Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 11

1. Plants contain much carbon.—Char a stick, and notice the black charcoal which is formed. Charcoal is an impure form of carbon.

2. Carbon dioxide gas is formed when wood burns.—Fasten a shaving or a splinter of wood on a piece of wire, light it, and lower it into a clean glass jar. When the wood has burned for a few seconds take it out, and pour a little clear lime-water into the jar. The lime-water turns milky. Similarly, pour a little lime-water into a jar in which nothing has been burning, and notice that it remains clear. There is evidently a difference in the nature of the air of the two jars. The difference is caused by the burning of the wood, during which some of the carbon unites with the oxygen of the air in the jar, forming an invisible gas, called carbon dioxide. Carbon dioxide can always be detected by the milkiness it causes in clear lime-water.

3. Carbon dioxide present in ordinary air.—Pour some clear lime-water into a blue saucer and let it stand exposed to the air for half an hour, then examine it. A white scum has formed on the surface of the lime-water. Stir with a glass rod; the solution becomes milky. The scum and the milkiness are produced by the union of the lime with carbon dioxide from the air. Carbon (in the form of carbon dioxide) is therefore present in the air.

4. No carbon in the food solution.—Examine again the list of elements (p. 27), which compose the mineral salts which have been found to replace satisfactorily the food which a plant obtains from the soil. There is no carbon in it. A plant evidently does not depend on the soil for the carbonaceous part of its food. From what other source can a plant obtain its carbon? Carbon has just been proved to be present in the air. Does the plant obtain its carbon from the air?

5. Green leaves contain starch after exposure to sunlight.—Take a green leaf from a plant which has been exposed to the sunlight, boil the leaf in water for a minute or two to kill it. Then put it in methylated spirit until the leaf-green is dissolved out. When the leaf is bleached rinse it in water, then put it into a dilute solution of iodine (p. 2) and notice that it becomes blue or purplish brown. The formation of this colour proves the presence of starch in the leaf.

6. Starch contains carbon.—Char a piece of laundry starch and observe the charcoal formed.

7. Green leaves do not contain starch after being left in the dark for 24 hours.—Keep a leafy plant in the dark for 24 hours and then test a leaf as in the previous experiment. No starch can be detected. Put the plant in the sunlight for an hour or two and test another leaf. It contains starch. Plainly, starch is only formed in leaves if they are exposed to light, and any starch previously present disappears when the plant is kept in the dark.

8. A green plant kept in air from which the carbon dioxide has been removed will not form starch in its leaves.—Obtain a large glass bottle, such as those used by confectioners, and fit it with a cork or india-rubber stopper through which passes a glass tube bent[5] as in Fig. 21. Care should be taken to make all the joints tight, and it may be necessary to soak the cork in melted paraffin to ensure this. Pack the bend of the tube loosely with pieces of soda lime, and in the bottle place a small jar containing lumps or a strong solution of caustic soda. When the apparatus is ready, place in the bottle a small plant or a leafy twig in water (fuchsia answers very well), which has been kept in the dark for 24 hours. The caustic soda in the jar very soon absorbs all the carbon dioxide which is present, and the soda lime in the bend of the tube prevents any carbon dioxide from getting into the bottle from the outside air. Place the jar in bright sunlight for a few hours and then test a leaf for starch. None can be detected. It is plain that one of the carbonaceous food stuffs—starch—is not formed in the leaves of plants unless the plant is grown (in the light) in air containing carbon dioxide.

9. Seedlings are at first independent of light.—Germinate pea or bean seeds in wet sand or sawdust in the dark. Notice that for some time the seedlings grow almost as well as when in the light. Soon, however, the stem becomes long, weak and straggling, and the leaves are pale in colour, even if the plant is supplied with mineral food.

10. The formation of sugar in a germinating pea-seed.—Take up a pea-seedling when the stem is one or two inches long, and chew the partly shrunken cotyledons. Notice the slightly sweet taste. Contrast this with the taste of an ungerminated seed.

A plant contains much carbon.—When a piece of wood or other part of a plant is strongly heated it first blackens or chars, showing the presence of a large proportion of charcoal or impure carbon. On continued heating, this carbon “burns away.” In the process of burning it unites with some of the oxygen of the air, and forms a colourless, invisible gas known as carbon dioxide. Though this substance cannot be seen, its presence can be easily detected by means of clear lime-water, which, when exposed to the gas, absorbs it and becomes milky owing to the formation of a white precipitate of chalk. If, for example, a splinter of wood is burnt in a glass jar, and a little clear lime-water is immediately afterwards poured into the jar and shaken up, a milkiness at once proclaims the presence of carbon dioxide.

The air contains carbon.—If lime-water is poured into a blue saucer and left exposed to the air for half an hour, a white scum of chalk is seen to have formed on its surface, showing that carbon dioxide is present in the atmosphere. It is important that the student should realise this presence of carbon—as invisible carbon dioxide gas—in the air. Although the proportion is very small, amounting to only 3 parts of carbon dioxide in 10,000 parts of fresh country air, it is of incalculable importance to plants, and indirectly to ourselves and all other animals.

A green plant obtains its carbon from the air.—Since the parts of a plant contain much carbon, and the food which a plant obtains from the soil need not contain any carbon; while the air, on the other hand, does contain carbon, it seems likely that a plant obtains its carbonaceous food from the air. This surmise is confirmed by experiments. One of the most easily recognisable of plant products containing carbon is starch, for it yields a very characteristic blue, or purplish-brown, colour when treated with iodine solution. By means of this test starch can easily be proved to be present in the green leaves of a plant which has been exposed to the air and sunlight. The leaf is first killed by being boiled in water for a minute or two, and then its green colouring matter is dissolved out by immersion in alcohol (methylated spirit). The bleached leaf is rinsed in water and then put in iodine solution, and the blue or purplish-brown colour which is formed shows the presence of starch. There is a marked difference when a leaf, which has been kept in the dark for twenty-four hours, is similarly tested. In this case no starch can be detected.

One compound of carbon—i.e. starch—may thus be recognised easily; and if we found that a leaf made no starch when supplied only with air from which the carbon dioxide had been removed, this fact would be strong evidence in favour of the conclusion that a green plant obtains its carbon from the carbon dioxide of the air. To test this, a large bottle is fitted, by means of a tightly fitting cork or stopper, with a tube containing lumps of soda lime, a substance which eagerly absorbs carbon dioxide from air. A small jar of caustic soda is placed inside the bottle (Fig. 21). A green plant or a leafy twig, which has been kept for twenty-four hours in the dark to free it from starch, is then put in the bottle, and the whole exposed to sunlight for a few hours. At the end of this time it is found, on testing the leaves, that no starch has been formed. By this, and other experiments, botanists have proved that green plants obtain all their carbon from the carbon dioxide of the air, and that sunlight is indispensable for the process. We shall examine this question more fully when we study leaves (Chapter III.).

Fig. 21.—Experiment to prove that leaves do not make starch

unless the air with which they are supplied contains carbon

dioxide. S, tube containing lumps of soda lime; S′, jar containing a solution of caustic soda.

The carbonaceous food of a young seedling.—Just as a bean or pea seedling is for a time independent of an outside supply of mineral food—its roots needing only to be supplied with water—so there is enough carbonaceous food also stored up in the seed to satisfy for some time the needs of the growing plant—the stored starch being gradually changed into sugar and absorbed. For this reason a young seedling will live healthily in the dark. When, however, the seed food is exhausted, or nearly so, the plant draws upon the store of carbon, which is present as carbon dioxide in the air, for a renewal of the starch and allied substances which are necessary to it. As it cannot make use of this atmospheric carbon in the dark, it must henceforth be supplied with sunlight or it will not thrive. Plants kept in the dark after their seed-food is exhausted are pale in colour and unhealthy. Their stems grow long and straggling (Fig. 22), but are usually too weak to stand upright.

EXERCISES ON CHAPTER II.

1. Make experiments to discover the effects, upon seedlings growing in a nutritive solution (p. 27), of each of the following modifications in the composition of the solution: (a) omit the sodium chloride; (b) omit the potassium nitrate; (c) omit the compound of iron; (d) omit the magnesium sulphate; (e) substitute sodium nitrate for potassium nitrate.

2. Explain how it is that a green plant cannot carry on its nutrition in darkness.(1892)

3. What part of its food does a green plant obtain from the air? In what form, and under what conditions, is it taken in? (1889)

4. Describe the method of water cultures, and give the general results of a set of experiments. (1898)

5. Give experimental proof that green plants require to be fed with combined nitrogen. (1897)

6. What are the necessary conditions for the formation of starch in a plant? Mention experiments which support your statements. (1896)

7. Explain the influence of light on a growing plant. Illustrate your answer by reference to the changes in a ripening and germinating bean. (King’s Schol. 1903)

8. How can it be proved experimentally that a green plant draws some, but not the whole, of its nourishment from the air? (1904)

Fig. 22.—Two mustard seedlings of equal age.

E, grown in the dark; N, grown in ordinary daylight.