THE BIOLOGICAL UNIT 45 



Thus a micron is cooi of a millimeter or approxi- 

 mately 1/25,000 of an inch. The micron is a unit 

 of measurement that is small enough to be applied 

 to bacteria. We find that the micrococci and the 

 width of bacilli average about i micron. The 

 bacteria which cause lockjaw have a width of 0.2 

 micron and a length of 5.2 microns; while some 

 of the spirilla may have a length of 40 microns. 



The distribution of bacteria is universal where- 

 ever there is food upon which they can exist. The 

 mere fact that they are found in so many different 

 situations, living equally well in the air, in the 

 upper layers of the soil and in the water, reveals 

 their wide adaptability. It also means that they 

 can live on an equally wide variety of foods. 



Many bacteria are able to move about from 

 place to place. Some move slowly while others 

 dart past rapidly. When studied under the micro- 

 scope, their motion seems very rapid on account of 

 the high magnification. While some of them have 

 been estimated to travel with the speed of an aero- 

 plane, most of them move very much more slowly. 

 If the comparison of the rate of speed of the 

 slower moving bacteria is made with an automo- 

 bile, the speed would be between 10 and 20 miles 

 an hour. 



These minute plants are extremely simple in 

 structure. The cell wall is usually surrounded by 

 a slimy or gelatinous capsule and it is not like the 



