THE SIMPLER ORGANISMS 



99 



media even as housewives have always sterilized fruit for 

 canning, sealing while hot ; but he may allow time for the 

 germination of any spores that are present and then may 

 sterilize again; thus the spores, as well as the active cells of 

 bacteria are killed. This is his method 

 of clearing the field. Then he sows in 

 his culture media the sort of bacteria he 

 wishes to study, and observes their hab- 

 \Ss~ , \^ /- its and manner of growth. 



f I In order to see bacteria, rather high 



powers of the compound microscope are 

 required, and even with the best instru- 

 ments little of internal structure is vis- 

 ible in them. There are three form- 

 types commonly found among them: a) 

 The spherical ccwws type, b) the rod like 

 bacillus type, and c) the spirillum type 

 (fig. 58). Under each of these form 

 types many different species occur, 

 which may differ in size and proportions, 

 in manner of grouping, in mode of cell 

 division, etc.; or, different species may 

 appear quite alike to the eye, and may 

 be distinguishable only by their manner 

 of growth in culture media. By proper 

 staining methods some of them show 

 locomotor flagella, that are quite invis- 

 ible unstained (fig. 58 b and e). 



Certain soil bacteria, of very great im- 

 portance to agriculture, cause minute 

 galls (known as tubercles) to grow upon 

 the roots of clover and other leguminous 

 plants. These are important because 

 they are able to derive their nitrogen 



FIG. 58. Bacteria 

 a, form types ; s, coc- 

 cus, t, bacillus; u, 

 spirillum types, b, 

 these forms stained, 

 some showing flag- 

 ella. others, none. 



c, types of division ; 

 v, ordinary cell divi- 

 sion ; w and x, simul- 

 taneous division of 

 longer filaments in- 

 to a number of cells. 



d, spore formation 

 in different forms, e, 

 two species of bac- 

 teria of the bacillus 

 type, showing differ- 

 ences of appearance, 

 both stained and un- 

 stained; y, the 

 typhoid bacillus ; z, 

 the bacillus of Asia- 

 tic cholera. 



