Footnote 24: A small amount of nitrogen gas is returned to the atmosphere by the action of the decomposing bacteria on the ammonia compounds in the soil. (See figure of nitrogen cycle.) Symbiosis.--We have seen that in the balanced aquarium the animals and plants, in a wide sense, form a sort of unconscious partnership. _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 algae, and, if we accept the term in a wide sense, all green plants and animals live in this relation of mutual give and take. Animals also frequently live in this relation to each other, as the crab, which lives within the shell of the oyster; the sea anemones, which are carried around on the backs of some hermit crabs, aiding the crab in protecting it from its enemies, and being carried about by the crab to places where food is plentiful. [Illustration: Life in the late stage of a hay infusion. _B_, bacteria, swimming or forming masses of food upon which the one-celled animals, the paramoecia, are feeding; _G_, gullet; _F.V._, food vacuole; _C.V._, contractile vacuole; _P_, pleurococcus; _P.D._, pleurococcus dividing. (Drawn from nature by J. W. Teitz.)] A Hay Infusion.--Still another example of the close relation between plants and animals may be seen in the study of a hay infusion. If we place a wisp of hay or straw in a small glass jar nearly full of water, and leave it for a few days in a warm room, certain changes are seen to take place in the contents of the jar; after a little while the water gets cloudy and darker in color, and a scum appears on the surface. If some of this scum is examined under the compound microscope, it will be found to consist almost entirely of bacteria. These bacteria evidently aid in the decay which (as the unpleasant odor from the jar testifies) is beginning to take place. As we have learned, bacteria flourish wherever the food supply is abundant. The water within the jar has come to contain much of the food material which was once within the leaves of the grass,--organic nutrients, starch, sugar, and proteins, formed in the leaf by the action of the sun on the chlorophyll of the leaf, and now released into the water by the breaking down of the walls of the cells of the leaves. The bacteria themselves release this food from the hay by causing it to decay. After a few days small one-celled animals appear; these multiply with wonderful rapidity, so that in some cases the surface of the water seems to be almost white with active one-celled forms of life. If we ask ourselves where these animals come from, we are forced to the conclusion that they must have been in the water, in the air, or on the hay. Hay is dried grass and may have been cut in a field near a pool containing these creatures. When the pool dried up,