FLOWERLESS PLANTS 



169 



when living in a fluid. Such movement, seems to be caused 

 by tiny lashlike threads of protoplasm called cilia. The cilia 

 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 un- 

 checked 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. 



Bacteria, highly magnified; 

 a, the germ of typhoid 

 fever, stained to show 

 the cilia; 6, a spiral 

 ciliated form; c, a rod- 

 shaped form, in chains; 

 d, a spherical form. — 

 a, b, from Engler and 

 Prantl. 



Method of Study. — Bacteria can be studied 

 only with the aid of the microscope. 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.^ 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. Now expose the material to the air of the schoolroom in 

 a shallow dish (known as a Petri di.sh) or a test tube in the case of beef 

 broth, for say five minutes. Then cover the dish or tube and put it away 

 in a warm place for a day or two. Little spots appear on the surface of 

 the gelatin or potato, or the beef broth becomes cloudy. 



Pure Culture. — The spots are colonies composed of millions of bacteria. 

 If now we wish to study one given form, it becomes necessary 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 may be on the needle point. Then the needle is 

 dipped in a colony containing the bacteria we wish to study. This mass of 

 bacteria is quickly transferred to another sterilized plate, and this plate is 

 immediately covered to prevent any other forms of bacteria from entering. 

 When we have succeeded in isolating the kind of bacteria in a given dish, we 

 are said to have a pure culture. 



Fermentation. — - Bacteria play an important part in the 

 process of fermentation. For example, bacteria act upon the 



^ For directions for making a culture medium, see Peabody, Manunl of Phi/.<^i- 

 ology. 



