an onion shows well, as do thin sections of a young stem, as the bean or pea. I have found one of the best places to study a tissue and the cells of which it is composed in the leaf of a green water plant, Elodea. In this plant the cells are large, and not only the outline of the cells, but the movement of the living matter within the cells, may easily be seen, and most of the parts described in the next paragraph can be demonstrated. Cells. A cell may be defined as a tiny mass of living matter, either living alone or forming the building material of a living thing. The living matter of which all cells are formed is known as protoplasm (from two Greek works meaning first form). When viewed under a high magnification of a compound microscope, it is a grayish, semifluid mass, seemingly almost devoid of any structure. A careful observer will find, however, that the material seems to be made of a ground mass of fluid with innumerable granules of nuclei), which is found. within the body of the cell. C.p. 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 surrounding it. This is seen when a plant or animal is placed in a liquid containing some dye such as logwood. 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 spaces to hold liquid cell sap (C.s.); just above the nucleus (N.l.) is a structure called the centrosome (c), which aids in cell division; within the nucleus are chromosomes (N.n.), which form a network; t.n., nucleolus; C.p., plastids; C.f., lifeless material in the cell. The chromosomes, which are believed to be always definite in number for every tissue cell, are of much interest to scientists. It is found that each time a cell splits to form two new cells, each chromosome splits lengthwise and the parts go in equal numbers into the nucleus of each of the two new cells thus formed. 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. The bulk of the nucleus is filled with a fluid, and in some nuclei a body known as a nucleolus is found; it does not, however, seem to be a constant structure. The protoplasm surrounding the nucleus is called cytoplasm. 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. The cytoplasm of a cell contain spaces, which are usually filled with a fluid known as cell sap. These spaces in the cytoplasm are given the name of vacuoles. Frequently nonliving materials are found within the cytoplasm of the cell. The cell is surrounded by a very delicate living structure called the cell membrane. Outside this membrane a wall is formed by the activity of the protoplasm in the cells of plants. These cell walls form wood. How Cells form Others. split into two new cells. In this process, which is of very great Cells grow to a certain size and then 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. Lastly, the cytoplasm separates, and two new cells are formed. This pro cess is known as fission. It is the usual method of growth found in the tissues of plants and animals. 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 named bacteria, several million could be placed on the 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 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 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 take in 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, manufacture food out of the dead organic and inorganic matter contained in the soil, air, and water, and then change this food into the living matter of their own bodies. This organic matter in turn may become food for animals. In the last 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 substance which forms the basis of all living things. Chemical Composition of Protoplasm. — 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 element nitrogen. It also contains the elements carbon, hydrogen, oxygen, and a little sulphur. Calcium, iron, silicon, sodium, potassium, phosphorus, 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 composed of the above-named elements. What is of far more importance 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. The properties of protoplasm are as follows: (1) 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 electricity. One of the simplest forms of plant life, the slime mold, a mass of naked protoplasm, if placed on a damp blotting paper, moistened at one end with an infusion of leaves, and at the other with a solution of quinine, will crawl to that part of the blotter most like its habitat, that is, moist leaves. 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. 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. (3) Protoplasm has the power of taking up food materials, of selecting the materials which can be used by it, and of rejecting the substances 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 projections of the root called root hairs, takes in only the material which will be of use in forming food or new protoplasm for the plant. An animal absorbs into its body only food material that can be used, rejecting material unfit for food. (4) Protoplasm grows, not as inorganic objects grow, from the outside,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 chem 1 Home Experiment. Make a strong solution of alum (two spoonfuls of powdered alum to half a glass of water). Suspend in the solution a thread with a pebble attached to the lower end. Notice where and how crystals of alum grow. ical 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. (5) Protoplasm, be it in the body of a plant or of an animal, uses oxygen. It breathes. Thus substances taken into the body are oxidized, and release energy for movement and the other activities of plants and animals. (6) Protoplasm has the power to rid itself of waste materials, especially those which might be harmful to it. A tree sheds its leaves, and as a result gets 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 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 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. We shall study this more in detail later. To sum up, 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 as it enters into the composition of a highly complex organism such as a tree, a dog, or a man. BOOKS FOR REFERENCE ELEMENTARY Sharpe, A Laboratory Manual. American Book Company. Atkinson, First Studies of Plant Life. Chap. XI. Ginn and Company. ADVANCED Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Com pany. Goodale, Physiological Botany. American Book Company. Green, Vegetable Physiology. J. and A. Churchill. Parker, An Elementary Course in Practical Biology. The Macmillan Company. Sedgwick and Wilson, General Biology. Henry Holt and Company. Verworn, General Physiology. The Macmillan Company. Wilson, The Cell in Development and Inheritance. The Macmillan Company. HUNT. ES. BIO. -3 |