BACTERIA AS PLANTS. 17 



WHAT ARE BACTERIA ? 



The most interesting facts connected with the 

 subject of bacteriology concern the powers and 

 influence in Nature possessed by the bacteria. 

 The morphological side of the subject is interest- 

 ing enough to the scientist, but to him alone. 

 Still, it is impossible to attempt to study the 

 powers of bacteria without knowing something of 

 the organisms themselves. To understand how 

 they come to play an important part in Nature's 

 processes, we must know first how they look and 

 where they are found. A short consideration of 

 certain morphological facts will therefore be 

 necessary at the start. 



FORM OF BACTERIA. 



In shape bacteria are the simplest conceivable 

 structures. Although there are hundreds of dif- 

 ferent species, they have only three general forms, 

 which have been aptly compared to billiard balls, 

 lead pencils, and corkscrews. Spheres, rods, and 

 spirals represent all shapes. The spheres may be 

 large or small, and may group themselves in va- 

 rious ways; the rods may be long or short, thick 

 or slender; the spirals may be loosely or tightly 

 coiled, and may have only one or two or may 

 have many coils, and they may be flexible or 

 stiff ; but still rods, spheres, and spirals comprise 

 all types (Fig. i). 



In size there is some variation, though not 

 very great. All are extremely minute, and never 

 visible to the naked eye. The spheres vary from 

 0.25 p. to 1.5 p. (0.000012 to 0.00006 inches). The 

 rods may be no more than 0.3 p. in diameter, 

 or may be as wide as 1.5 /A to 2.5 y, and in length 



