water was used to wash the milk cans. Once in the milk, the bacteria multiplied rapidly, so that the milkman gave out cultures of typhoid in his milk bottles. Proper safeguarding of our water and milk supply is necessary if we are to keep typhoid away.

Tetanus, or Blood Poisoning. The bacterium causing blood poisoning is another toxin-forming germ. It lives in the earth and enters the body through 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 battlefield, using powerful antiseptics in order to kill any bacteria that might find lodgment in the exposed wounds.

Other Diseases. Many other diseases have been traced to bacteria. Diphtheria is one of the best known. As it is a throat disease, it may easily be conveyed from one person to another by kissing, putting into the mouth objects which have come in contact with the mouth of the patient having diphtheria, or by food into which the germs have been carried. Another disease which probably causes more misery in the world than any other germ disease is syphilis. It is estimated that 80 per cent of blindness in newborn children is due to this cause. Grippe, pneumonia, whooping cough, and colds are believed to be caused by bacteria. Other diseases, as malaria, yellow fever, sleeping sickness, and probably smallpox, scarlet fever, and measles, are due to the attack of one-celled animal parasites. Of these we shall learn later in the chapter on Protozoa.

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 from food, water, by breathing in the germs in the air, or by taking them into the blood through a cut or a wound or a body opening.

It is evident that as individuals we may each do something to

prevent the spread of germ diseases, especially in our homes. We may keep our bodies, especially our hands and faces, clean. Sweeping and dusting may be done with damp cloths so as not to raise a dust; our milk and water, when from a suspicious supply, should be sterilized, that is, the germs contained killed by boiling or pasteurizing for a few minutes. Wounds through which bacteria might obtain foothold in the body should be washed with some antiseptic, a substance like corrosive sublimate (1 part to 1000 water) or carbolic acid (1 part to 40 water), which kills the germs. In a later chapter we shall learn more of how we may coöperate with the authorities to combat disease and make our city or town a better place to live in.1

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company.

Conn, Bacteria, Yeasts, and Molds in the Home. Ginn and Company.

Conn, Story of Germ Life. D. Appleton and Company.

Davison, The Human Body and Health. American Book Company.

Frankland, Bacteria in Daily Life. Longmans, Green, and Company.

Prudden, Dust and its Dangers. G. P. Putnam's Sons.

Prudden, The Story of the Bacteria. G. P. Putnam's Sons.

Ritchie, Primer of Sanitation. World Book Company.

ADVANCED

Conn, Agricultural Bacteriology. P. Blakiston's Sons and Company.

Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Company.

De Bary, Comparative Morphology and Biology of the Fungi, Mycetozoa, and Bacteria

Clarendon Press.

Duggar, Fungous Diseases of Plants. Ginn and Company.

Hough and Sedgwick, The Human Mechanism.

Ginn and Company.

Muir and Ritchie, Manual of Bacteriology. The Macmillan Company.

Newman, The Bacteria. G. P. Putnam's Sons.

Sedgwick, Principles of Sanitary Science and Public Health. The Macmillan Com

pany.

1 Teachers may take up parts or all of Chapter XXIX at this point. I have found it advisable to repeat much of the work on bacteria after the students have taken up the study of the human organism.

XIV. THE RELATIONS OF PLANTS TO ANIMALS

Problem XXIV. The general biological relations existing between plants and animals. (Laboratory Manual, Prob. XXIV.) (a) A balanced aquarium.

(b) Relations between green plants and animals.

(c) The nitrogen cycle.

(d) A hay infusion.

Study of a Balanced Aquarium.

Perhaps the best way for us.

to understand the interrelation between plants and animals is to study an aquarium in which plants and animals live and in which a balance has been established between the plant life on one side and animal life on the other. Aquaria containing green pond weeds, either floating or rooted, a few snails, some tiny animals known as water fleas, and a fish or two will, if kept near a light window, show this relation.

We have seen that green plants under favorable conditions of sunlight, heat, moisture, and with a supply of raw food materials, give off oxygen as a by-product while manufacturing food in the green cells. We know the necessary raw materials for starch manufacture are carbon dioxide and water, while nitrogenous material is necessary for the making of proteids within the plant. In previous experiments we have proved that carbon dioxide is given off by any living thing when oxidation occurs in the body. The crawling snails and the swimming fish give off carbon dioxide, which is dissolved in the water; the plants themselves, night and day, oxidize food within their bodies, and so must pass off some carbon dioxide. The green plants in the daytime use up the carbon dioxide obtained from the various sources and, with the water taken in, manufacture starch. While this process is going on, oxygen is given off to the water of the aquarium, and this free oxygen is used by the animals.

But the plants are continually growing larger. The snails and fish, too, eat parts of the plants. Thus the plant life gives food

to at least part of the animal life within the aquarium. The animals give off certain nitrogenous wastes of which we shall learn more later. These materials, with other nitrogenous matter from the dead parts of the plants or animals, form the part of the raw material of the proteid food manufactured within the plant. The animals eat the plants and give off organic waste, from which the

plants make their food and living matter. The plants give off oxygen to the animals, and the animals give carbon dioxide to the plants. Thus a balance exists between the plants and animals in the aquarium.

What goes

Relations between Green Plants and Animals. on in the aquarium is an example of the relation existing between all green plants and all animals. Everywhere in the world green plants are making food which becomes, sooner or later, the food of

animals. Man may not feed upon the leaves of plants, but he eats fruits and seeds in one form or another. Even if he does not feed directly upon plants, he eats the flesh of herbivorous animals,

which in turn feed directly upon plants. And so it is the world. over; the plants are the food-makers and supply the animals. Green plants also give a very considerable amount of oxygen to the atmosphere every day, which the animals may make use of.

released through the agency of bacteria, which live upon the roots of certain plants. These bacteria are the only organisms that can take nitrogen from the air. Thus, in spite of all the nitrogen of the atmosphere, plants and animals are limited in the amount