. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 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-
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 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 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 ) 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 f
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