Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 15

1. In sunlight leaves make starch.—Expts. 5, 7, and 8 (Sec. 6) have already proved (a) that leaves of a plant growing in ordinary air and exposed to the sunlight make starch; (b) that in the dark this starch somehow disappears; (c) that in air destitute of carbon dioxide leaves are unable to make starch even in sunlight.

2. The parts of a leaf which are not exposed to light do not make starch.—Keep a plant, say of tropæolum—or, if not convenient, a single leaf (Fig. 31)—in the dark for 24 hours to free the leaves from starch. Split a small cork and pin the halves on opposite sides of a leaf, and then expose the plant to bright sunlight for an hour or two. (If a single leaf is used let the end of the stalk dip into water.) Take off the cork, kill the leaf with boiling water, dissolve out the green colouring matter with methylated spirit, rinse, and test with iodine solution. The part from which the light was excluded remains bleached, and therefore contains no starch; while the rest of the leaf becomes blue or purplish brown owing to the presence of starch.

3. Parts of a leaf which are not green do not form starch in sunlight.—Take a variegated leaf from a plant (e.g. the variegated geranium or maple) which has been in bright sunlight for some hours. Apply the usual test for starch. The parts which were originally green contain starch; the originally white parts remain bleached.

4. Leaves supplied with carbon dioxide, and exposed to sunlight, give off oxygen gas.—(a) Take a bunch of fresh watercress or any green water-weed and put it in a beaker or glass jar. Cover the plant with an inverted funnel which is shorter than the beaker. Now fill the beaker with ordinary tap water or river water (not distilled water), so that the end of the neck of the funnel is covered. Completely fill a narrow test tube with water, close it with the thumb, and invert it over the neck of the funnel. If this has been done carefully the test tube will still be full of water. Expose the arrangement (Fig. 29) to bright sunlight, and notice the bubbles of gas which are given off from the plant and collect at the top of the tube. When a few inches of gas have collected, raise the test tube, close it with the thumb whilst still under water, and hold it mouth upwards. In the meantime, light a splinter of wood with the other hand. When it is well burning, blow out the light, remove the thumb from the test tube, and plunge the glowing splinter into the gas. It bursts into flame again, showing that the gas is oxygen.

Fig. 29.—Experiment to prove that green leaves supplied with carbon dioxide, and exposed to sunlight, give off oxygen gas.

Fig. 30.—Experiment to prove that tap-water, or river-water, contains dissolved carbon dioxide.

(b) Repeat the experiment, (i) placing the apparatus in the dark, (ii) without using any plant. No gas collects in the test tube.

(c) To show that the water used contains carbon dioxide in solution, completely fill a gallon can or a large flask with similar water, and attach a cork and a delivery tube which has also been filled with water—dipping the end of the tube into a little clear lime-water (Fig. 30). Put the same quantity of lime-water into another vessel for comparison, and then heat the can. Gas is given off, and as it bubbles through the lime-water the liquid is gradually turned milky.

5. Leaves wither in sunlight unless supplied with water.—(a) Cut off a leafy twig and leave it exposed to sunlight for an hour or two; notice the change in the appearance of the leaves.

(b) Put a similar twig in the dark for the same length of time; again notice the leaves. Is the difference due to a difference in light or to one of heat? (c) To test this, keep, if possible, a similar twig in the dark in a warm place. Do the leaves wither as much as in (a)?

(d) Smear with vaseline the under surface of some of the leaves of such a twig and again expose to sunlight. Do the smeared leaves remain fresh longer than the others?

(e) Cut off the end of a twig with a sharp knife whilst it is under water, and leave it exposed to sunlight, dipping in water. The leaves remain fresh. How do you explain these differences?

6. In sunlight, leaves give off water.—Take a piece of cardboard about 4 in. square and make a small hole in the middle. Pass the end of a leafy twig through the hole and make up with wax any chinks between the twig and the card. Put the card on a tumbler containing water, so that the end of the twig dips under water; and invert on the card—covering the leafy end of the twig—a second tumbler which is clean and dry. Put the apparatus in the sunshine and notice the mistiness (or even visible drops of water) forming on the inside of the upper tumbler. Where does this moisture come from?

7. The skin of a leaf is perforated by little pores.—Dip a fresh laurel leaf into boiling water in a beaker or tumbler. Can you see bubbles of air escaping from the leaf? Are they to be seen on both surfaces of the leaf, or only on one? Which?

Examine both surfaces of box leaves with a strong lens, and try to see the little dots (pores) on the lower surface.

The student who has performed the experiments described in this section, and who has thought about the results obtained, cannot but have gained some insight into the main duties of leaves. The meaning of these results must now be discussed.

The formation of starch in leaves.—When the green leaves of a plant are exposed to sunlight in ordinary air—that is in air containing a certain proportion of carbon dioxide—the leaf forms starch in its interior, and the starch can be detected by applying the iodine test (p. 34). When part of a leaf is protected from the light, as by pinning the halves of a split cork on opposite sides of it (Fig. 31), no starch is formed in the shaded parts, but only in the regions which are exposed to the light. Further, if a variegated leaf is treated in the same way, starch can be detected only in those parts of the leaf which were originally green; the parts which were white are free from starch. It is plain that it is the green colouring matter which puts the energy of the sunlight at the disposal of the leaf and enables it to manufacture starch.

AB

Fig. 31.—A, Tropæolum leaf, on which have been pinned the halves of a split cork (C). (×½.)

B, the same leaf tested for starch with iodine solution, after exposure to sunlight for an hour. The part shielded from the light remains bleached; the rest of the leaf has turned blue.

At least three conditions are therefore necessary for the formation of starch in leaves: (1) the green colouring matter; (2) sunlight; (3) carbon dioxide.

Oxygen is liberated when leaves form starch.—Carbon dioxide gas, which has been seen to be indispensable for the manufacture of starch in leaves, consists of carbon, or charcoal, chemically united with the gas oxygen. The green-stuff of the interior of the leaf makes the starch by causing this carbon to combine with water which has come up from the roots, but it returns to the air the unnecessary oxygen. Water plants, the leaves of which are not directly exposed to the air, use carbon dioxide which the water has dissolved from the air. They also give off the surplus oxygen, after fixing the carbon. This is the explanation of the bubbles of gas which, in sunlight, are often seen rising from the plants in an aquarium. By such an arrangement as is described in Expt. 9, 4, this evolved gas can be collected, and proved to be oxygen.

Fig. 32.—Experiment to

prove that green leaves,

exposed to sunlight,

give off water.

The use of water to the leaves.—It is common knowledge that if a twig is allowed to become dry its leaves hang limply and wither; but if the twig is allowed to dip into water the leaves will keep fresh and crisp for a considerable time. This necessity for supplying the twig with water seems to indicate that leaves give off water, and that this is so may be proved by a few simple experiments. Two tumblers may be arranged as in Fig. 32: separated by a card through which passes the end of a leafy twig. The end of the twig dips into water in the lower tumbler. In order to prevent water vapour from passing from the lower tumbler to the upper, the chinks between the twig and the card are sealed with paraffin wax.

When this arrangement is placed in the sunlight, a dew soon collects on the inside of the inverted upper tumbler. This water must have been given off in the form of vapour from the leaves. That the loss of water from leaves is due rather to the light than to the heat of the sunshine may be shown by keeping leafy twigs in the dark. The leaves keep fresh much longer than when placed in the light, even if they are kept in as warm a place.

The pores of the leaf-surface.—An ordinary leaf remains fresh much longer if its lower surface is smeared with vaseline. The explanation of this lies in the fact that the waterproof skin of a leaf is perforated by a multitude of little pores, especially on the lower surface. In most leaves, indeed, the pores are confined to the lower surface. Smearing the surface with vaseline blocks up these pores and thus prevents the escape of water vapour from the interior of the leaf.

These little mouths (known as stomata)[6] open in the light and close in the dark. During the daytime, therefore, the air (containing its small proportion of carbon dioxide) has free access to the interior of the leaf through the stomata, and, on the other hand, any water which the leaf does not require can escape in the form of vapour. A leaf requires water not only because all its mineral food (p. 29) is brought to it dissolved in water, but also because water as well as carbon dioxide is required for the manufacture of the starch and other plant-foods.

How plant-food is distributed.—The water which comes up from the roots is distributed to the various parts of the leaf through the veins. These are therefore not only supports, which stretch out the soft leaf-stuff to the light and air, but they also form a very complete network of irrigating channels or water pipes. Further, the starch and other foods which a leaf makes are drained off into the stem through other minute channels which are bound up with the water pipes. The starch, for example, is changed into a kind of sugar which dissolves in water and drains away. From the stem, the food solutions are distributed to all the parts where growth is taking place.

EXERCISES ON CHAPTER III.

1. Make drawings of as many cases as possible of economy of leaf surface.

2. Grow various plants, e.g. Tropæolum, Geranium, Fuchsia, Mustard, etc., in the window, and notice the effect which the direction of the light has upon the positions of the leaves.

3. Smear with vaseline the lower surfaces of various growing leaves, and on the following day test the leaves for starch, comparing each with an unsmeared leaf from the same plant.

4. How is the transpiration of water from a green leaf effected and controlled? Discuss the uses of transpiration. (1897)

5. Put the same quantity of water into each of two similar test-tubes, and let the end of a leafy twig dip into one. Weigh the tubes, place them together in the sun for an hour, weigh again, and estimate roughly the weight of water lost by one square inch of leaf surface per hour. Compare various plants in this respect. Repeat the experiments, (a) in a moderate light, (b) in the dark.

6. Make a list of plants in which the leaves are so arranged as (a) to conduct rain-water towards the base of the main stem, (b) to cause rain-water to fall to the ground from the outside of the foliage. Try to discover whether the difference has any relation to the arrangement of the roots.

7. Under what conditions can plants use carbon dioxide as a source of food? Mention experimental and other proofs of the principal statements made. (1905)

8. What part of its food does a green plant obtain from the air? In what form and under what conditions is it taken in? (King’s Scholarship, 1905)