176 HOW PLANTS BENEFIT AND HARM MANKIND 



movement seems to be caused by tiny lashlike threads of proto- 

 plasm called cilia. The ciha project from the body, and by a rapid 

 movement cause locomotion to take place. Bacteria reproduce 

 with almost incredible rapidity. It is estimated that a single 

 bacterium, by a process of division called fission, will give rise to 

 over 16,700,000 others in twenty-four hours. Dr. Prudden has 

 estimated that such a bacterium, if allowed to develop unchecked 

 for five days, would fill all the oceans of this earth to a depth of one 

 mile. Under unfavorable conditions they stop dividing and form 

 spores, in which state they remain until conditions of temperature 

 and moisture are such that growth may begin again. 



i Method of Study. — In order to get a nimaber of bacteria of a 

 gi^'en kind to study, it becomes necessarj' 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 The material used as 

 a growth medium is at first 

 sterilized by heating to such a 

 temperature as to kill all life 

 that might be there. If the 

 material is exposed to the air 

 of the schoolroom in a shallow 

 dish (known as a Petri dish), or 

 in a test tube in the case of 

 beef broth, for say five minutes, 

 and if then the dish or tube is 

 covered and put away in a 

 warm place for a day or two, little spots will appear on the surface 

 of the gelatin or potato, or the beef broth will become cloudy. 

 ', Pure Culture. — The spots are colonies composed of milhons of 

 bacteria. If now we wish to study one given form, it becomes nec- 

 essary to isolate them from the others on the plate. 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 maybe on the 



' For directions tor making a culture medium, see Peabody, Manual of Physiology. 

 Culture tubes may be obtained, already prepared, from Parke, Davis, and Company. 



A Petri dish culture of bacteria; the colo- 

 nies of bacteria are the little spots of 

 various size and color. 



