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.