92 ELEMENTARY BOTANY. 



isolated or in cell-families, and multiply usually by transverse 



fission of the cells ; most of them 

 have a motile and a motionless 

 stage. The unicellular Bacteria 

 sometimes form a jelly-like mass 

 or colony, and this is called the 

 zobglosa stage. Representatives of 

 the genus Bacterium have an oval 

 or cylindric-oblong shape. The 

 Fig. 117. common agent of putrefaction is 



Bacterium termo, which consists of 

 very small cylindrical cells. The genus Micrococcus consists 

 of spherical cells. M. prodigiosus causes the blood-like patches 

 on bread, paste, etc. 71/. diphtheriticus causes diphtheria. M. 

 nitrificans decomposes the ammoniacal compounds found in 

 the soil. There are two genera with filiform cells : Bacillus, 

 with the filament straight, and Vibrio, with the filament curved 

 or undulated. B. subtilis is the butjTic ferment, B. anthracis 

 causes the anthrax or splenic fever, and B. tuberculosus causes 

 pulmonary consumption. The genus Spirillum has spirally 

 twisted cells. S. cholerse asiaticx produces Asiatic cholera. 



The compound microscope with high powers is required for a study of the 

 Bacteria. The results of their work can be seen almost everywhere, al- 

 though the individual plants themselves are always microscopic. 



9. The role or part performed by these micro-organisms or 

 Bacteria, in the economy of nature and in our everyday life is 

 very important. Many forms flourish in the soil, and without 

 their agency plant food would not be made available. Nume- 

 rous species disintegrate organic matter and deliver the ele- 

 ments back to the mineral kingdom. The souring of milk, 

 the maturing of cream, the ripening of cheese, and many 

 operations incident to our civilization are directly dependent 

 on the presence of the Bacteria. Comparatively few species 

 are injurious to man and other animals. Diseases caused by 

 Bacteria are designated as " germ diseases." They are con- 

 tagious and often epidemic. Most of the fevers belong to this 



