. Introduction to botany. Botany. 200 i:nteoduction to botany. their size are almost meaningless. When we say that average rod-shaped bacteria are about y^^-qo inch (^^-^ mm.) long, and 5-3^-0^ inch (yuV-q mm.) in thickness, we are describing dimensions so small that we cannot appreciate them. It must also be kept in mind that many kinds of bacteria are smaller than the dimensions given above. These figures mean more when we calculate the number of average bacteria that might be contained in a vessel measuring a cubic inch, or when we measure the thickness of a finger nail and estimate how man
. Introduction to botany. Botany. 200 i:nteoduction to botany. their size are almost meaningless. When we say that average rod-shaped bacteria are about y^^-qo inch (^^-^ mm.) long, and 5-3^-0^ inch (yuV-q mm.) in thickness, we are describing dimensions so small that we cannot appreciate them. It must also be kept in mind that many kinds of bacteria are smaller than the dimensions given above. These figures mean more when we calculate the number of average bacteria that might be contained in a vessel measuring a cubic inch, or when we measure the thickness of a finger nail and estimate how many bacte- ria, piled one upon another, would be required to make a column as high as the finger nail is thick. Such estimates will make it quite clear that bacteria are small enough to be everywhere and yet escape our attention. Bacteria are of three different forms. Most of them are rod-shaped, or of the bacillus form, some are spherical, and still others are spiral in form (fig. 165). The rod bacteria vary in length and in diameter. Similarly, spherical l)ac- teria vary in size, and some spiral forms may consist of less than one complete spiral, others of a dozen or more spirals. Little is known about the structure of these minute plants. The definitely organized cell wall contains a small amount of protoplasm, which sometimes forms extruding cilia (fig. 165). By means of these cilia some kinds of bacteria are able to swim, and some of them move with a speed (over two thousand times their own length in one hour) which is remarkably rapid in proportion to their small size. Definite nuclei have not been seen in bacteria, but it may be that certain granular fragments represent the nucleus. AV^e are more interested, however, in how bacteria live than in their structure. Fig. 165. A group of bacteria illustrating type forms A, spherical bacteria gathered into a colony, and B, spherical bacte- ria arranged in a cliain; C, D, E, and F, rod, or bacillus, bacteria, E and F having cilia
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