154. The Growth of Plants is a Chemical Process carried on in the Leaf by the Sunlight. - We have seen that the plant takes in water, carbonic acid, and ammonia; that it decomposes these substances; and that from their elements it elaborates all the organic compounds which enter into its own structure. The leaf is the laboratory in which this chemical process is conducted, and since it is only in sunshine or bright daylight that the work goes on, it is evident that the sunlight is the agency by which it is accomplished.

"The sun's rays, acting on the green parts of the leaf, give to them the power of absorbing water, carbonic acid, and ammonia, and of constructing from the materials thus obtained the woody fibre, starch, sugar, and other compounds of which the plant consists. We have analyzed the woody fibre, and we know that it is composed of charcoal and water. Nineteen ounces of wood contain nine ounces of charcoal and ten ounces of water. Moreover, the amount of charcoal required to make nineteen ounces of wood is contained in thirty-three ounces of carbonic acid. If, then, we add together thirty-three ounces of carbonic acid and ten ounces of water, and subtract from this sum twenty-four ounces of oxygen, we shall have just the composition of wood. This is what the sun's light accomplishes in the leaves of the plant. It decomposes the carbonic acid, and unites its carbon to the elements of water to form the wood." -Cooke.

What is here stated to be true of wood is equally true of other vegetable products. If in their production carbonic acid and water alone take part, we have such substances as woody fibre, starch, sugar, and gum; and these make up nine-tenths of all vegetable structures. If the ammonia is likewise employed in the process, we have nitrogenized products, like albumen and caseine.

The force which the sunbeam exerts in the decomposi

tion of carbonic acid in the leaf is very remarkable, since it overcomes the intense affinity of oxygen for carbon.

"In order to decompose carbonic acid in our laboratories, we are obliged to resort to the most powerful chemical agents, and to conduct the process in vessels composed of the most resisting materials, under all the violent manifestations of light and heat, and we then succeed in liberating the carbon only by shutting up the oxygen in a still stronger prison; but under the quiet influences of the sunbeam, and in that most delicate of all structures, a vegetable cell, the chains which unite together the two elements fall off, and, while the solid carbon is retained to build up the organic structure, the oxygen is allowed to return to its home in the atmosphere. There is not in the whole range of chemistry a process more wonderful than this. We return to it again and again, with ever increasing wonder and admiration, amazed at the apparent inefficiency of the means, and the stupendous magnitude of the result. When standing before a grand conflagration, witnessing the display of mighty energies there in action, and seeing the elements rushing into combination with a force which no human agency can withstand, does it seem as if any power could undo that work of destruction, and rebuild those beams and rafters which are disappearing in the flames? Yet in a few years they will be rebuilt. This mighty force will be overcome; not, however, as we might expect, amidst the convulsion of nature, or the clashing of the elements, but silently, in a delicate leaf waving in the sunshine."- COOKE.

155. Plants purify the Air for the Respiration of Animals. We have learned (144) that by the various forms of combustion going on in nature, eight thousand millions of pounds of oxygen are daily removed from the atmosphere; and that an immense amount of carbonic

acid is pouring back into the air as one of the chief products of this combustion. The atmosphere, then, is continually losing the element which is essential to the support of animal life, and receiving in its place a gas which, even when largely diluted with air, is a deadly poison to animals.

But we have also learned that during the daytime plants are constantly drinking in this carbonic acid, and giving out an equal bulk of pure oxygen. They are thus purifying the air for the respiration of animals. They breathe in what animals breathe out, and breathe out what animals breathe in; so that the air is kept in a fit state to sustain both forms of organic life.

An Oriental fable narrates that a nightingale and a rose-tree were once imprisoned in a cage of glass, and long lived together there, happy in each other's society. But at last the rose-tree withered and died; and the bird soon pined away, and perished with grief at the loss of its beloved companion. The fancy of the poet is in perfect harmony with the facts of science, and prettily illustrates the dependence of animals upon plants for the very breath of life.

156. Plants prepare Food for Animals. We have seen that the plant draws its food directly from the earth and the air. It has the power of forming organic compounds out of inorganic matter. The animal, on the other hand, has no such power. It can elaborate its tissues only from organic matter, which it receives readymade from the vegetable kingdom. It may get this matter either directly from the plant, as when it eats vegetable food, or, indirectly, as when it eats animal food; but in either case the origin is the same. By the process of digestion the organic compounds originally prepared by the plant are converted into bones, muscles, nerves, or whatever else enters into the struc

ture of the animal. In this process they do not undergo any radical change, but merely "become parts of more finely organized tissues." We find in the blood albumen and caseine, having precisely the same composition as when prepared from potatoes, and the substance of the muscles does not differ essentially from the gluten of wheat-flour.

157. The Relations of Plants and Animals to each other and to the Air. - The plant, then, is a producer of organic materials; the animal, a consumer of these materials. "While the plant is a true apparatus of reduction, the animal is a true apparatus of combustion, in which the substances it derived from the vegetable are burnt, and restored to the atmosphere in the form of carbonic acid, water, and ammonia, ready to be again absorbed by the plant and to repass through the phases of organic life. Our bodies are furnaces, furnaces continually burning, whose fuel is our own flesh, and the smoke of whose fires is the food of the plant.

"When the foundations of the globe were laid, there were collected in the atmosphere all the essential elements of organized beings. From this inexhaustible storehouse the plant absorbs water, carbonic acid, and ammonia, which were placed there for its use, and which have been made to serve as its nourishment and food. It is the special office of the plant to elaborate from these few mineral substances, and a small amount of earthy salts, all the materials of organized beings. The animal receives these crude materials already prepared, and builds with them its various tissues; but no sooner are the cell-walls finished, and the structure ready to discharge its vital functions, than it is consumed by almost the very act which gave it life. The carbonic acid, water, and ammonia are restored to the atmosphere, and the cycle is complete." - COOKE.

NATURAL VEGETABLE PRODUCTS.

158. Starch. Starch has already been mentioned as one of the most important products of vegetable growth. Its chemical composition is precisely the same as that of woody fibre, C12H20O10. It consists of round or oval granules, varying considerably in size in different vegetables. The largest are about th, and the smallest less than 30th of an inch in diameter. In the potato (Figure 24) they are considerably larger than in wheat

flour (Figure 25). These granules consist of concentric layers, formed one after another, about a nucleus or centre. Lines, indicating these layers, can sometimes be seen on the surface of the larger grains, as in those of potato starch in the figure. Since the grains from the same kind of plant are tolerably uniform in size and shape, while they vary much in different species, the microscope will show to what plant starch-grains belong. Adulterations of arrowroot, and of other starchy substances, may thus be detected.

Starch is heavier than water, and is insoluble in cold water, alcohol, or ether. If, however, it be placed in water at 150°, the grains swell up and burst, forming a paste or jelly. When long boiled in water, starch is converted into the soluble dextrine (162).

The test for starch is iodine, with which it forms a compound of a deep blue color (60). Bromine gives starch a brilliant orange tint.