Some Ways in which Plants affect Animals.--We have been studying the life of plants in order better to understand the life of animals and men. We have seen first that green plants play indirectly a tremendous part in man's welfare by supplying him with food. We have found that the colorless plants directly affected his welfare by causing disease, and by causing decay, thus making usable the nitrogen locked up in dead bodies of plants and animals, and by some even supplying nitrogen from the atmosphere. The dependence of animals upon plants has been shown and the interdependence of plants on animals has also been seen in cross-pollination and in the supply of raw food materials to plants by animals. 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. [Illustration: A balanced aquarium. Explain the term "balanced."] 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 their 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 proteins 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, at all times, 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 there. [Illustration: This diagram shows that plants and animals on the earth hold the same relation to each other as plants and animals in a balanced aquarium. Explain the diagram in your notebook.] [Illustration: The carbon and oxygen cycle in the balanced aquarium. Trace by means of the arrows the carbon from the time plants take it in as CO{2} until animals give it off. Show what happens to the oxygen.]