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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 proteids, 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 onecelled 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, which may have been cut in a field near a pool containing these creatures. When these pools dried up, the wind may have scattered some of these little organisms in the dried mud or dust. Some may exist in a dormant state on the hay, the water serving to awaken them to active life. In the water,

too, there may have been some living cells, plant and animal. 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 onecelled animals may appear, some of which prey upon others. Consequently a struggle for life begins, 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 results.

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company.

ADVANCED

Eggerling and Ehrenberg, The Fresh Water Aquarium and its Inhabitants. Henry Holt and Company.

Furneaux, Life in Ponds and Streams.

Parker, Biology. The Macmillan Company.

XV. THE PROTOZOA

Problem XXV. The study of a one-celled animal. (Laboratory Manual, Problem XXV.)

(a) In its relations to its surroundings.

(b) As a cell. (Optional.)

(c) In its relations to man.

We have seen that perhaps the simplest plant would be exemplified by one of the tiny bacteria we have just read about. A typical one-celled plant, however, would contain green coloring matter or chlorophyll, and would have the power to manufacture its own food under conditions giving it a moderate temperature, a supply of water, oxygen, carbon dioxide, and sunlight. Such a simple plant is the pleurococcus, the "green slime" seen on the shady sides of trees, stones, or city houses. This plant would meet one definition of a cell, as it is a minute mass of protoplasm containing a nucleus. It is surrounded by a wall of a woody material which covers a delicate membrane formed by the activity of the living matter within the cell. It also contains granules of protoplasm colored green, called chloroplasts. Of their part in the manufacture of organic food we have already learned. Such is a simple plant cell. Let us now examine a simple animal cell in order to compare it with that of a plant.

The Paramecium. The one-celled animal most frequently found in hay infusions is the paramecium, or slipper animalcule (so-called because of its shape).

This cell is elongated, oval, or elliptical in outline, but somewhat flattened. Seen under the low power of the microscope, it appears to be extremely active, rushing about now rapidly, now more slowly, but seemingly always taking a definite course. more pointed end of the body (the anterior) usually goes first.

The

1 This shows one practical reason why plant food often contains more indigestible matter than animal food of same bulk.

If it pushes its way past any dense substance in the water, the cell body is seen to change its shape as it squeezes through.

The cell body is almost transparent, and consists of semifluid protoplasm which has a granular grayish appearance under the microscope. This protoplasm appears to be bounded by a very delicate membrane through which project numerous delicate threads of protoplasm called cilia. (These are usually invisible under the microscope.)

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Paramecium.

Greatly

The locomotion of the paramecium is caused by the movement of these cilia which lash the water like a multitude of tiny oars. The cilia also send tiny particles of food into a funnel-like opening, the gullet on one side of the cell. Once within the cell body, the particles of food materials are gathered into little balls within the almost transparent protoplasm. These masses of food seem to be inclosed within a little area containing fluid, called a vacuole. Other vacuoles appear to be clear; these are spaces in which food has been digested. One or two larger vacuoles may be found; these are the contractile vacuoles; their purpose seems to be to pass off waste material from the cell body. This is done by pulsation of the vacuole, which ultimately bursts, passing fluid waste to the outside. Solid wastes are passed out of the cell in somewhat the same manner. The nucleus of the cell is not easily visible in living specimens. In a cell that has been stained it has been found to be a double structure, consisting of one large and one small portion, called respectively the macronucleus and the micronucleus.

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magnified. From side. F.V., food vacuole; C.V., contractile vacuole; M, mouth; N, nucleus; W.V., water vacuole. (After Sedgwick and Wilson.)

Response to Stimuli. In the paramecium, as in the one-celled plants, the protoplasm composing the cell can do certain things. Protoplasm responds, in both plants and animals, to certain agencies acting upon it, coming from without; these agencies we call stimuli. Such stimuli may be light, differences of temperature, presence of food, electricity, or other factors of its surroundings.

Plant and animal cells may react differently to the same stimuli. In general, however, we know that protoplasm is irritable to some of these factors. To severe stimuli, protoplasm usually responds by contracting, another power which it possesses. We know, too, that plant and animal cells take in food and change the food to protoplasm, that is, that they assimilate food; and that they may waste away and repair themselves. Finally, we know that new plant and animal cells are reproduced from the original bit of protoplasm, a single cell.

Reproduction of Paramecium. Sometimes a paramœcium may be found in the act of dividing by the process known as fission, to form two new cells, each of which contains half of the original cell. This is a method of asexual reproduction. The original cell

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may thus form in succes-
sion many hundreds of
cells in every respect like
the original parent cell.

Frequently another
method of reproduction
may be observed. This
is called conjugation, and
somewhat resembles the
same process in the simple
plants. Two cells of equal
size attach themselves
together as shown in the
Figure. Complicated

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changes take place in the nuclei of the two cells thus united, which results in an equal exchange of part of the material forming the nucleus. After a short period of rest the two cells separate. The stage of conjugation we believed in the plants to be a sexual stage. There seems every reason to believe that it is a like stage in the life history of the paramœcium.

Amoeba. In order to understand more fully the life of a simple bit of protoplasm, let us take up the study of the amaba, a type

1 Amœbæ may be obtained from the hay infusion, from the dead leaves in the bottom of small pools, from the same source in fresh-water aquaria, from the roots of

of the simplest form of life known, either plant or animal. Unlike the plant and animal cells we have examined, the amoeba has no fixed form. Viewed under the compound microscope, it has the appearance of an irregular mass of granular protoplasm. Its form is constantly changing as it moves about. This is due to the pushing out of tiny projections of the protoplasm of the cell, called pseudopodia (false feet). The

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Amoeba, showing the changes which take place during division. The dark body in each Figure is the nucleus; the transparent circle, the contractile vacuole; the outer, clear portion of the body the ectoplasm; the granular portion, the endoplasm; the granular masses, food vacuoles. Much magnified.

being called endoplasm. In the central part of the cell is the nucleus. This important organ is difficult to see, except in cells that have been stained.

The locomotion is accomplished, according to Professor Jennings of Johns Hopkins University, by a kind of rolling motion, "the upper and lower surfaces constantly interchanging positions." The pseudopodia are pushed forward in the direction which the animal is to go, the rest of the body following.

duckweed or other small water plants, or from green algae growing in quiet localities. No sure method of obtaining them can be given.

HUNT. ES. BIO.-13

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