16 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Size. Bacteria are the smallest plants. The microscope, 

 and usually the compound microscope, is necessary for the 

 study of the individual cells. It is, therefore, convenient 

 to use the microscopic unit of measurement. The micron 

 (abbreviated /*) is defined as one one-thousandth of a milli- 

 meter, or one ten-thousandth of a centimeter, or approxi- 

 mately one twenty-five-thousandth of an inch. Most bac- 

 teria are 0.5 to 5/x in diameter, and from 0.5 to 10/x in 

 length. Probably the average bacterial cell does not have 

 a volume much greater than one cubic micron. The mi- 

 nuteness of these organisms is emphasized when it is re- 

 called that there are one trillion cubic microns in a cubic 



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 o.o 



Ox c/^ 



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FIG. 2. INVOLUTION FORMS, a. Normal shapes, b. Involution 

 forms. 1. Acetobacter. 2. Mycobacterium. 3. Rhizobium, 



centimeter, therefore, approximately this number of bac- 

 teria of average size would be required to fill a space of one 

 cubic centimeter. The specific gravity of the cell is usually 

 only slightly greater than that of water. It would require, 

 therefore, nearly a trillion of such bacteria to weigh a gram. 

 When water is heavily polluted it may sometimes contain 

 one million bacteria per cubic centimeter. This number 

 of bacteria, of average size, would not occupy more than 

 one millionth of the space within that cubic centimeter. 



