But the plants are continually growing larger. The snails and fish, too, eat parts of the plants. Thus the plant life gives food to the animals 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 part of the raw material used for protein manufacture in the plant. This nitrogenous matter is prepared for use by several different kinds of bacteria which first break the dead bodies down and then give it to the plants in the form of soluble nitrates. The green plants manufacture food, the animals eat the plants and give off organic waste, from which the plants in turn 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. Make a table to show this balance. [Illustration: The relations between green plants and animals.] Relations between Green Plants and Animals.--What goes 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 does not feed to a great extent upon leaves, but he eats roots, stems, fruits, and seeds. When he does not feed directly upon plants, he eats the flesh of plant eating 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 use. [Illustration: The nitrogen cycle. Trace the nitrogen from its source in the air until it gets back again into the air.] The Nitrogen Cycle.--The animals in their turn supply much of the carbon dioxide that the plant uses in starch making. They also supply some of the nitrogenous matter used by the plants, part being given the plants from the dead bodies of their own relatives and part being prepared from the nitrogen of the air 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 available. And the available supply is used over and over again, perhaps in nitrogenous food by an animal, then it may be given off as organic waste, get into the soil, and be taken up by a plant through the roots. Eventually the nitrogen forms part of the food supply in the body of the plant, and then may become part of its living matter. When the plant dies, the nitrogen is returned to the soil. Thus the usable nitrogen is kept in circulation.[24]