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AGE AT WHICH PLANTS CAN BEAR FRUIT.

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in biennials a longer period is required before this condition is arrived at; and in shrubs and trees a still greater age must be acquired. The American Aloe will not flower before it is thirty years old, under the most favourable conditions; and, under unfavourable circumstances, the age at which it fructifies is so much increased as to have given rise to the vulgar belief that it flowers only after a hundred years. This curious subject has been little investigated, and we have no comparative statements of the ages at which different species begin to bear; but the fact is certain. It is often, however, in the power of man to advance or retard these periods artificially. Whatever produces excessive vigour in plants is favourable to the formation of leaf-buds, and unfavourable to the production of flower-buds; while, on the other hand, such circumstances as tend to diminish luxuriance, and to check rapid vegetation, without affecting the health of the individual, are more favourable to the production of flower-buds than of leaf-buds. Thus, a plant in a sterile soil and exposed situation flowers sooner and more abundantly than one in a rich and shaded place; young vigorous plants flower later and less abundantly than old ones. In India and China fruit-trees are made to bear by cutting their roots, or exposing them periodically to dryness; and in this country the same practice is observed, especially with the Fig tree. An apparent exception to this law is found in the fact that a seedling fruit-tree may be made, by grafting upon an old stock, to bear flowers at an earlier age than it otherwise would have done; for the effect of grafting it thus is certainly not to render it less vigorous, but the contrary. But it is probable that all these facts arise out of one common law, which is, that the period when a plant begins to flower depends upon the presence in its system of a sufficient quantity of secreted matter fit for the maintenance of the flowers when produced. Under ordinary circumstances, a considerable part of all the nutritious secretions elaborated by the leaves are expended in the production of new leaves; but after a time, a greater supply is formed than the leaves require, and the residue collects in the system; as soon as this residue has arrived at the necessary amount, flowers may begin to form. If the sterile branch

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AGE AT WHICH PLANTS CAN BEAR FRUIT.

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of a tree is ringed, it ceases to be sterile; and this can only be accounted for upon the supposition that the secreted matter of the branch, instead of being conveyed away into the trunk and roots, is stopped by the annular incision, above which it is compelled to accumulate. If a tree that is unproductive be transplanted, it begins to bear; in this case the operation injures its roots, sap is therefore less abundantly supplied in the succeeding season to the leaves; the leaves are therefore less able to grow than they previously were, and they consequently do not consume the nutritious matter lying in the branches, and which they would have expended, had they been able to grow with their former vigour; hence the nutritious matter accumulates, and flower-buds are formed. In this country, if a fruittree has its crop destroyed one year, it bears the more abundantly the next; owing, no doubt, to the accumulation in its system of that nutritive matter which would not have been present there, had the crop which was destroyed been allowed to grow and the reverse of this is well known to be the fact; an excessive crop one year being followed by a scanty crop the succeeding year. So, when a young seedling fruit-tree is made to bear prematurely by grafting it upon an old stock, the effect of which will apparently not be to diminish its vigour, it may be conceived that, in the first place, the seedling will receive a considerable quantity of nutritive matter from the old stock, where it had been already collected, and that thus the supply will be greater than the consumption, however large the latter may be; and, secondly, that, at the time of union of itself with the stock, there will be sufficient interruption of continuity in the bark to oppose some obstacle to the descent from the seedling of whatever matter it may have received or

One of the effects of ringing has been observed to consist in the formation of numerous barren shoots below the wound, while fertile shoots appear above it. This is conformable to the theory of the formation of flowers being determined by a superabundance of nutritious matter in a given place. The bark below the annular excision is cut off from a supply of the sap elaborated by the leaves above it; and, at the same time, in consequence of the obstruction of the wound to the ascent of the crude sap, an unusual supply of the latter is forced towards the buds in the bark below the wound, which buds, being chiefly fed with crude sap, push forth into branches and leaves, but bear no flowers.

USES OF THE PARTS.

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formed. Hence, it is an axiom in vegetable physiology, that the production of flower-buds depends upon the presence of nutritive matter in sufficient abundance for their support.

The use of the calyx and corolla is too uncertain and unimportant to demand much notice. The calyx is usually regarded as a protecting organ, and the corolla as a part for the embellishment of the sexes. They neither appear to be of much physiological importance; more especially not the corolla, or it would not be absent in such large numbers of plants.

The use of the stamens is to effect the fertilisation of the young seed contained in the pistil. To this end, the pollen of the anther must be applied to the stigma, the result of which is, that an embryo, the rudiment of a future plant, is generated in the inside of the young seed, and, when mature, is capable of multiplying the species. It is, however, to be observed, that the seed, when ripe, will not renew the species from which it is derived, with all its individual peculiarities; the seed of a Green Gage Plum, for instance, will not, with any certainty, produce a plant having the sweet green fruit of that variety, but it may produce a Plum whose fruit is red and acid. All that the seed will certainly do is to produce a new individual of the Plum species: the peculiarities of individuals are perpetuated by other means, and especially by leaf-buds. (See Book II.)

It has been remarked that the freshness of flowers may be much prolonged by any circumstance which hinders the act of fertilisation. Orchidaceous plants in hothouses are an instance of this. In general, from the absence of insects, or of those other disturbing causes to which Orchidaceae are exposed in their native places, the pollen cannot come into contact with the stigma, and so long as this is prevented the flowers of many species will retain their freshness for weeks, as if in expectation of that event for which they were created. But as soon as the act of fecundation is accomplished, that is to say, from twelve to twentyfour hours after the pollen touches the stigma, the flowers collapse, the bright colours become dim, the ovary begins to enlarge, and the beauty of the flower is gone. The same fact has been noticed in the Night Flowering Cereus, whose flowers will retain their beauty during the day after blossoming, provided the stigma is removed.

If the pistil of one species be fertilised by the pollen of another species, which may take place in the same genus, or if

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HYBRIDS AND CROSSBREDS.

two distinct varieties of the same species be in like manner intermixed, the seed which results from the operation will be intermediate between its parents, partaking of the qualities of both father and mother. In the first case the progeny is hybrid, or mule; in the second it is simply crossbred.

In general, crossbreds are capable of producing fertile seed, and thus of perpetuating one of the species from which they sprang. Hybrids, on the contrary, are often sterile, and therefore incapable of yielding seed.

Reasoning from a few facts, and from the analogy of the higher orders in the animal kingdom, it has been believed that all vegetable hybrids are sterile; and, when sterility is not the consequence of the intermixture of two species, it has been thought that such species are not naturally distinct, however different their appearance. But facts prove that undoubted hybrids may be fertile; and when we consider that plants are not analogous to the higher orders of animals, but to the lowest, concerning whose habits we know little, it is obvious that no analogical inferences can be safely established.

CHAPTER VII.

OF THE MATURATION OF THE FRUIT.

CHANGES IT UNDERGOES.-IS FED BY BRANCHES UPON ORGANIZABLE MATTER FURNISHED BY LEAVES.-PHYSIOLOGICAL USE OF THE FRUIT.-NATURE OF SECRETIONS.-THE CHANGES THEY UNDERGO.-EFFECT OF HEAT.— OF SUNLIGHT.-OF WATER.-SEEDS.-ORIGIN OF THEIR FOOD.-CAUSE OF THEIR LONGEVITY. -OF THEIR DESTRUCTION.

THEIR VIGOUR.

DIFFERENCE IN

AFTER the fertilization of the seed has taken effect, the pistil by itself, or the pistil and surrounding parts, go on growing; alter their appearance, as well as size; acquire new qualities of colour, texture, flavour, &c.; and become the fruit.

A flower being a kind of branch, as has been already shown (see page 83), and the fruit being the advanced stage of a flower, it follows that a fruit is also a kind of branch. It has originally the same organic connexion with the plant as other branches, and like them requires to be supplied with food, in the absence of which it perishes or languishes.

It may be conceived that, as the fruit is an altered state of a leaf, its physiological action will resemble that of a leaf, in proportion as it retains its organic similitude; and this is found to happen, a fruit decomposing carbonic acid, &c., under the influence of light, so long as it retains its original green foliaceous character. In the Pea, for example, whose pod is green until it begins to die, the action is always similar to that of a leaf, but in the Peach, whose texture becomes pulpy and unlike that of a leaf, the physiological action eventually ceases to be that of the latter organ.

But although a fruit has, like a leaf, the power of forming secretions by elaborating the sap which is attracted into it, yet

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