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 seed 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 seed, 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

FIG. 7.

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 extremities 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 This is an im

plants, except in a state of solution.

portant fact to the farmer, whose endeavour it is to replenish the exhausted soil, by means of manures, 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

FIG. 8.

extraordinary form. They are distinguished from roots by the production of leaf-buds. Some are slender, and creep along the ground, as in strawberries. On the other hand, the bulbs of the onion and lily are reckoned stems; and even the potato, whose leaf-buds are familiarly called eyes, belongs rather to the stem than to the root. It is, in fact, a kind of subterranean branch. Stems are divided, according to their mode of growth, into three classes, corresponding exactly to the three classes of plants which are determined by the structure of the seed. In our forest trees, and all those plants already described as dicotyledonous, the stem increases outwards, a new layer of woody matter being deposited every year immediately below the bark. When the trunk of a tree is sawn across, these layers are distinctly visible, and by their number the age of the tree can be exactly ascertained.

Such stems are called exogenous. The stems of monocotyledonous plants, on the other hand, increase inwardly, and are therefore called endogenous. In their case, the oldest and hardest part of the stem is the outside; they have no separable bark, and, generally speaking, no branches. Similar to them in these last respects are the stems of acotyledonous plants, which, altering little in thickness after being once formed, and increasing chiefly by additions to the summit, have received the name of acrogenous.

The leaves of plants play a very important part in carrying out the process of nutrition. Like the stem, they are of various forms, and (what is remarkable enough) a classification of plants according to these forms would nearly coincide with the two coincident classifications already referred to. In most exogenous plants, the ribs and veins of the leaves cross each other, so as to form a sort of net

work, while in endogenous plants the veins are more or less parallel, and a variety of this latter form is characteristic of acrogenous plants. Leaves, as well as roots, are provided with little mouths; not, however, at their extremities merely, but over their whole surface. By means of these they extract nourishment from the air, and also exhale moisture, sometimes in such quantities as to affect the climate of a country. Hence the destruction of a forest not unfrequently makes a climate drier than it was before. The value of leaves in purifying the atmosphere, by the absorption and decomposition of carbonic acid gas, has been already noticed. But there is still another function of great importance which these organs perform. The fluids imbibed by the roots are conveyed through the stem to the leaves, in whose living laboratory, acting under the stimulating influence of air and light, they are transformed into those organic compounds on which the plant feeds. Thus prepared, they are then distributed over every part of the organism.

How can we sufficiently admire the wisdom which has endowed a structure apparently so simple, and withal so beautiful, with powers as various and wonderful as they are essential to the welfare of the plant! We examine the organs, and we see the result of their action, but who can tell the real nature of those processes by which the ends of their existence are attained? It was an empty boast of the proud naturalist, which he ordered to be inscribed on his statue-"A genius equal to the majesty of nature;" for it has been well remarked, that a single blade of grass was sufficient to confound his pretensions.

QUESTIONS FOR EXAMINATION.

In what respects are plants useful to man? What purpose is served by their beauty? What lesson should we learn from it? How many species of plants are known? What effect does the breathing of animals produce on the surrounding air? How is this effect counteracted? What would happen if it were not counteracted? Of what parts do seeds usually consist? What is the use of the cotyledons, and what becomes of them as the plant grows? From which part of the seed does the root of the plant proceed? Which part sprouts upwards? Explain the words dicotyledonous, monocotyledonous, and acotyledonous, with examples. Name some plants which are propagated by spores.

What is the influence on germination of moisture? of air? of heat? of light? What are the organs of nutrition in plants? What are the uses of the root? How does it accomodate itself to the soil? Which parts of it imbibe nourishment? In what form must such nourishment be? What is the use of manure? Give examples of peculiarly shaped stems. Classify stems according to their mode of growth. What kind of leaves usually grow on exogenous stems? on endogenous stems? What is the effect of leaves on climate? What purposes do they serve in nourishing the plant?

EXCHANGE.

In the earlier stages of society, exchanges were effected by direct giving and taking of commodity for commodity, or, as it is termed, barter; but great and serious difficulties attended this system, difficulties ever more deeply felt as exchanges multiply and become more various: the baker may not want the shoemaker's shoes, if the latter want his bread; but the latter may not want as much bread as equals the value of a pair of shoes, and payment by a half or a third of a pair of shoes is impossible. A medium of exchange, accordingly, is introduced; usually, the precious metals. Exchange, thus facilitated by the adoption of a medium which all are ready to receive, and by which most minute proportions of value may be easily represented, proceeds with vastly increased rapidity.

Exchanges becoming thus continually more frequent and complicated, it is found convenient and advantageous, on the principle of the division of labour, that a class of men should devote themselves to conduct the business of exchange solely, the work of production being left to others. By the introduction of merchants, who do not themselves produce, a greater amount of production is attained, on the whole, than would be possible if all both produced and exchanged without their intervention.

But for facility and frequency of exchange, even at home, rapidity, and ease, and safety of communication are indispensable; good roads, swift conveyances, canals, and ultimately railways arise, with their adjuncts of carriers and couriers, and post-establishments, and telegraphs of even greater ingenuity and efficiency.