24 POSITION—RELATIONSHIPS 



Bacteria are among the smallest of organisms, so small 

 that it requires the highest powers of the microscope for their 

 study and the use of a special unit for their measurement. 

 This unit is the one-thousandth part of a millimeter and is 

 called a micro-millimeter or micron. Its symbol is the Greek 

 letter mn {/jl). The micron, plural microns or micra, is used 

 generally for measuring other microscopic objects as well as 

 bacteria. 



The size varies widely among different kinds but is fairly 

 constant in the same kind. The smallest described form is 

 said to be only 0.18 m long by 0.06 /jl thick and is just visible 

 with the highest power of the microscope, though it is 

 possible and even probable that there are forms still smaller 

 which cannot be seen. Some large rare forms may measure 

 40 fi in length, but the vast majority are from 1 /x to 4 ^t or 

 5 M long and from one-third to one-half as wide. 



From the description outlined in the above paragraphs a 

 bacterium might be said to be a microscopic, wiiceUular plant, 

 without chlorophyl, ivhich reproduces by dividing transversely. 



