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out ample scope for its exercise. It is well, then, that we should study the vegetable creation with those sentiments of gratitude which a bounty so generous ought always to inspire.
Figures cannot express, nor imagination conceive, the immense number of individual plants with which the earth is clothed. Even of distinct species, there are known at present upwards of 120,000, and there is little doubt that many more will yet be discovered. They exhibit the most wonderful diversity of size and form. The mould which gathers on decaying bodies consists of vegetables too small to be individually distinguished by the naked eye; while some of the largest forest trees rise to the height of 200 feet, and the banyan covers so vast an area, that a considerable army may repose under its shade.
Plants perform a very important function in purifying the atmosphere. Into this reservoir there are constantly poured immense quantities of carbonic acid gas, produced by the burning of fire, and the breathing of animals. By these same processes oxygen is consumed, so that the air, were there no means of compensation, would soon become deficient in oxygen, and overcharged with carbonic acid. This is, in fact, what usually happens in a close, ill-ventilated room, in which people are crowded, or fires kept burning. The effects of such a vitiation of the whole atmosphere would be most disastrous. Oxygen is essential to the maintenance of life; and in proportion as the necessary supply of this substance in the atmosphere is diminished, the vital functions of animals go on languidly, and their existence becomes comfortless. Apart altogether from the noxious qualities of the carbonic acid which would take its place, the mere abstraction of the oxygen of the air would soon be followed by the death of every living thing. Now all these consequences are averted by the influence of vegetable respiration. The leaves of plants inhale carbonic acid from the air, decompose it, and appropriate the carbon, letting the oxygen of the acid go free. Thus a perfect equilibrium is maintained among the constituents of the
atmosphere. It is observable, however, that for this respiratory action, plants require sunlight. In the dark the elimination of oxygen ceases, and it is said (though some are of a different opinion) that a small quantity of carbonic acid is evolved. Be this as it may, there can be no reasonable doubt that we have here a constant circulation of benefits between the two great provinces of organized nature. The plant purifies the air which the animal has poisoned, and by the same process extracts from it its own carbonaceous food.
EVERY one has seen the little seed cast into the ground, and watched with eagerness the first appearance of the tender blade, which gradually developed into plant and flower. If we examine of what such a seed consists, we shall find that it contains, in most cases, provision for two grand purposes; first, for the safety of the germ or embryo, and, secondly, for the temporary support of the future plant. The germ, delicate and brittle beyond all other substances, is folded up within one or two leaves or lobes, called cotyledons. Either included in these, or immediately surrounding them, is a supply of nutritious matter to feed the little organism till it is able to draw nourishment from the earth in sufficient quantity for its demands.
When the seed is put into the ground, and there exposed to the influence of heat, air, and moisture, this dead nutritious matter begins to undergo certain chemical changes, whereby further heat is produced, and the living embryo soon puts forth its vital energy. From one part of it, called the radicle, a little rootlet, a (Fig. 6), pushes down into the soil; and another part, known as the plumule, sends out the sprout, c, which by and by appears above ground, and becomes gradually larger and larger, till it attains the dimensions and form of a full grown plant. The cotyledons, bb, also sometimes rise above ground, and, assuming a
green colour, perform the functions of leaves, till the ordinary leaves are developed; this is well seen in the lupin and turnip. In other cases, as in the bean and pea, they remain buried in the soil, the nourishment they contain being gradually absorbed, until they shrivel up, and finally disappear. In the same way the nutritive matter which surrounds the cotyledons is also absorbed.
Those plants which have seeds with two cotyledons are called dicotyledonous. They form by far the most numerous class, including all our ordinary timber trees, and an immense number
of smaller plants. The seeds of others have but one cotyledon, which may usually be seen wrapped round the little plant in the earlier stages of its growth. These are called monocotyledonous. To this class belong the grasses, including all the varieties of grain, so valuable to man as the staple portion of his vegetable food. Among trees, it is represented by the palm, and other natives of tropical regions. There is still another class of plants, which have no seeds properly so called, but are propagated by minute cellular bodies called spores or sporules. Among these are ferns, mosses, lichens, and sea-weed. Spores have no cotyledons, and hence the plants of this third class are called acotyledonous.
The phenomena of germination are essentially the same, though different in detail, in all these classes. Both seed and spore contain the rudiments of the plants which are to spring from them, both are alike endowed with a principle of vitality, and, in the case of both, that principle will lie dormant, unless circumstances favour its development. If we keep a seed without moisture, or bury it so deep in the
earth that air shall find no access to it, the little germ, whether it live or die, can never send forth sprouts. Heat is also necessary to its activity, but the temperature which different seeds require is far from uniform. Light, on the other hand, retards germination; hence seeds are sown at a moderate depth below the surface of the soil, that they may have the benefit of darkness, without being excluded from the action of the air.
It is remarkable how long some of the harder seeds may retain their vitality. The germinative powers, when not stimulated into action by the circumstances in which the sccd is placed, remain unimpaired for years, and sometimes even for centuries, ready to awaken into life at the first opportunity.
NUTRITIVE ORGANS OF PLANTS.
WHEN the little plant has consumed the nourishment stored up for it in the secd, it becomes dependent on such food as it is able to extract from the surrounding earth and air. For the purpose of gathering thence the materials suitable for its support, converting them into an available form, and finally assimilating them to its own substance, it is endowed with three separate sets of organs,-—the root, bb (Fig. 7), the stem, c, and the leaves, d d. The external appearance of these is so various, and, in their more usual forms, so well known, that a description is at once difficult and unnecessary. It may be well, however, to notice some less obvious, but extremely interesting particulars regarding them.
The root serves the double purpose of fixing the plant firmly in its place, and collecting nourishment from the soil. Its fibres accordingly shoot out in every direction, insinuating themselves, without injury, between stones and other obstacles, or winding round them till a less resisting medium is reached. If nourishment is not readily found
near the base of the stem, they extend themselves in whatever direction they receive most encouragement. Sometimes, on meeting with reservoirs of water, as in wells or drains, they spread and multiply with amazing rapidity.
The extremitics of the fibres are soft and porous, and are therefore called spongioles, a a (Fig. 7). These, like so many little mouths, absorb liquid matter from the soil, and convey it upwards to the stem, from which, again, it passes to the leaves, there to undergo the elaborating processes, by means of which it is to become suitable nourishment for the
plant. Hence it appears that no substance can be taken up by the roots of plants, except in a state of solution. This is an important fact to the farmer, whose endeavour it is to replenish the exhausted soil, by means of nianures, with that nutriment which the growth of crops has extracted from it. For it is clear that he must either supply the nutritious substances themselves in a soluble state, or some chemical agent which will act on the insoluble ingredients of the soil, and render them fit for the support of vegetable life.
The stems of plants are not always apparent; sometimes they are concealed underground, sometimes disguised in an