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

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 correlation.

Symbiosis. Plants and animals are seen in a general way to be of mutual advantage to each other. Some plants, called lichens, show this mutual

partnership in the following interesting way. A lichen is composed of two kinds of 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 between them. In most lichens the alga, a green plant, forms starch and nourishes the fungus.

W. C. Barbour.

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, 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

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

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.

A Hay Infusion. Still another example of the close relation between plants and animals may be seen in the study of a hay

Life in a 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, pleuococcus; P.D., pleurococcus dividing.

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; the water after a little while 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 taking place. As we have learned, bacteria flourish wherever the food supply is