mineral matters containing nitrogen, sulphur, and phosphorus obtained from the soil, the living plant manufactures protein. 35. Do green plants give off a gas in sunlight? - Laboratory Study No. 18. Into a glass cylinder containing water fresh from the faucet put a small amount of water plant (Elodea, Spirogyra, or Milfoil), holding it to the bottom of the tall jar by means of a weight if necessary. Stand the cylinder in direct sunlight. 1. Describe the preparation of the experiment. 2. What do you observe coming off from the plant? (These bubbles of gas have been proved to be composed of oxygen.) 36. Do green plants give off a gas when deprived of sunlight? - Laboratory Study No. 19. Place the glass cylinder prepared as directed above in darkness for several hours (or, still better, a second cylinder should be used for comparison). 1. In what respects do Experiments 18 and 19 differ? 2. Do you see any bubbles as long as the cylinder is kept in the dark? 3. Under what condition, therefore, does a green plant give off oxygen? 37. The oxygen supply for animals. We have seen that starch is made of carbon dioxid (CO2) and water (H2O). By repeated experiments biologists have proved that in the process of manufacturing carbohydrates more oxygen is present in the CO2 and H2O than is needed. This is the oxygen we have seen given off by the green water plant in sunlight. Every green plant gives off oxygen into the air when manufacturing carbohydrates. Hence, in this process the carbon dioxid is constantly being taken from the air and a fresh supply of oxygen set free. CHAPTER III THE STRUCTURE OF PLANTS 38. The parts of a plant. - Laboratory Study No. 20. Materials: A well-developed bean plant or other seedling or a weed, for each two pupils; one or more plants with flowers and if possible with fruits for demonstration. Nearly all the plants with which we are most familiar FIG. 4. Roots, stems, leaves, flowers, and fruits of a buttercup plant. consist of at least three kinds of parts, namely, roots, stems, and leaves (Fig. 4). 1. Name and describe as to color and form the parts of the plant that grew beneath the ground. 2. How does the stem differ from the root as to color and direction of growth? What parts of the plant above ground are attached to the stem? 3. How does the main part of the leaf 4. Make a drawing, natural size, of the 39. Organs and functions. From our laboratory study we have learned that a common plant consists of roots, stems, and leaves, and that at certain seasons of the year flowers and fruits are present. To each of these various parts is given the name organ. Roots are useful to a plant, for one thing, because they hold it in the ground, while stems support the leaves, flowers, and fruits. In fact every organ of a plant has some work to do, and this work is called its function. Hence, we may define an organ as a part of a plant that has a certain function or functions to perform. 40. Microscopic structure of plants. When one examines by the aid of a compound microscope a small portion of any of the organs of common plants, one finds that each organ is composed of many smaller portions too minute to be seen with the unaided eye. These tiny divisions are called cells. We shall now attempt to become familiar by the use of the compound microscope with the appearance of several kinds of plant cells. 41. Study of plant cells. Laboratory Study No. 21. Materials: (1) Slides prepared as follows: Cut a layer of an onion bulb into small squares, and strip off from the inner surface of each square a very thin layer. Place it on a glass slide and add a drop of water. (If it is desirable to keep the slides for several hours, put glycerin diluted with water over the onion cells.) Cover each thin membrane with a cover glass.1 (2) Prepared or freshly cut thin sections of roots, stems, and leaves. 1. By the aid of the low power of the compound microscope, examine the slide prepared as directed in (1) under materials. Note that the thin membrane is composed of a large number of tiny spaces each in 1 The authors are indebted to Miss Elsie M. Kupfer, Head of Department of Biology of Wadleigh High School, New York City, for suggesting this admirable material for cell study. a. closed by lines more or less dark in color called cell-walls. (These parts are usually seen more clearly if the light is largely excluded by closing the diaphragm in the stage.) Describe the general appearance of the membrane, stating of what it is composed. b. State whether or not the cells in the various parts of the membrane differ in size and shape. 2. Within the cell-body, often near the center of the cell, is usually a tiny object called the nucleus. Describe the location, shape, and color of the nucleus. 3. All the parts of the cell between the cell-wall and the nucleus constitute the cell-body. Describe the location and the appearance of the cell-body. 4. Make a drawing of three or four adjacent cells, several times as large as they appear under the microscope. Label cell-wall, cell-body, cell-nucleus. 5. (Demonstration.) Secure some growing sprays of Elodea (a common water plant). Pull off one of the youngest leaves near the tip end, put it on a slide with a drop of water, and cover with a cover glass. Let the preparation stand in a warm place for a time: Examine with the high power the cells of which this leaf is composed. FIG. 5.-Elodea plant. Within each cell note some green bodies b. Carefully watch the chlorophyll bodies in several cells and describe any movements you see. These movements show that the substance of the cell is in motion, and is carrying the chlorophyll bodies along with it. c. Make a drawing at least 2 inches long of one of the cells with its chlorophyll bodies. Label cell-wall, chlorophyll bodies, and show by arrows the direction of their movements. 6. (Demonstration.) Examine with the low power of the microscope the sections of root, stem, and leaf, or study Figures 11, 12, 15, 22. What have you learned of the microscopic structure of root, stem, and leaf? (Optional.) Make a drawing of four or five cells from each of the organs studied. 42. Cells and protoplasm. - Under the microscope cells at first appear to be only plane surfaces surrounded by lines. In reality, however, each cell has not only length and breadth, but also thickness, and each cell is covered on all sides by a cell-wall which is composed of a lifeless substance known as cellulose. This wall is often so transparent that we can look through it and see the cell-body and nucleus within (Fig. 6). protoplasm cell-nucleus.... .cell wall The discovery of these minute bodies of which organs are composed was not made until about the middle of the last century (1838). With the rather imperfect microscopes then in use the two discoverers, Schleiden and Schwann, could see the cell-walls only, and they did not know, as we now know, that the most important part of the cell is not the lifeless wall of cellulose, but the living substance which is found inside the cell-wall, making up a large part of the cell-body and cell-nucleus. FIG. 6.- Plant cell. The spaces in the cell-body are filled with cell-sap. |