PLANTS IN THEIR RELATION TO HUMAN WELFARE 141 



water. The spiral bacteria roll 

 over and over, and advance in a 

 spiral path like a corkscrew. Other 

 forms have rapid movements, but 

 it is not known how they are ac- 

 complished. 



It is very difficult to get any 

 clear notion of the extreme minute- 

 ness of bacteria. It means but 

 little to say that the rod-shaped 

 forms are -^nr of an inch in length. 

 The imagination may be somewhat 

 assisted if we remember that fifteen 

 hundred of them arranged in a 

 procession end to end would 

 scarcely equal the diameter of a 

 pin head. 



148. Reproduction of bacteria. 

 When conditions are favorable, 

 the production of new cells goes 

 on with marvelous rapidity. The 

 process is something as follows : 

 The tiny cells take in through the 

 cell wall some of the food materials 

 that are about them, change this 

 food into protoplasm, and thus 

 increase somewhat in size. The 

 limit is soon reached, however, and 

 the bacterium begins to divide 

 crosswise into halves. The mother 

 cell thus forms two daughter cells 

 by making a cross partition (cell wall of cellulose) between the 

 two parts. (See Fig. 71, C.) If the daughter cells cling to- 



FIG. 71. Various forms 

 bacteria. 



A, a colony of spherical bac- 

 teria (coccus, plur. cocci) ; 

 B, rod-shaped bacteria (ba- 

 cillus, plur. bacilli) ; C, rod- 

 shaped bacteria dividing ; 

 D, rod-shaped bacteria, each 

 containing a spore ; E, spiral 

 bacteria (spirillum, plur. 

 spirilla) each with cilia. 



