CHAPTER XXXIII 

 BACTERIA 



196. Nature of Bacteria. Bacteria, often called germs, are 

 the smallest of living things. They can be seen only with the 

 highest powers of the compound microscope and some are so 

 small that fifty thousand in single file would not make a line 

 more than an inch long. Bacteria are really plants, their 

 nearest relations being the seaweeds, mushrooms, puffballs, 

 and yeasts, but they are much smaller than any of these. 

 Each consists of a single cell, the shape of which varies with 

 the species. All, however, resemble certain types enough to 

 enable us to classify them as round, rod-shaped, and twisted 

 or corkscrew-like forms. Bacteria occur almost everywhere; 

 in the air we breathe, in the water we drink, in the food we 

 eat, in the soil underfoot, and even in the bodies of plants 

 and animals. They multiply very rapidly by cell division 

 and in some cases may double their numbers every half hour. 

 It is due to their activities that wood rots, food ferments, 

 and cider turns to vinegar. 



197. Helpful and Harmful Species. Although the bacteria 

 are plants, they lack the green coloring matter of ordinary 

 plants and thus are unable to make food for themselves. 

 Like the animals, they require food already made, and this 

 they take from the bodies of plants and animals, living or 

 dead. As to the manner in which food is obtained, they are 

 divided into two classes, the parasites, which obtain their food 

 from living things, and the saprophytes, which feed only upon 

 dead organic matter. In using the food, they act like other 

 living things, breaking it down into simpler substances. In 



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