A Civic Biology Part 17

Footnote 23: Teachers may take up parts or all of Chapter XXIV 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.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American Book Company.

Bigelow, _Introduction to Biology_. The Macmillan 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.

Overton, _General Hygiene_. American Book 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.

Sharpe, _Laboratory Manual in Biology_, pages 123-132.

American 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.

Hutchinson, _Preventable Diseases_. Houghton, Mifflin and Company.

Lee, _Scientific Features of Modern Medicine_. Columbia University Press.

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 Company.

XII. THE RELATIONS OF PLANTS TO ANIMALS

_Problems.--To determine the general biological relations existing between plants and animals._ _(a) As shown in a balanced aquarium._ _(b) As shown in hay infusion._

SUGGESTIONS FOR LABORATORY WORK

_Demonstration of life in a "balanced" and "unbalanced"

aquarium._--Determination of factors causing balance.

_Demonstration of hay infusion._--Examination to show forms of animal and plant life.

Tabular comparison between balanced aquarium and hay infusion.

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.

[Ill.u.s.tration: 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 _pa.s.s 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.

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: 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.]

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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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]

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 partners.h.i.+p. _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 sh.e.l.l 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.

[Ill.u.s.tration: Life in the late stage of a hay infusion. _B_, bacteria, swimming or forming ma.s.ses 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 gla.s.s 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 sc.u.m appears on the surface. If some of this sc.u.m 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 gra.s.s,--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 gra.s.s and may have been cut in a field near a pool containing these creatures. When the pool dried up, the wind may have scattered some of these little organisms in the dried mud or dust. Some may have existed in a dormant state on the hay and the water awakened them to active life. In the water, too, there may have been some living cells, plants and animals.

At first the multiplication of the tiny animals within the hay infusion is extremely rapid; there is food in abundance and near at hand. After a few days more, however, several kinds of one-celled animals may appear, some of which prey upon others. Consequently a struggle for life takes place, which becomes more and more intense as the food from the hay is used up.

Eventually the end comes for all the animals unless some green plants obtain a foothold within the jar. If such a thing happens, food will be manufactured within their bodies, a new food supply arises for the animals within the jar, and a balance of life may result.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American Book Company.

Sharpe, _A Laboratory Manual for the Solution of Problems in Biology_, pp. 133-138. American Book Company.

ADVANCED

Eggerlin and Ehrenberg, _The Fresh Water Aquarium and its Inhabitants_. Henry Holt and Company.

Furneaux, _Life in Ponds and Streams_. Longmans, Green, and Company.

Parker, _Biology_. The Macmillan Company.

Sedgwick and Wilson, _Biology_. Henry Holt and Company.