22 AGRICULTURAL BACTERIOLOGY 



vided with a single flagellum at one end. If two or more 

 fiagella are present at one end, the term lophotrichous is 

 used. When clusters of flagella are at either end, 

 amphitrichous is applied. And when the flagella are 

 distributed over the entire cell, it is known as pcritri- 

 chous. 



The flagella are exceedingly delicate, and are so small 

 that they are not visible in ordinary microscopic prepara- 

 tions. To make them apparent, the preparation is treated 

 in such a way as to precipitate something upon them, thus 

 increasing their apparent diameter. These locomotor ap- 

 pendages, which are usually much longer than the cell itself, 

 are not found on the cocci. Many of the bacilli and nearly 

 all of the spirilla are motile. 



The space that can be traversed by even the most ac- 

 tively motile cell is small, probably not more than an inch 

 or two in an hour. In comparison to their size the motion 

 is quite rapid. Some have been shown to travel twenty 

 times their own length in one second, a relatively much 

 faster rate than that of a speeding race-horse. Some of 

 the motile spirilla can reverse their direction without the 

 turning about of the cell. In others, the cell must turn 

 to reverse the direction of movement. It should be remem- 

 bered that in the case of the peritrichous forms there must 

 be coordination of movement among all of the flagella of a 

 single cell. These arrangements give some idea of the com- 

 plexity of the bacterial cell. 



Size of bacteria. The unit of measurement in micros- 

 copy is the micron, which is one thousandth of a milli- 

 meter, or approximately one twenty-five thousandth of an 

 inch. It is usually expressed by the Greek letter /JL. The 

 great majority of the bacteria are from 0.5 to 5 microns in 

 length or diameter. 



It is difficult to obtain any appreciation of the minute- 



