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 seum, 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,! 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 
1JIt has been estimated by Migula, a good authority, that there are at 
least 1272 distinct species of bacteria. 
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