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granates, of Roses and Black Currants, and the like, may, therefore be set down to pure invention.
It is, nevertheless, apparently true, that bigeners, that is to say, mules between different genera, have in some few cases been artificially obtained. Kölreuter obtained such between Malvaceous plants; Gærtner, between Daturas and Henbane and Tobacco; Wiegman, between a Garden Bean and a Lentil; and there are other well-attested cases. But all such productions were as short-lived and sickly as they were monstrous.
As this power of creating mule plants fertile for two or three generations incontestably exists, it is not to be wondered at, that in wild nature hybrid varieties should be far from uncommon. Among the most remarkable cases are, the Cistus Ledon, constantly produced between C. monspessulanus and laurifolius; and Cistus longifolius, between C. monspessulanus and populifolius; in the wood of Fontfroide, near Narbonne, mentioned by Bentham. The same acute botanist ascertained that Saxifraga luteopurpurea of Lapeyrouse, and S. ambigua of De Candolle, are only wild accidental hybrids between S. aretioides and calyciflora : they are only found where the two parents grow together; but there they form a suite of intermediate states between the two. Gentians, having a similar origin, have also been remarked upon the mountains of Europe; and altogether about forty cases of wild reputed species of the genera Ranunculus, Anemone, Hypericum, Scleranthus, Drosera, Potentilla, Geum, Medicago, Galium, Centaurea, Stachys, Rhinanthus, Digitalis, Verbascum, Gentiana, Mentha, Quercus, Salix, and Narcissus, have been collected by Schiede, Lasch, and De Candolle ; to which far too many may be added from the works of species-making botanists. It is impossible not to believe that a great proportion of the reputed species of Rosa, Rubus, and other intricate genera, have had a hybrid origin.
In a practical point of view, I am inclined to believe that the power of obtaining mule varieties by art is one of the most important means that man possesses of modifying the works of nature, and of rendering them better adapted to his purposes. In our gardens some of the most beautiful flowers have such an origin; as, for instance, the roses obtained between R. indica and moschata, the different mule Potentillæ and Cacti, the splendid Azaleas raised between A. pontica and A. nudiflora coccinea, and the magnificent American-Indian Rhododendrons. By crossing varieties of the same species, the races of fruits and of culinary vegetables have been brought to a state as nearly approaching perfection as we can suppose possible. And if similar improvements have not taken place in a more important department, namely, the trees that afford us timber, experience fully warrants the belief that, if proper means were adopted, improved varieties of as much consequence might be introduced into our forests, as have already been created for our gardens.
It is, however, to be regretted that those who occupy themselves with experiments of this kind do not confine them to woody or perennial plants which can be perpetuated by cuttings. Mule annuals have the great fault of perishing almost as soon as they are obtained, and they serve no other purpose than that of encumbering the records of science with accounts of so called species which, from their transitory existence, can never be re-examined. .
These, however, are considerations which belong to Horticulture rather than to Botany. The reader who would make himself acquainted with the practical bearing of the subject should study Mr. Herbert's work on Amaryllidaceæ, p. 335. to 380.
The cause of the frequent sterility of mule plants is at present unknown. Sometimes, indeed, a deficiency of pollen may be assigned; but in many cases there is no perceptible difference in the healthiness of structure of the fertilising organs of a mule plant and of its parents. I know of no person who has attempted to prove this by comparative anatomical observations, except Professor Henslow, of Cambridge; who, in an excellent paper upon a hybrid Digitalis, investigated anatomically the condition of the stamens and pistil, both of his hybrid and its two parents, with great care and skill. The result of his enquiry was, that no appreciable difference could be detected.
OF THE FRUIT.
The fruit is mechanically destined as a mere protection to the seed; it constitutes the principal part of the food, especially in winter, of birds and small animals; it is often more ornamental than the flowers themselves, and it contributes most materially to the necessities and luxuries of mankind. When ripe, it falls from the plant, and, borne down by its weight, lies on the ground at the foot of the individual that produced it: here its seeds vegetate, when it decays, and a crop of new individuals arises from the base of the old one. But, as plants produced in such a manner would soon choke and destroy each other, nature has provided a multitude of ways for their dispersion. Many are carried to distant spots by the animals which eat them : others, such as the samara, and the pappus of Compositæ, provided with a sort of wing, fly away upon the wind to seek a distant station; others scatter their seeds abroad by an explosion of the pericarp, caused by a sudden contraction of the tissue; many, falling upon the surface of streams, are carried along by the current; while others are dispersed by a variety of methods which it would be tedious to enumerate.
The fruit, during its growth, is supported at the expense of the sap generally: but most especially of that which had been previously accumulated for its maintenance. This is less apparent in perennial or ligneous plants than in annual ones, but is capable of demonstration in both. Knight has well observed, that in annual fruit-bearing plants, such as the Melon, if a fruit is allowed 10 form at a very early period of the life of the plant, as, for instance, in the axil of the third leaf, it rarely sets or arrives at maturity, but falls off soon after beginning to swell, from want of an accumulation of food for its support; while, if the same plant is not allowed to bear fruit until it has provided a considerable supply of food, as will be the case after the leaves are fully formed, and have been some little time in action, the fruit which may then set swells rapidly, and speedily arrives at the highest degree of perfection of which it may be susceptible. And in woody trees, also, a similar phenomenon is observable: it is well known to gardeners, that, if a season occurs in which trees in a state of maturity are prevented bearing their usual crops, the succeeding year their fruit is unusually fine and abundant; owing to their having a whole year's extra stock of accumulated sap to feed upon.
The cause of the fruit attracting food from surrounding parts is probably to be sought in the phenomenon called endosmose. All the sap that may be at first impelled into the fruit by the action of vegetation, not being able to find an exit, collects within the fruit, and, in consequence of evaporation, becomes gradually more dense than that in the surrounding tissue: it will then begin to attract to itself all the more aqueous fluid that is in communication with it ; and the impulse, once given in this way to the concentration of the sap in particular points, will continue until the growth of the fruit is completed, and its tissue so much gorged as to be incapable of receiving any more food, when it usually falls off.
No one has studied the effects of fruit upon the atmosphere, and the nature of the chemical changes it undergoes, with more success than Théodore de Saussure and Bérard, an account of whose discoveries I partly translate and partly condense from De Candolle. According to the first of these original observers, “ Fruits, while green, whether leafy or fleshy, act much as leaves either in the sun or in shade, and differ from those organs principally in the intensity of their action. In the night they destroy the oxygen of their atmosphere, and replace it with carbonic acid, which they partially absorb again. This absorption is generally less in the open air than under a receiver ; and, their volume remaining the same, they consume more oxygen in darkness when distant from ripeness, than when they are approaching that state. If exposed to the sun, they disengage altogether or in part the oxygen which they inhaled during the night, and preserve no trace of this acid in their own atmosphere. If many fruits are detached from the plant, they thus add oxygen to air which contains no carbonic acid. When their vegetation is very feeble, or extremely languid, they vitiate the air under all circumstances, but less in the sun than in the shade. Green fruits detached from a plant, and exposed successively to the action of the sun and of darkness, change it but little or not at all either in purity or in volume. The trifling variations that may be remarked in this respect depend either upon the greater or less faculty which they have of elaborating carbonic acid, or on their composition, which is modified according to the degree of their ripeness. Thus Grapes, in a state of verjuice, appear to assimilate in small quantity the oxygen of the carbonic acid which they form in the air where they vegetate both day and night; while, on the contrary, Grapes nearly ripe give back almost entirely, during the day, to their own atmosphere, the oxygen of the carbonic acid they have formed in darkness. If there is no deception in this circumstance, which, although feeble, appears to have been constant, it marks the passage from the acid to the sweet state, by indicating that the acidity of verjuice depends upon the fixing of the oxygen of the air, and that this acidity disappears when the fruit no longer seeks for carbon in the air or in carbonic acid. Green fruits decompose, either entirely or in part, not only the carbonic acid they have produced during the night, but, in addition, such quantity as may be artificially added to their atmosphere. When this last experiment is tried with fruits which are not watery, and which, like Apples and Grapes, elaborate carbonic acid slowly, one sees that they absorb in the sun a much larger proportion of gas than the same volume of water in a similar mixture; afterwards they disengage the oxygen of the carbonic acid absorbed, and thus appear to elaborate it in their interior.