CHAPTER XIV 

 THE BACTERIA " 



186. What are the bacteria? When a dish of water in 

 which cut flowers have been kept for several days is carefully 

 examined, a thin film, or scum, may often be found on the sur- 

 face of the water. This surface film usually consists of many 

 millions of bacteria. The bacteria have thrived upon the plant 

 substances which were dissolved in the water. The bacteria 

 also produce the unpleasant odors that often arise from such 

 a dish of cut flowers. If but a few bacteria had gathered at 

 the water's surface, they could not be seen except by the use 

 of very great magnification, but when such large numbers 

 accumulate in one place, they may easily be seen, just as it is 

 easier to see a pile of sand than it is to see a few grains of sand. 



The bacteria are extremely simple one-celled plants. The 

 fact that they are plants was not generally recognized until 

 within the past few decades. Although this group of plants 

 is still imperfectly known, much has been learned of their very 

 great hygienic importance. It has been popularly supposed 

 that all or nearly all of the bacteria are causes of disease, 

 although it is now believed by scientists that very few of 

 them are disease-producing. On the other hand, there are 

 many kinds of bacteria that live in such ways as greatly to 

 aid in important processes upon which men depend. 



187. The size and structure of the bacteria. The bacteria 

 are so small, and there are so many kinds of them, 1 that it is 

 not easy to give a clear answer to the question as to what 

 they are. Indeed, they are so small that the figures given for 



1 It lias been estimated by Migula, a good authority, that there are at 

 least 1272 distinct species of bacteria. 



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