PLANTS WITHOUT CHLOROPHYLL 



141 



(d) exposed at the level of a dirty and much-used city street. 



(e) exposed at the level of a well-swept and little-used city street. 

 (/) exposed in a city park. 



(g) exposed in a factory building. 



(h) dirt from hands placed in dish. 



(i) rub interior of mouth with finger and touch surface of dish. 



(j) touch surface of dish with decayed vegetable or meat. 



(k) touch surface of dish with dirty, coin or bill. 



(I) place in dish two or three hairs from boy's head. 



This list might be prolonged indefinitely. 



Now let us place all of the dishes together in a moderately warm 

 place (a closet in the schoolroom will do) and watch for results. 

 After a day or two little spots, 

 brown, yellow, white, or red, will 

 begin to appear. These spots, which 

 grow larger day by day, are colonies 

 made up of millions of bacteria. 

 But probably each colony arose 

 from a single bacterium which got 

 into the dish when it was exposed 

 to the air. 



How we may isolate Bacteria of 

 Certain Kinds from Others. In 

 order to get a number of bacteria 

 of a given kind to study, it becomes 

 necessary to grow them in what is 



known as a pure culture. This is done by first growing the 

 bacteria in some medium such as beef broth, gelatin, or on 

 potato. 1 Then as growth follows the colonies of bacteria appear 

 in the culture media or the beef broth becomes cloudy. If now 

 we wish to study one given form, it becomes necessary to isolate 

 them from the others. This is done by the following process: 

 a platinum needle is first passed through a flame to sterilize it ; 

 that is, to kill all living things that may be on the needle point. 



1 For directions for making a culture medium, see Hunter, Laboratory Problems 

 in Civic Biology. Culture tubes may be obtained, already prepared, from Parke, 

 Davis, and Company or other good chemists. 



Colonies of bacteria growing in 

 a petri dish. 



