrating threads having been 

 lost; C, in the spore condition. After 

 Brefeld. 



well as some of the rows 

 of bodies are in active 

 motion ; others are quiet. The rod-shaped bodies belong to 

 the group of bacteria. Even among the rod-shaped bacteria 

 in the infusion there may be two or three different kinds ; 

 but the differences between them are slight, and we may con- 

 sider them under the name of what will probably be the most 

 abundant species, Bacillus subtilis (Fig. 6). 



18. The Structure of a Bacillus. Each rod-shaped body 

 is a cell ; Bacillus subtilis is a plant composed of a single cell. 



