A STUDY OF LIVING SUBSTANCE 33 



ent forms. Some are rod-shaped (like a firecracker), some 

 are spherical, others are egg-shaped, or spiral-shaped like 

 a corkscrew. Each bacterium is a tiny bit of translucent 

 protoplasm, inclosed in a cell wall of cellulose. Thus 'far no 

 nucleus has been discovered in any kind of bacteria. Be- 

 cause of their cellulose walls, and because of their likeness 

 to certain low forms of green plants, biologists now regard 

 these organisms as plants rather than animals. 



Some of the rod-shaped bacteria have one or more long 

 hairlike projections from the ends, called cil'-i-a, which give 

 the germs still further resemblance to firecrackers. These 

 cilia lash about furiously, and thus drive the cells through 

 the water. The spiral bacteria roll over and over, and ad- 

 vance in a spiral path like a corkscrew or spiral spring. 

 Other forms have rapid movements, but it is not known 

 how they are accomplished. 



Size of Bacteria. It is very difficult to get any clear 

 notion of the extreme minuteness of bacteria. It means but 

 little to say that the rod-shaped forms are one five-thou- 

 sandth 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 

 reach across the head of a pin. 



Reproduction of Bacteria. When conditions are favor- 

 able, 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. If the daughter cells cling together, a chain or a 

 mass is formed. Oftentimes they separate entirely from 

 each other. In either case the whole mass of bacteria is 

 called a colony. 



