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If we walk out any afternoon in the fall of the year, we notice the many forms of plant life that fill the waste places along the waysides and make their way into the cultivated gardens and fields, driving out the rightful inhabitants. We call such plants weeds. Let us study some common weed such as the yellow-blossomed butter and eggs or the ubiquitous shepherd’s purse, with the intention of finding out how such plants are so well fitted to live. If we think of the plant as a mass of living matter, we at once are struck with the evident fact that the living ma terial has taken on very different forms in dif ferent parts of the plant. The root below the surface of the ground differs considerably in form from the stem, which in turn differs in structure from the leaves.
Functions of the Parts of a Plant
Still more prominent are the structures we call flowers and fruits. Each of these structures differs from each other part, and each has a different work or function to perform for the plant. The root holds the plant firmly in the ground and takes in water ; the~SKm holds the leaves up to the lighl; the leaves, under certain conditions, manufacture food for the plant; the flowers form the fruits; the fruits hold the seeds, which in turn reproduce young plants of the same kind.
Each part ofa plant or animal, having a separate work or function,is known as an organ. Most of organs; plants andhence animals any are living composed thing, even the simplest single living cell, has come to be called an organism. If we look rather carefully from all sides at the organ called the leaf, we find that the materials of which it is composed do not ap pear to be everywhere the same. The leaf is much thinner and more delicate in some parts than in others. Holding the flat, expanded blade to the branch is a little stalk, the pctioje, which extends into tne blade of the leaf as a se ries of little veins which evidently form a framework for the flat blade somewhat as the sticks of a kite hold the paper in place. In the same manner the veins, if cut crosswise and mounted on a glass slide under the compound microscope,1 show that they are made up of building material which, although microscopic in size, yet differs considerably from other material in the same part of the vein. The smallest units of building material of the plant or ani mal disclosed by the compound microscope are called cells. The organs of a plant .or. animal are built of these tiny structures.
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 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 cells which collectively allow of movement, the so-called muscles; the material that forms the framework to which the muscles are attached, the bony tissues; and many others.
A cell may be defined as the smallest bit of living matter that can live alone. All plant and animal cells appear to be alike in the fact that every living cell possesses a structure known as the nucleus, which is found within the body of the cell. The nucleus is composed of living matter like the rest of the cell, although it seems to differ in some chemical way from that part of the cell surround ing it. This is seen when a plant or animal is placed in a liquid containing some dye such as log wood. Certain bodies in the nucleus take up the stain much more readily than the rest of the living matter of the cell, taking on a deep black color. They are thus called the chromosomes (color-bearing bodies).
Diagram of a cell (after Wilson). The cell protoplasm contains cell food (C.I.) ; spaces contain liquid cell sap (C.s.) ; just above the nucleus (N.l.) is a struc ture called the centrosome (c), which aids in cell division; within the nucleus are chromosomes (N.n.), which form a network; «.n., nucleolus.
The living matter of which all cells are formed is known as protoplasm (from two Greek words meaning first form). The bulk of the nucleus is filled with a fluid; in some nuclei a body known as a nucleolus is found; it does not seem to be a constant structure. The protoplasm surrounding the nucleus is called cytoplasm be cause it makes up the body of the cell. The nucleus plays a very important part in the life of a cell. 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 so that an equal number go to each of the two cells formed from the old cell. Lastly, the cytoplasm 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.
The protoplasm in some cells collects into little bodies called plastids. In plant cells the plastids are frequently colored green. This green coloring matter, which is found only in plant cells, is called chlorophyll and green plastids are called chlorophyll bodies or chloroplasts. The cytoplasm of a cell contains spaces, which are usually filled with a fluid known as cell sap or cytoplasm. These spaces in the cytoplasm are given the name of vacuoles. Frequently non-living materials are found within the cytoplasm of the cell.
The cell is surrounded by a very delicate living structure called the cell membrane. This is so thin that it is impossible to get a microscope of power enough to throw any light on its structure. Outside this membrane a wall is formed by the activity of the protoplasm of the cell. The cell wall is usually much heavier and more conspicuous in the cells of plants. In the cells of the pith it was the wall of cellulose or wood that you saw under the microscope.
Protoplasm, when viewed under a high magnification of a compound microscope, is a grayish, almost fluid mass, seemingly almost devoid of any structure. A careful observer will, however, find that the material seems to be made of a ground mass of fluid with innumerable granules of various size and form floating in the fluid portion. Other observers believe protoplasm to consist of a fluid groundwork with in numerable tiny threads scattered through it, each thread being more or less firmly united with other threads of the mass. Still other scientists hold that protoplasm has essentially the structure of an emulsion or froth or foam. To them the fine structure resolves itself into a collection of very minute bub bles. Doubtless all of the observers are right in part, for protoplasm doubt less assumes all of the above-mentioned forms in different plants and animals and under different conditions. But we must also bear in mind that when we make observations on protoplasm it may be already dead when we examine it — and therefore undoubtedly greatly changed in structure — or else we may view it under conditions which are far from the normal con ditions under which it usually exists as living matter. Finally, the instru ment we call the microscope, although seeming to be nearly perfect, may not always give to our eye an exact representation of what is under its lenses.
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 that in the case of the plant cells we call bacteria, several million could be placed on a dot of this letter i. 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 so irregular that description is impossible. 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 m ay live alone or they may form collections of cells as tissues. Some plants are so simple in structure as to be formed of only one kind of tissue cells. Usually living organisms are composed of several groups of such tissues. Examples have been given. It is only necessary to call attention to the fact that such collections of tissues may form organisms so tiny as to be barely visible to the eye; as, for instance, some water-loving flowerless plants or many of the tiny animals living in fresh water or salt water, such as the hydra, small worms, and tiny crustaceans. 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.
Relation to Organic and Inorganic Matter
The inorganic matter covering the earth, as air and water, and forming the great mass of its bulk, is made use of by plants and animals. The latter make their homes in earth, air, or water ; they breathe the oxygen of the atmosphere; they use the water for drinking; but in the main their food consists of organic matter. Plants, on the other hand, use the elements contained in the soil, air, and water, not only for food, but also to make the living matter of their own bodies. In some mysterious way, of which we shall later learn something, plants take up inorganic and organic substances from the soil and air and transform them into organic matter. This organic matter in turn becomes food for animals. In thejast chapter we found that the classes of substances in an animal or plant and the organic food substances have a similar composition. Let us now consider chemically the sub stance which forms the basis of all living things.
Living matter, when analyzed by chemists in the laboratory, seems to have a very complex chemical composition. It is somewhat like a proteid in that it always contains the ele ment nitrogen. It also contains the elements carbon, hydrogen, oxygen, and a little sulphur, and perhaps phosphorus. Calcium, iron, silica, sodium, potassium, and other mineral matters are usually found in very minute quantities in its composition. We believe that the matter out of which plants and animals are formed, although a very complex building material and almost impossible of correct analysis, is nevertheless composed of cer tain chemical elements which are always present. To this living matter the name protoplasm has been given. Protoplasm, then, is made up of certain chemical elements, combined in definite proportions. What is of far more impor tance to us is the fact that it is distinguished by certain properties which it possesses and which inorganic matter does not’ possess.
Properties of Protoplasm
Plants and animals are largely made up of living matter. Let us study its properties:
1. It responds to influences or stimulation from without its own siihst.anp.fi. Both plants and animals are sensitive to touch or stimulation by light, heat, or electricity. Leaves 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. Many other instances might be given. Protoplasm is thus said to be irritable.
2. Protoplasm has the power to move and to contract. Muscular movement is a familiar instance of this power. Plants move their leaves and other organs. One-celled animals change their form.
3. Protoplasm has the power of taking up food materials, of se lecting the materials which can be used by it, and of rejecting the sub stances that it cannot use. A commercial sponge, the dried skeleton of an animal, if placed in water, will swell up with the water absorbed by it, but the water thus taken in is not used by the dead skeleton. Protoplasm, however, in the tiny parts of the root of a plant called the root hairs, takes in only the material which will be of use in forming food or new protoplasm for the plant. An animal selects only such food as it wants, and refuses to eat material that it does not use as food.
4. Protoplasm grows, not as inorganic objects grow, from the out side,1 but by a process of taking in food material and then changing it into living material. To do this 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 well developed as the most complex forms of life.
5. Protoplasm, be it in the body of a plant or an animal, uses oxygen. It breathes. Thus the food substances taken into the body are oxidized, and either release energy for growth, move ment, etc., or form new protoplasm.
6. Protoplasm has the power to rid itself of waste materials, especially those which might be harmful to it. A tree sheds its leaves partly to get rid of the accumulation of mineral matter in the leaves. Plants and animals alike pass off the carbon dioxide which results from the very processes of living, the oxidation of foods or parts of their own bodies. Animals eliminate wastes containing nitrogen through the skin and the kidneys.
7. 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 de creasing; reproduction is constantly taking place. In a general way it is possible to say that plants and animals reproduce in a very similar manner. We shall study this more in detail later.
To sum up, then, we find that living protoplasm has the properties of sensibility, motion, growth, and reproduction alike in its simplest state as a one-celled plant or animal and when it enters into the composition of a highly complex organism such as a tree, a dog, or a man.
Source: Hunter, Elements of Biology (1907)
Modern Biology, Lesson Assessment