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 twentyfour 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.

b

Bacteria, highly magnified;
a, the germ of typhoid
fever, stained to show
the cilia; b, 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 dish) 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

1 For directions for making a culture medium, see Peabody, Manual of Physiology.

alcohol in cider, and by a chemical process change the alcohol to vinegar. Others, called the lactic acid bacteria, bring about the souring of milk by acting on the sugar found in the milk. Thus they aid in butter and cheese making. Others give the flavor to cheese or butter. Some are of great economic importance, as we have already seen, in their relation to the roots of some plants, where they fix nitrogen in such a form that it can be used by the plants as food. Bacteria seem to prefer to feed upon substances that contain nitrogen, this element being necessary to

Microscopic appearance of ordinary milk showing fat globules and bacteria. The cluster of bacteria on left side are lactic-acid-forming germs. (H. L. Russell, Wis. Bul. No. 62.)

form protoplasm. They are found in great numbers upon all nitrogenous foods, as milk, meats, fish, etc. Feeding upon such substances, they decompose them and cause decay to take place. Typhoid. Such bacteria as are parasitic in the human body may cause disease. Sometimes the harm is done by certain poisons called ptomaines, which are formed by the bacteria during the process of growth. The disease we call typhoid fever is thus caused. The bacterium causing the fever is taken into the body through the mouth in water, milk, or other food. Once in the intestine the bacteria multiply very rapidly, producing a ptomaine. This poison gets into the blood and is passed to all parts of the

body, causing the symptoms of the disease we call typhoid. To prevent the spread of this disease, we must guard our water and milk supply most carefully, in order to prevent any of the bacteria which cause the disease from gaining entrance into the body.

Tetanus or Blood Poisoning. The bacterium causing blood poisoning is another ptomaine-forming germ. It lives in the earth and enters the body by means of cuts or bruises. It seems to thrive best in less oxygen than is found in the air. It is therefore important not to close up with court plaster wounds in which such germs may have found lodgment. It, with typhoid, is responsible for four times as many deaths as bullets and shells in time of battle. The wonderfully small death rate of the Japanese army in their war with Russia was due to the fact that the Japanese soldiers always boiled their drinking water before using it, and their surgeons always dressed all wounds on the battle field, using powerful antiseptics in order to kill any bacteria that might find lodgment in the exposed wounds.

Tuberculosis. Another bacterium that is responsible for nearly one seventh of all the yearly deaths in the world is the so-called tubercle bacillus. It causes the disease called tuberculosis. This disease is not caused by a ptomaine, but by the growth of the bacteria in the lungs, and other parts of the body, where they eat away the tissues of the lungs and form little masses of new tissue, called tubercles. Tuberculosis may be contracted by taking the bacteria into the throat or lungs in the air we breathe. Although there are always some of the germs in the air of an ordinary city street, and though we doubtless take some of these germs into our bodies every day, yet the bacteria seem able to gain a foothold only under certain conditions. It is only when the tissues are in a worn-out condition, when we are run down," as we say, that the parasite may obtain a foothold in the lungs. Even if the disease gets a foothold, it is quite possible to cure it if it is taken in time. The germ of tuberculosis is killed by exposure to bright sunlight and fresh air. Thus the course of the disease may be arrested, and a permanent cure brought about, by a life in the open air, the patient sleeping out of doors, taking plenty of nourishing food and moderate exercise.

66

Many other diseases have been traced to bacteria. Diphtheria and asiatic cholera are the best known. Grippe, pneumonia, whooping cough, and colds are believed to be caused by bacteria. Other diseases, as malaria, yellow fever, and probably smallpox, scarlet fever, and measles, are due to the presence in the blood of a one-celled animal parasite.

Methods of Fighting Germ Diseases. As we have seen, diseases produced by bacteria may be caused by the bacteria being transferred from one person directly to another, or the disease may obtain a foothold in the body in food, or water, by breathing in the germs in the air, or by taking them into the blood through a cut or wound.

In the prevention of germ diseases we must fight the germ by attacking the parasites directly with poisons that will kill them (such poisons are called germicides or disinfectants), and we must strive to make the persons coming in contact with the disease unlikely to take the disease. This insusceptibility or immunity may be either natural or acquired. Immunity may be acquired by means of such treatment as the anti-toxin treatment for diphtheria. This treatment, as the name denotes, is a method of neutralizing the poison (toxin) caused by the bacteria in the system. It was discovered a few years ago that the serum of the blood of

an animal immune to diphtheria is capable of neutralizing the poison produced by the diphtheria-causing bacteria. Horses are rendered immune by giving them large doses of the diphtheria toxin or poison. The serum of the blood of these horses is then used to inoculate the patient suffering from or exposed to diphtheria, and thus the disease is checked or prevented altogether.

LICHENS.-Lichens may be found incrusting rocks, tree trunks, or other exposed localities. They have in general a grayish color, although they may be red, yellow, or black. The form of the body is usually that of a thallus, being

Stages in the formation of the lichen thallus, showing the relation of the threadlike fungus to the green cells of the alga. After Bornet.

flat and irregular. One of the commonest of the lichens (Physcia), found on stone walls or tree trunks, produces cup-shaped bodies from the thallus, in which spores are formed. Most lichens have definite spore-producing structures which protrude from the upper surface of the thallus. A lichen is of interest to us chiefly because it shows a partnership to exist between certain green plants, called the algae, and the fungi. A lichen is thus composed of two plants, one at least of which may live alone, but which have formed a partnership for life, and have divided the duties of such life bctween them. In most lichens the alga, a green plant, forms starch and ⚫ nourishes the fungus. The fungus, in turn, produces spores, by means of which new lichens are started in life. The body of the lichen is usually protected by the fungus, which is stronger in structure than the green part of the combination. This process of living together for mutual advantage is called symbiosis. Some animals thus combine with plants; for example, the tiny animal known as the hydra with certain of the one-celled algæ. Animals also frequently live in this relation to each other.

Algæ. The algae are a very diverse collection of plants, containing some of the smallest and simplest as well as some of the

largest plants in the world. The tiny one-celled Pleurococcus is an example of the former; the giant kelps of the Pacific Ocean, which attain a length of over one thousand feet, of the latter. The body of the alga is a thallus, which may be platelike, circular, ribbon-formed, threadlike, or filamentous. It may even be composed of a single cell. A large number of the algae inhabit the water, either fresh or salt. In color they vary from green through the shades of blue-green to yellow, brown, and red. These colors are best seen in the seaweeds, all of which, however, contain chlorophyll. In the red