Читать книгу A Civic Biology, Presented in Problems - George W. Hunter - Страница 10

A weed—notice the unfavorable environment.

Functions of the Parts of a Plant.—We are all familiar with the parts of a plant—the root, stem, leaves, flowers, and fruit. But we may not know so much about their uses to the plant. Each of these structures differs from every other part, and each has a separate work or function to perform for the plant. The root holds the plant firmly in the ground and takes in water and mineral matter from the soil; the stem holds the leaves up to the light and acts as a pathway for fluids between the root and leaves; the leaves, under certain conditions, manufacture food for the plant and breathe; the flowers form the fruits; the fruits hold the seeds, which in turn hold young plants which are capable of reproducing adult plants of the same kind.

The Functions of an Animal.—As we have already seen, the grasshopper has a head, a jointed body composed of a middle and a hind part, three pairs of jointed legs, and two pairs of wings. Obviously, the wings and legs are used for movement; a careful watching of the hind part of the animal shows us that breathing movements are taking place; a bit of grass placed before it may be eaten, the tiny black jaws biting little pieces out of the grass. If disturbed, the insect hops away, and if we try to get it, it jumps or flies away, evidently seeing us before we can grasp it. Hundreds of little grasshoppers on the grass indicate that the grasshopper can reproduce its own kind, but in other respects the animal seems quite unlike the plant. The animal moves, breathes, feeds, and has sensation, while apparently the plant does none of these. It will be the purpose of later chapters to prove that the functions of plants and animals are in many respects similar and that both plants and animals breathe, feed, and reproduce.

Section through the blade of a leaf. e, cells of the upper surface; d, cells of the lower surface; i, air spaces in the leaf; v, vein in cross sections; p, green cells.

Organs.—If we look carefully at the organ of a plant called a leaf, we find that the materials of which it is composed do not appear to be everywhere the same. The leaf is much thinner and more delicate in some parts than in others. Holding the flat, expanded blade away from the branch is a little stalk, which extends into the blade of the leaf. Here it splits up into a network of tiny "veins" which evidently form a framework for the flat blade somewhat as the sticks of a kite hold the paper in place. If we examine under the compound microscope a thin section cut across the leaf, we shall find that the veins as well as the other parts are made up of many tiny boxlike units of various sizes and shapes. These smallest units of building material of the plant or animal disclosed by the compound microscope are called cells. The organs of a plant or animal are built of these tiny structures.

Several cells of Elodea, a water plant. chl., chlorophyll bodies; c.s., cell sap; c.w., cell wall; n., nucleus; p., protoplasm. The arrows show the direction of the protoplasmic movement.

Tissues.[2]—The cells which form certain parts of the veins, the flat blade, or other portions of the plant, are often found in groups or collections, the cells of which are more or less alike in size and shape. Such a collection of cells is called a tissue. Examples of tissues are the cells covering the outside of the human body, the muscle cells, which collectively allow of movement, bony tissues which form the framework to which the muscles are attached, and many others.

A cell. ch., chromosomes; c.w., cell wall; n., nucleus; p., protoplasm.

Cells.—A cell may be defined as a tiny mass of living matter containing a nucleus, either living alone or forming a unit of the building material of a living thing. The living matter of which all cells are formed is known as protoplasm (formed from two Greek words meaning first form). If we examine under a compound microscope a small bit of the water plant Elodea, we see a number of structures resembling bricks in a wall. Each "brick," however, is really a plant cell bounded by a thin wall. If we look carefully, we can see that the material inside of this wall is slowly moving and is carrying around in its substance a number of little green bodies. This moving substance is living matter, the protoplasm of the cell. The green bodies (the chlorophyll bodies) we shall learn more about later; they are found only in plant cells. All plant and animal cells appear to be alike in the fact that every living cell possesses a structure known as the nucleus (pl. nuclei), which is found within the body of the cell. This nucleus is not easy to find in the cells of Elodea. Within the nucleus of all cells are found certain bodies called chromosomes. These chromosomes in a given plant or animal are always constant in number. These chromosomes are supposed to be the bearers of the qualities which we believe can be handed down from plant to plant and from animal to animal, in other words, the inheritable qualities which make the offspring like its parents.

How Cells form Others.—Cells grow to a certain size and then split into two new cells. In this process, which is of very great importance in the growth of both plants and animals, the nucleus divides first. The chromosomes also divide, each splitting lengthwise and the parts going in equal numbers to each of the two cells formed from the old cell. In this way the matter in the chromosomes is divided equally between the two new cells. Then the rest of the protoplasm separates, and two new cells are formed. This process is known as fission. It is the usual method of growth found in the tissues of plants and animals.

Stages in the division of one cell to form two. Which part of the cell divides first? What seems to become of the chromosomes?

Cells of Various Sizes and Shapes.—Plant cells and animal cells are of very diverse shapes and sizes. There are cells so large that they can easily be seen with the unaided eye; for example, the root hairs of plants and eggs of some animals. On the other hand, cells may be so minute, as in the case of the plant cells named bacteria, that several million might be present in a few drops of milk. The forms of cells may be extremely varied in different tissues; they may assume the form of cubes, columns, spheres, flat plates, or may be extremely irregular in shape. One kind of tissue cell, found in man, has a body so small as to be quite invisible to the naked eye, although it has a prolongation several feet in length. Such are some of the cells of the nervous system of man and other large animals, as the ox, elephant, and whale.

Varying Sizes of Living Things.—Plant cells and animal cells may live alone, or they may form collections of cells. Some plants are so simple in structure as to be formed of only one kind of cells. Usually living organisms are composed of several groups of different kinds of cells. It is only necessary to call attention to the fact that such collections of cells may form organisms so tiny as to be barely visible to the eye; as, for instance, some of the small flowerless plants or many of the tiny animals living in fresh water or salt water. On the other hand, among animals, the bulk of the elephant and whale, and among plants the big trees of California, stand out as notable examples. The large plants and animals are made up of more, not necessarily larger, cells.

What Protoplasm can Do.—It responds to influences or stimulation from without its own substance. Both plants and animals are sensitive to touch or stimulation by light, heat or cold, certain chemical substances, gravity, and electricity. Green plants turn toward the source of light. Some animals are attracted to light and others repelled by it; the earthworm is an example of the latter. Protoplasm is thus said to be irritable.

Protoplasm has the power to contract and to move. Muscular movement is a familiar instance of this power. Movement may also take place in plants. Some plants fold up their leaves at night; others, like the sensitive plant, fold their leaflets when touched.

Protoplasm can form new living matter out of food. To do this, food materials must be absorbed into the cells of the living organism. To make protoplasm, it is evident that the same chemical elements must enter into the composition of the food substances as are found in living matter. The simplest plants and animals have this wonderful power as certainly developed as the most complex forms of life.

Protoplasm, be it in plant or animal, breathes and throws off waste materials. When a living thing does work oxygen unites with food in the body; the food is burned or oxidized and work is done by means of the energy released from the food. The waste materials are excreted or passed out. Plants and animals alike pass off the carbon dioxide which results from the oxidation of food and of parts of their own bodies. Animals eliminate wastes containing nitrogen through the skin and the kidneys.

Protoplasm can reproduce, that is, form other matter like itself. New plants are constantly appearing to take the places of those that die. The supply of living things upon the earth is not decreasing; reproduction is constantly taking place. In a general way it is possible to say that plants and animals reproduce in a very similar manner.

The Importance of Reproduction.—Reproduction is the final process that plants and animals are called upon to perform. Without the formation of new living things no progress would be possible on the earth. We have found that insects help flowering plants in this process. Let us now see exactly what happens when pollen is placed by the bee on the stigma of another flower of the same kind. To understand this process of reproduction in flowers, we must first study carefully pollen grains from the anther of some growing flower.

Pollen grains of different shapes and sizes.

Pollen.—Pollen grains of various flowers, when seen under the microscope, differ greatly in form and appearance. Some are relatively large, some small, some rough, others smooth, some spherical, and others angular. They all agree, however, in having a thick wall, with a thin membrane under it, the whole inclosing a mass of protoplasm. At an early stage the pollen grain contains but a single cell. A little later, however, two nuclei may be found in the protoplasm. Hence we know that at least two cells exist there, one of which is called the sperm cell; its nucleus is the sperm nucleus.

A pollen grain greatly magnified. Two nuclei are found (n, n') at this stage of its growth.

Growth of Pollen Grains.—Under certain conditions a pollen grain will grow or germinate. This growth can be artificially produced in the laboratory by sprinkling pollen from well-opened flowers of sweet pea or nasturtium on a solution of 15 parts of sugar to 100 of water. Left for a few hours in a warm and moist place and then examined under the microscope, the grains of pollen will be found to have germinated, a long, threadlike mass of protoplasm growing from it into the sugar solution. The presence of this sugar solution was sufficient to induce growth. When the pollen grain germinates, the nuclei enter the threadlike growth (this growth is called the pollen tube; see Figure). One of the nuclei which grows into the pollen tube is known as the sperm nucleus.

Three stages in the germination of the pollen grain. The nuclei in the tube in (3) are the sperm nuclei. Drawn under the compound microscope.

Fertilization of the ovule. A flower cut down lengthwise (only one side shown). The pollen tube is seen entering the ovule. a, anther; f, filament; pg, pollen grain; s, stigmatic surface; pt, pollen tube; st, style; o, ovary; m, micropyle; sp, space within ovary; e, egg cell; P, petal; S, sepal.

Fertilization of the Flower.—If we cut the pistil of a large flower (as a lily) lengthwise, we notice that the style appears to be composed of rather spongy material in the interior; the ovary is hollow and is seen to contain a number of rounded structures which appear to grow out from the wall of the ovary. These are the ovules. The ovules, under certain conditions, will become seeds. An explanation of these conditions may be had if we examine, under the microscope, a very thin section of a pistil, on which pollen has begun to germinate. The central part of the style is found to be either hollow or composed of a soft tissue through which the pollen tube can easily grow. Upon germination, the pollen tube grows downward through the spongy center of the style, follows the path of least resistance to the space within the ovary, and there enters the ovule. It is believed that some chemical influence thus attracts the pollen tube. When it reaches the ovary, the sperm cell penetrates an ovule by making its way through a little hole called the micropyle. It then grows toward a clear bit of protoplasm known as the embryo sac. The embryo sac is an ovoid space, microscopic in size, filled with semifluid protoplasm containing several nuclei. (See Figure.) One of the nuclei, with the protoplasm immediately surrounding it, is called the egg cell. It is this cell that the sperm nucleus of the pollen tube grows toward; ultimately the sperm nucleus reaches the egg nucleus and unites with it. The two nuclei, after coming together, unite to form a single cell. This process is known as fertilization. This single cell formed by the union of the pollen tube cell or sperm and the egg cell is now called a fertilized egg.

Development of Ovule into Seed.—The primary reason for the existence of a flower is that it may produce seeds from which future plants will grow. After fertilization the ovule grows into a seed. The first beginning of the growth of the seed takes place at the moment of fertilization. From that time on there is a growth of the fertilized egg within the ovule which makes a baby plant called the embryo. The embryo will give rise to the adult plant.

The fruit of the locust, a bean-like fruit. p, the attachment to the placenta; s, the stigma.

A Typical Fruit—the Pea or Bean Pod.—If a withered flower of any one of the pea or bean family is examined carefully, it will be found that the pistil of the flower continues to grow after the rest of the flower withers. If we remove the pistil from such a flower and examine it carefully, we find that it is the ovary that has enlarged. The space within the ovary has become nearly filled with a number of nearly ovoid bodies, attached along one edge of the inner wall. These we recognize as the young seeds.

The pod of a bean, pea, or locust illustrates well the growth from the flower. The pod, which is in reality a ripened ovary with other parts of the pistil attached to it, is considered as a fruit. By definition, a fruit is a ripened ovary and its contents together with any parts of the flower that may be attached to it. The chief use of the fruit to the flower is to hold and to protect the seeds; it may ultimately distribute them where they can reproduce young plants.

The Necessity of Fruit and Seed Dispersal to a Plant.—We have seen that the chief reason for flowers, from the plant's standpoint, is to produce fruits which contain seeds. Reproduction and the ultimate scattering of fruits and seeds are absolutely necessary in order that colonies of plants may reach new localities. It is evident that plants best fitted to scatter their seeds, or place fruits containing the seeds some little distance from the parent plants, are the ones which will spread most rapidly. A plant, if it is to advance into new territory, must get its seeds there first. Plants which are best fitted to do this are the most widely distributed on the earth.

The development of an apple. Notice that in this fruit additional parts besides the ovary (o) become part of the fruit. Certain outer parts of the flower, the sepals (s) and receptacle, become the fleshy part of the fruit, while the ovary becomes the core. Stages numbered 1 to 7 are in the order of development.

How Seeds and Fruits are Scattered.—Seed dispersal is accomplished in many different ways. Some plants produce enormous numbers of seeds which may or may not have special devices to aid in their scattering. Most weeds are thus started "in pastures new." Some prolific plants, like the milkweed, have seeds with a little tuft of hairlike down which allows them to be carried by the wind. Others, as the omnipresent dandelion, have their fruits provided with a similar structure, the pappus. Some plants, as the burdock and clotbur, have fruits provided with tiny hooks which stick to the hair of animals, thus proving a means of transportation. Most fleshy fruits contain indigestible seeds, so that when the fruits are eaten by animals the seeds are passed off from the body unharmed and may, if favorably placed, grow. Nuts of various kinds are often carried off by animals, buried, and forgotten, to grow later. Such are a few of the ways in which seeds are scattered. All other things being equal, the plants best equipped to scatter seeds or fruits are those which will drive out other plants in a given locality. Because of their adaptations they are likely to be very numerous, and when unfavorable conditions come, for that reason, if for no other, are likely to survive. Such plants are best exemplified in the weeds of the grassplots and gardens.

[2] To the Teacher.—Any simple plant or animal tissue can be used to demonstrate the cell. Epidermal cells may be stripped from the body of the frog or obtained by scraping the inside of one's mouth. The thin skin from an onion stained with tincture of iodine shows well, as do thin sections of a young stem, as the bean or pea. One of the best places to study a tissue and the cells of which it is composed is in the leaf of a green water plant, Elodea. In this plant the cells are large, and not only their outline, but the movement of the living matter within the cells, may easily be seen, and the parts described in the next paragraph can be demonstrated.

Reference Books

elementary

Hunter, Laboratory Problems in Civic Biology. American Book Company.

Andrews, A Practical Course in Botany, pages 250–270. American Book Company.

Atkinson, First Studies of Plant Life, Chaps. XXV-XXVI. Ginn and Company.

Bailey, Lessons with Plants, Part III, pages 131–250. The Macmillan Company.

Coulter, Plant Life and Plant Uses. American Book Company.

Dana, Plants and their Children, pages 187–255. American Book Company.

Lubbock, Flowers, Fruit, and Leaves, Part I. The Macmillan Company.

Newell, A Reader in Botany, Part II, pages 1–96. Ginn and Company.

advanced

Bailey, Plant Breeding. The Macmillan Company.

Campbell, Lectures on the Evolution of Plants. The Macmillan Company.

Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Company.

Darwin, Different Forms of Flowers on Plants of the Same Species. Appleton.

Darwin, Fertilization in the Vegetable Kingdom, Chaps. I and II. Appleton.

Darwin, Orchids Fertilized by Insects. D. Appleton and Company.

Müller, The Fertilization of Flowers. The Macmillan Company.