_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.
[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.]
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]
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,
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.