Читать книгу A Civic Biology, Presented in Problems - George W. Hunter - Страница 14

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Problem.—Where, when, and how do green plants make food?

(a) How and why is moisture given off from leaves?

(b) What is the reaction of leaves to light?

(c) What is made in green leaves in the sunlight?

(d) What by-products are given off in the above process?

(e) Other functions of leaves.

Laboratory Suggestions

Demonstration.—Water given off by plant in sunlight. Loss of weight due to transpiration measured.

Laboratory exercise.—

(a) Gross structure of a leaf.

(b) Study of stoma and lower epidermis under microscope.

(c) Study of cross section to show cells and air spaces.

Demonstration.—Reaction of leaves to light.

Demonstration.—Light necessary to starch making.

Demonstration.—Air necessary to starch making.

Demonstration.—Oxygen a by-product of starch making.

Apple twigs split to show the course of colored water up the stem.

What becomes of the Water taken in by the Roots?—We have seen that more than pure water has been absorbed through the root hairs into the roots. What becomes of this water and the other substances that have been absorbed? This question may be partly answered by the following experiments.

Passage of Fluids up the Stem.—If any young growing shoots (young seedlings of corn or pea, or the older stems of garden balsam, touch-me-not, or sunflower) are placed in red ink (eosin), and left in the sun for a few hours, the red ink will be found to have passed up the stem. If such stems were examined carefully, it would be seen that the colored fluid is confined to collections of woody tubes immediately under the inner bark. Water evidently rises in that part of the stem we call the wood.

Experiment to prove that water is given off through the leaves of a green plant.

Water given off by Evaporation from Leaves.—Take some well-watered potted green plant, as a geranium or hydrangea, cover the pot with sheet rubber, fastening the rubber close to the stem of the plant. Next weigh the plant with the pot. Then cover it with a tall bell jar and place the apparatus in the sun. In a few minutes drops of moisture are seen to gather on the inside of the jar. If we now weigh the potted plant, we find it weighs less than before. Obviously the loss comes from the water lost, and evidently this water escapes as vapor from either the stem or leaves.

The skeleton of a leaf. M.R., the midrib; P., the leafstalk; V., the veins.

The Structure of a Leaf.—In the experiment with the red ink mentioned above we will find that the fluid has gone out into the skeleton or framework of the leaf. Let us now examine a leaf more carefully. It shows usually (1) a flat, broad blade, which may take almost any conceivable shape; (2) a stem which spreads out in the blade (3) in a number of veins.

Section through the blade of a leaf as seen under the compound microscope. S, air spaces, which communicate with in the outside air; V, vein cross section; S.T., breathing hole (stoma); E, outer layer of cells; P, green cells.

The Cell Structure of a Leaf.—The under surface of a leaf seen under the microscope usually shows numbers of tiny oval openings. These are called stomata (singular stoma). Two cells, usually kidney-shaped, are found, one on each side of the opening. These are the guard cells. By change in shape of these cells the opening of the stoma is made larger or smaller. Larger irregular cells form the epidermis, or outer covering of the leaf. Study of the leaf in cross section shows that these stomata open directly into air chambers which penetrate between and around the loosely arranged cells composing the underpart of the leaf. The upper surface of leaves sometimes contains stomata, but more often they are lacking. The under surface of an oak leaf of ordinary size contains about 2,000,000 stomata. Under the upper epidermis is a layer of green cells closely packed together (called collectively the palisade layer). These cells are more or less columnar in shape. Under these are several rows of rather loosely placed cells just mentioned. These are called collectively the spongy tissue. If we happen to have a section cut through a vein, we find this composed of a number of tubes made up of, and strengthened by, thick-walled cells. The veins are evidently a continuation of the tubes of the stem out into the blade of the leaf.

Evaporation of Water.—During the day an enormous amount of water is taken up by the roots and passed out through the leaves. So great is this excess at times that a small grass plant on a summer's day evaporates more than its own weight in water. This would make nearly half a ton of water delivered to the air during twenty-four hours by a grass plot twenty-five by one hundred feet, the size of the average city lot. According to Ward, an oak tree may pass off two hundred and twenty-six times its own weight in water during the season from June to October.

From which Surface of the Leaf is Water Lost?—In order to find out whether water is passed out from any particular part of the leaf, we may remove two leaves of the same size and weight from some large-leaved plant[14]—a mullein was used for the illustrations given below—and cover the upper surface of one leaf and the lower surface of the other with vaseline. The leaf stalks of each should be covered with wax or vaseline, and the two leaves exactly balanced on the pans of a balance which has previously been placed in a warm and sunny place. Within an hour the leaf which has the upper surface covered with vaseline will show a loss of weight. Examination of the surface of a mullein leaf shows us that the lower surface of the leaf is provided with stomata. It is through these organs, then, that water is passed out from the tissues of the leaf.