size, and color, which it afforded. In 1787, a degenerate sort of pea was growing in his garden, which had not recovered its former vigor even when removed to a better soil. Being thus a good subject of experiment, the male organs of a dozen of its immature blossoms were destroyed, and the female organs left entire. When the blossoms had attained their mature state, the pollen of a very large and luxuriant grey pea was introduced into the one-half of them, but not into the other. The pods of both grew equally; but the seeds of the half that were unimpregnated withered away without having augmented beyond the size to which they had attained before the blossoms expanded. The seeds of the other half were augmented and matured as in the ordinary process of impregnation; and exhibited no perceptible difference from those of other plants of the same variety; perhaps because the external covering of the seed was furnished entirely by the female. But when they were made to vegetate in the succeeding spring, the effect of the experiment was obvious. The plants rose with great luxuriance, indicating in their stem, leaves, and fruit, the influence of this artificial impregnation; the seeds produced were of a dark grey. By impregnating the flowers of this variety with the pollen of others, the color was again changed, and new varieties obtained, superior in every respect to the original on which the experiment was first made, and attaining in some cases, to a height of more than twelve feet. (Phil. Trans. 1789.) Knight thinks his experiments on this subject afford examples of superfotation, a phenomenon, the existence of which has been admitted amongst animals, but of which the proof amongst vegetables is not yet quite satisfactory. Of one species of superfotation he has certainly produced examples; that is, when, by mpregnating a white pea-blossom with the pollen both of a white and grey pea, white and grey seeds were obtained. But of the other species of superfotation, in which one seed is supposed to be the joint issue of two males, the example is not quite satisfactory. Such a production is perhaps possible, and further experiments may probably ascertain the fact, but it seems to be a matter of mere curiosity, and not apparently connected with any views of utility.

1600. The practicability of improving the species, is rendered strikingly obvious by these experiments; and the ameliorating effect is the same, whether by the male or female; as was ascertained by impregnating the largest and most luxuriant plants with the pollen of the most diminutive and dwarfish, or the contrary. By such means any number of varieties may be obtained, according to the will of the experimenter, amongst which some will no doubt be suited to all soils and situations. Knight's experiments of this kind were extended also to wheat; but not with equal success. For though some very good varieties were obtained, yet they were found not to be permanent. But the success of his experiments on the apple-tree were equal to his Hopes. This was, indeed, his principal object, and no means of obtaining a successful issue were left untried. The plants which were obtained in this case were found to possess the good qualities of both of the varieties employed, uniting the greatest health and luxuriance with the finest and best-flavored fruit.

1601. Improved varieties of every fruit and esculent plant may be obtained by means of artificial impregnation, or crossing, as they were obtained in the cases already stated. Whence Knight thinks, that this promiscuous impregnation of species has been intended by nature to take place, and that it does in fact often take place, for the purpose of correcting such accidental varieties as arise from seed, and of confining them within narrower limits. All which is thought to be countenanced from the consideration of the variety of methods which nature employs to disperse the pollen, either by the clastic spring of the anthers, the aid of the winds, or the instrumentality of insects. But although he admits the existence of vegetable hybrids, that is, of varieties obtained from the intermixture of different species of the same genus, yet he does not admit the existence of vegetable mules, that is, of varicties obtained from the intermixture of the species of different genera; in attempting to obtain which he could never succeed, in spite of all his efforts. Hence he suspects that where such varieties have been supposed to take place, the former must have been mistaken for the latter. It may be said, indeed, that if the case exists in the animal kingdom, why not in the vegetable kingdom? to which it is, perhaps, difficult to give a satisfactory reply. But from the narrow limits within which this intercourse is in all cases circumscribed, it scarcely seems to have been the intention of nature that it should succeed even among animals. Salisbury is of a different opinion, and considers (Hort. Trans. i. 364.) that new species may be created both by bees and the agency of man; and the recent experiments of Herbert, Sweet, and others, seem to confirm this opinion. Sweet's experience leads him to conclude that the plants of all orders strictly natural may be reciprocally impregnated with success, and he has already, in the nursery-gardens of Messrs. Colville, produced many new geraniæ and rhoderaceæ.

1602. A singular or anomalous effect of crossing, or extraneous impregnation, is the change sometimes undergone by the seed or fruit which is produced by the blossom impregnated. These effects are not uniform results, but they are of frequent occurrence, and have attracted notice from a very early period. John Turner observes (Hort. Trans. v. 65.) that Theophrastus and Pliny (Theophrast. Hist. Plant. 1. ii. c. 4.; Plinii Hist. Nat. 1. xvii. c. 25.) seem to allude to it, and that the notion was entertained by Bradley, who, in his New Improvements in Planting and Gardening, after giving directions for fertilising the female flowers of the hazel with the pollen of the male, says, "By this knowledge we may alter the property and taste of any fruit, by impregnating the one with the farina of another of the same class, as, for example, a codlin with a pearmain, which will occasion the codlin so impregnated to last a longer time than usual, and be of a sharper taste; or if the winter fruit should be fecundated with the dust of the summer kinds, they will decay before their usual time; and it is from this accidental coupling of the farina of one kind with the other, that in an orchard, where there is a variety of apples, even the fruit gathered from the same tree differs in its flavor and times of ripening; and, moreover, the seeds of those apples so generated, being changed by that means from their natural qualities, will produce different kinds of fruit, if they are sown. Turner, after quoting several instances, and, among others, one from the Philosophical Transactions "concerning the effect which the farina of the blossoms of different sorts of apples had on the fruit of a neighboring tree," states upwards of six cases of hybridised apples, that had come within his own observation, and concludes with the remark, that if there does exist in fruits such a liability to change, it will at once be evident to the intelligent cultivator how much care is requisite in growing melous, cucumbers, &c. to secure their true characters, even without reference to saving seed for a future crop. In the same volume of the Horticultural Transactions (p. 234.), an account is given of different-colored peas being produced in the same pod by crossing the parent blossom. All these facts seem to contradict the generally received opinion, that crossing only affects the next generation; here it appears to affect the embryo offspring; and a gardener who had no keeping apples in his orchard, might communicate that quality in part to his summer fruit by borrowing the use of a neighbour's blossoms from a late variety. It is probable, however, that such counter-impregnations do not take place readily; otherwise the produce of a common orchard would be an ever-varying round of monstrosities.

SECT. VIII. Changes consequent upon Impregnation.

1603. The peculiar changes consequent upon impregnation, whether in the flowers or fruit, may be considered as external and internal.

1604. External changes. At the period of the impregnation of the ovary the flower has attained to its ultimate state of perfection, and displayed its utmost beauty of coloring and richness of perfume. But as it is now no longer wanted, so it is no longer provided for in the economy of vegetation. Its period of decline has cominenced; as is indicated, first by the decay of the stamens, then of the petals, and then of the calyx, which wither and shrink up, and finally detach themselves from the fruit altogether, except in some particular cases in which one or other of them becomes permanent and falls only with the fruit. The

stigma exhibits also similar symptoms of decay, and the style itself often perishes. The parts contiguous to the flower, such as the bractes and floral leaves, are sometimes also affected; and finally the whole plant, at least in the case of annuals, begins to exhibit indications of decay. But while the flower withers and falls, the ovary is advancing to perfection, swelling and augmenting in size, and receiving now all the nutriment by which the decayed parts were formerly supported. Its color begins to assume a deeper and richer tinge; its figure is also often altered, and new parts are even occasionally added-wings, crests, prickles, hooks, bloom, down. The common receptacle of the fruit undergoes also similar changes, becoming sometimes large and succulent, as in the fig and strawberry; and sometimes juiceless and indurated, as in compound flowers.

1605. Internal changes. If the ovary is cut open as soon as it is first discoverable in the flower, it presents to the eye merely a pulpy and homogeneous mass. But if it is allowed to remain till immediately before the period of its impregnation, it will now be found to be divisible into several distinct parts, exhibiting an apparatus of cells, valves, and membranes, constituting the pericarp, and sometimes the external coats of the seed. In this case the umbilical cord is also to be distinguished; but the embryo is not yet visible. These changes, therefore, are to be attributed merely to the operation of the ordinary laws of vegetable developement, and are not at all dependent upon impregnation. But impregnation has no sooner taken place than its influence begins to be visible; the umbilical cord, which was formerly short and distended, is now generally converted into a long and slender thread. Sometimes the position of the seed is altered. Before impregnation the seeds of caryophyllus aromaticus and netrosideros gummifera, are horizontal; after impregnation they become vertical. Before impregnation the magnolia seeds are erect; after impregnation they become inverted and pendulous. The figure of the seed is often also altered in passing from its young to its mature state; changing from smooth to angular, from tapering to oval, from oval to round, and from round to kidney-shaped. But all seeds are not brought to maturity, of which the rudiments may exist in the ovary. Lagacia and hasselquistia, produce uniformly the rudiments of two seeds, of which they mature but one. But the principal changes resulting from impregnation are operated in the seed itself, which, though previously a homogeneous and gelatinous mass, is now converted into an organised body, or embryo. Such are the phenomena, according to the description of Gartner, accompanying or following the impregnation of all flowers producing seeds; exceptions occur where the fecundation is spurious and incomplete; where the ovary swells, but exhibits no traces of perfect seed within, as often happens in the vine and tamus; or when barren and fertile seeds are intermingled together in the same ovary. This proceeds from some defect either in the quantity or quality of the pollen; but rather in the quality, as it is not always plants having the most pollen that produce the most seeds. The two stamens of the orchidæ fecundate 8000 seeds, and the five stamens of tobacco fecundate 900: while the 50 stamens of barringtonia, the 230 of thea, and the 80 of the caryophilli, fecundate only two or three ovaries

SECT. IX. The Propagation of the Species.

1606. As the life of the vegetable, like that of the animal, is limited to a definite period, and as a continued supply of vegetables is always wanted for the support of animals, what we call art, or nature operating by means of the animal man, has taken care to institute such means as shall secure the multiplying and perpetuating of the species in all possible cases.

1607. Equivocal generation. It was long a vulgar error, countenanced even by the philosophy of the times, that vegetables do often spring up from the accidental mixture of putrid water and earth, or other putrid substances, in the manner of what was called the equivocal generation of animals; or at the very least, that the earth contains the principle of vegetable life in itself, which, in order to develope, it is only necessary to expose to the action of the air. The former alternative of the error has been long ago refuted; the latter has lost its hold, having been also refuted by Malpighi, who proved that the earth produces no plant without the intervention of a seed, or, of some other species of vegetable germ deposited in it by nature or by art.

1608. Propagation by seeds. When the seed has reached maturity in the due and regular course of the developement of its several parts, it detaches itself sooner or later from the parent plant, either singly or along with its pericarp, and drops into the soil, where it again germinates and takes root, and springs up into a new individual. Such is the grand means instituted by nature for the replenishing and perpetuating of the vegetable kingdom.

1609. Dispersion of seed. If seeds were to fall into the soil merely by dropping down from the plant, then the great mass of them, instead of germinating and springing up into distinct plants, would grow up only to putrefy and decay; to prevent which consequence nature has adopted a variety of the most efficacious contrivances, all tending to the dispersion of the seed. The first means to be mentioned, is that of the elasticity of the pericarp of many fruits, by which it opens when ripe, with a sort of sudden spring, ejecting the seed with violence, and throwing it some considerable distance from the plant. This may be exemplified in a variety of cases; the seeds of oats when ripe are projected from the calyx with such violence, that in a fine and dry day you may even hear them thrown out with a slight and sudden snap in passing through a field that is ripe. The pericarp of the Dorsiferous Ferns (fig. 259 a.) is furnished with a sort of peculiar elastic ring (b), intended, as it would appear, for the very purpose of projecting the seeds. The capsules of the cucumber, geranium geum, and fraxinella, discharge their seeds also when ripe with an elastic jerk. But the pericarp of impatiens, which consists of one cell with five valves, exhibits perhaps one of the best examples of this mode of dispersion. If it is accidentally touched when ripe it will immediately burst open, while the valves, coiling themselves up in a spiral form, and springing from the stem, discharge the contained seeds, and scatter them all around. The bursting of the pericarp of some species of pines is also worthy of notice. The pericarp, which is a cone, remains on the tree till the summer succeeding that on which it was produced, the scales being still closed. But when the hot weather has commenced and continued for some time, so as to dry the cone thoroughly, the scales open of their own

252

240

accord with a sudden jerk, ejecting the contained seeds: and if a number of them happen to burst together, which is often the case, the noise is such as to be heard at some considerable distance. The twisted awn of avena fatua (fig. 240.), or wild oat, as well as that of geranium cicutarium, and some others, seems to have been intended particularly for the purpose of aiding the further dispersion of the seed, after being discharged from the plant or pericarp. This spiral awn or spring, which is beset with a multitude of fine and minute hairs, possesses the property of contracting by means of drought, and of expanding by means of moisture. Hence it remains of necessity in a perpetual state of contraction or dilatation, dependent upon change of weather; from which, as well as from the additional aid of the fine hairs, which act as so many fulcra, and cling to whatever object they meet, the seed to which it is attached is kept in continual motion till it either germinates or is destroyed. The awn of barley, which is beset with a multitude of little teeth all pointing to its upper extremity, presents also similar phenomena. For when the seed with its awn falls from the ear and lies flat upon the ground, it is necessarily extended in its dimensions by the moisture of the night, and contracted by the drought of the day. But as the teeth prevent it from receding in the direction of the point, it is consequently made to advance in the direction of the base of the seed, which is thus often carried to the distance of many feet from the stalk on which it grew. If any one is yet sceptical with regard to the travelling capacity of the awn, let him only introduce an awn of barley with the seed uppermost between his coat and shirt sleeve at the wrist, when he walks out in the morning, and by the time he returns to breakfast, if he has walked to any great distance, he will find it up at his armpit. This journey has been effected by means of the continued motion of the arm, and consequently of the teeth of the awn acting as feet to carry it forward.

1610. Where distance of dispersion is required, nature is also furnished with a resource. tance from their place of growth is that of the instrumentality of animals. Many seeds are thus carried to One of the most common modes by which seeds are conveyed to a disa distance from their place of growth merely by their attaching themselves to the bodies of such animals as may happen accidentally to come in contact with the plant in their search after food; the hooks or hairs with which one part or other of the fructification is often furnished serving as the medium of attachment, and the seed being thus carried about with the animal till it is again detached by some accidental cause, and at last committed to the soil. This may be exemplified in the case of the bidens and myosotis, in which the hooks or prickles are attached to the seed itself; or in the case of galium aparine and others, in which they are attached to the pericarp; or in the case of the thistle and the burdock, in which they are attached to the general calyx. Many seeds are dispersed by animals in consequence of their pericarps being used as food. This is often the case with the seeds of the drupe, as cherries, sloes, and haws, which birds often carry away till they meet with some convenient place for devouring the pulpy pericarp, and then drop the stone into the soil. though even with the view of feeding on the seed itself, as in the case of nuts hoarded up by squirrels, And so also fruit is dispersed that has been hoarded for the winter, which are often dispossessed by some other animal, that not caring for the hoard scatters and disperses it. Sometimes the hoard is deposited in the ground itself, in which case part of it is generally found to take root and to spring up into plants. Though it has been observed that the ground-squirrel often deprives the kernel of its gerin before it deposits the fruit it collects. Crows have been also observed to lay up acorns and other seeds in the holes of fence-posts, which being either forgot or accidentally thrust out, fall ultimately into the earth and germinate. But sometimes the seed is even taken into the stomach of the animal, and afterwards deposited in the soil, having passed through it unhurt. This is often the case with the seed of many species of berry, such as the mistletoe, which the thrush swallows and afterwards deposits upon the boughs of such trees as it may happen to alight upon. The seeds of the loranthus americanus, another parasitical plant, are said to be deposited in like manner on the branches of the cocoloba grandiflora, and other lofty trees; as also the seeds of phytolacca decandra, the berries of which are eaten by the robin, thrush, and wild pigeon. And so also the seeds of currants or roans are sometimes deposited, after having been swallowed by blackbirds or other birds, as may be seen by observing a currant-bush or young roan-tree growing out of the cleft of another tree, where the seed has been left, and where there may happen to have been a little dust collected by way of soil; or where a natural graft may have been effected by the insinuation of the radicle into some chink or cleft. It seems indeed surprising that any seeds should be able to resist the heat and digestive action of the stomach of animals; but it is undoubtedly the fact. Some seeds seem even to require it. The seeds of magnolia glauca, which have been brought to this country, are said to have generally refused to vegetate till after undergoing this process, and it is known that some seeds will bear a still greater degree of heat without any injury. Spallanzani mentions some seeds that germinated after having been boiled in water: and Du Hamel gives an account of some others that germinated even after having been exposed to a degree of heat measuring 255° of Fahrenheit. In addition to the instrumentality of brute animals in the dispersion of the seed might be added also that of man, who, for purposes of utility or of ornament, not only transfers to his native soil seeds indigenous to the most distant regions, but sows and cultivates them with care.

Some

1611. The agency of winds is one of the most effective modes of dispersion instituted by nature. seeds are fitted for this mode of dispersion from their extreme minuteness, such as those of the mosses, lichens, and fungi, which float invisibly on the air, and vegetate wherever they happen to meet with a suitable soil. Others are fitted for it by means of an attached wing, as in the case of the fir-tree and liriodendron tulipifera, so that the seed, in falling from the cone or capsule, is immediately caught by the wind, and carried to a distance. Others are peculiarly fitted for it by means of their being furnished with an aigrette or down, as in the case of the dandelion, goat's-beard, and thistle, as well as most plants of the class Syngenesia; the down of which is so large and light in proportion to the seed it supports, that it is wafted on the most gentle breeze, and often seen floating through the atmosphere in great abundance at the time the seed is ripe. Some have a tail, as in clematis vita alba. Others are fitted for this mode of dispersion by means of the structure of the pericarp, which is also wafted along with them, as in the case of staphylea trifolia, the inflated capsule of which seems as if obviously intended thus to aid the dispersion of the contained seed by its exposing to the wind a large and distended surface with but little weight. And so also in the case of the maple, elm, and ash, the capsules of which are furnished, like some seeds, with a membranous wing, which when they separate from the plant the wind immediately lays hold of and drives before it.

to

1612. The instrumentality of streams, rivers, and currents of the ocean, is a further means adopted by nature for the dispersion of the seeds of vegetables. The mountain-stream or torrent washes down the valley the seeds which may accidentally fall into it, or which it may happen to sweep from its banks when it suddenly overflows them. The broad and majestic river, winding along the extensive plain, and traversing the continents of the world, conveys to the distance of many hundreds of miles the seeds that may have vegetated at its source. Thus the southern shores of the Baltic are visited by seeds which grew

in the interior of Germany, and the western shores of the Atlantic by seeds that have been generated in the interior of America. "But fruits indigenous to America and the West Indies have sometimes been found to be swept along by the currents of the ocean to the western shores of Europe. The fruit of mimosa scandens, dolichos pruriens, guilandina bonduc, and anacardium occidentale, or cashew-nut, have been thus known to be driven across the Atlantic to a distance of upwards of 2000 miles; and although the fruits now adduced as examples are not such as could vegetate on the coast on which they were thrown, ewing to soil or climate, yet it is to be believed that fruits may have been often thus transported to climates or countries favorable to their vegetation.

1612. Propagation by gems. Though plants are for the most part propagated by means of seeds, yet many of them are propagated also by means of gems; that is, bulbs and buds.

1614. The caulinary bulb is often the means of the propagation of the species: it generally appears in the axil of the leaves, as in dentaria bulbifera and lilium bulbiferum; or between the spokes of their umbels, as in allium canadense; in the midst of the spike of flowers, as in polygonum viviparum and poa alpina As plants of this last kind are mostly alpine, it has been thought to be an institution or resource of nature to secure the propagation of the species in situations where the seed may fail to ripen. 1615. The bud, though it does not spontaneously detach itself from the plant and form a new individual, will yet sometimes strike root and develope its parts if carefully separated by art and planted in the earth: but this is to be understood of the leaf-bud only, for the flower-bud, according to Mirbel, if so treated, always perishes.

1616, Propagation by the leaves. The species may sometimes be propagated even by means of the leaves; as in the aloe, sea-onion, and some species of arum, which if carefully deposited in the soil will grow up into new plants, by virtue, no doubt, of some latent gem contained in them. The fungi and Lehens, according to Gærtner, are all gemmiferous, having no sexual organs, and ro pollen impregnating a germ In the genus Lycoperdon, the gelatinous substance that pervades the cellular tissue is converted into a proliferous powder; in clavaria, the fluid contained in the cavities of the plant is converted into a proliferous powder also; and in the agarics, hypnum, and poletus, vesicles containing sobolifercus granules are found within the lamina, pores, or tubes. Hedwig, on the contrary, ascribes to the fungi a sexual apparatus, and maintains that the pollen is lodged in the volva. But here it is to be recollected, as in the cases of the scutelle of the lichens, that all fungi are not furnished with a volva, and consequently not furnished with pollen. The confervæ and ulvæ, together with the genera Blasia and Riccia, are also, according to Gartner, propagated only by gems; while marchantia, anthoceros, jungermannia, and lycoperdon, are said to be propagated both by gems and seeds.

1617. Runners are young shoots issuing from the collar or summit of the root, and creeping along the surface of the soil; but producing a new root and leaves at the extremity, and forming a new individual, by the decay of the connecting link, as in the strawberry.

1618. Stips. The process of raising perennials by slips is well known to gardeners, and should perhaps be regarded as an extension of the old plant, rather than as the generation of a new one; though it serves the purpose of the cultivator equally well as a plant raised from seed, with the additional advantage of bearing fruit much sooner. But how is the root generated which the slip thus produces? If the trunk of a tree is lopped, and all its existing buds destroyed, then there will be protruded from between the wood and bark a sort of protuberant lip or ring formed from the proper juice, and from which there will spring a number of young shoots. The formation of the root in the case of the slip is effected in the same inan ner, the moisture of the soil encouraging the protrusion of buds at and near the section; and the bud that would have been converted into a branch above ground is converted into a root below.

1619 Layers. The mode of propagation by layers is practised upon trees that are delicate, and which cannot readily be propagated by means of slips; in which case the root is generated nearly as in the former case, the soil stimulating the protrusion of buds which are converted into roots. In many plants, such as the currant and laurel, this is altogether a natural process, effected by the spontaneous bending down of a branch to the surface of the soil.

120. Suckers or off-sets. Many plants protrude annually from the collar a number of young shoots, encircling the principal stem and depriving it of a portion of its nourishment, as in the case of most fruit-trees. Others send out a horizontal root, from which there at last issues a bud that ascends above the soil and is converted into a little stem, as in the case of the elm-tree and syringa. Others send out a horizontal shoot from the collar or its neighborhood; or a shoot that ultimately bends down by its own weight till it reaches the ground, in which it strikes root and again sends up a stem as in the currant-bush and Laurel The two former are called suckers or off-sets, though the term off-set should perhaps be restricted to the young bulbs that issue and detach themselves annually from bulbous roots. The latter is not designated by any particular name, but may be regarded as a sort of natural layer, resembling also, in some respects, the runner; from which, however, it is distinguished in that it never detaches itself spontaneously from the parent plant, as is the case also with the two former. But if either of them is artificially detached, together with a portion of root or a slice of the collar adherring to it, it will now bear transplanting, and will constitute a distinct plant.

1621, Grafting and budding. The species is also often propagated, or at least the variety is multiplied, by means of grafting, which is an artificial application of a portion of the shoot or root of one tree or plant to the stem, shoot, branch, or root of another, so that the two shall coalesce together and form but one plant. The shoot which is to form the summit of the new individual is called the scion; the stem to which it is affixed is called the stock; and the operation, when effected, the graft. As the graft is merely an extension of the parent plant from which the scion came, and not properly speaking a new individual, so it is found to be the best method of propagating approved varieties of fruit-trees without any danger of altering the quality of the fruit, which is always apt to be incurred in propagation from seed, but never in propagating from the scion. The scion will also bear fruit much sooner than the tree that is raised from seed; and, if effected on a proper stock, will be much more hardy and vigorous than if left on the parent plant. And hence the great utility of grafting in the practice of gardening. Till lately, grafting was confined to the ligneous plants, but it is now successfully practised on the roots and shoots of herbaceous vegetables; and the dahlia is grafted by the root; the melon on the gourd; the love-apple on the potatoe; the cauliflower on the cabbage, &c. by the shoot. A very ingenious tract has been pubJished on this subject, entitled, Essai sur la Greffe de l'herbe des plantes et des arbres, par Monsr. Le Baron de Tachoudy, Bourgeois de Glaris. Paris, 1819.

SECT. X. Causes limiting the Propagation of the Species.

1622. Though plants are controlled chiefly by animals, yet they also control one another. From the various sources of vegetable reproduction, but particularly from the fertility and dispersion of the seed, the earth would soon be overrun with plants of the most prolific species, and converted again into a desert, if it were not that nature has set bounds to their propagation by subjecting them to the controul of man, and to the depredations of the great mass of animals; as well as in confining the germination of their seeds to certain and peculiar habitations arising from soil, climate, altitude, and other circumstances. In order to form an idea of the manner in which these act upon vegetation; imagine that every year an enormous quantity of seeds, produced by the existing

vegetables, are spread over the surface of the globe, by the winds and other causes already mentioned, all of these seeds which fall in places suitable for their vegetation, and are not destroyed by aniinals, germinate and produce plants; then among these plants, the strongest, and largest, and those to which the soil is best suited, develope themselves in number and magnitude so as to choke the others. Such is the general progress of nature, and among plants, as among animals, the strong florish at the expense of the weak. These causes have operated for such a length of time, that the greater number of species are now fixed and considered as belonging to certain soils, situations, and climates, beyond which they seldom propagate themselves otherwise than by the hands of man.

SECT. XI. Evidence and Character of Vegetable Vitality.

1623. The power of counteracting the laws of chemical affinity is reckoned the best and most satisfactory evidence of the presence and agency of a vital principle as inherent in any subject. This principle, which seems first to have been instituted by Humboldt, is obviously applicable to the case of animals, as is proved by the process of the digestion of the food, and its conversion into chyle and blood; as well as from the various secretions and excretions effected by the several organs, and effecting the growth and developement of the individual, in direct opposition to the acknowledged laws of chemical affinity, which, as soon as the vital principle is extinct, begin immediately to give indication of their action in the incipient symptoms of the putrefaction of the deady body. But the rule is also applicable to the case of vegetables, as is proved by the intro-susception, digestion, and assimilation of the food necessary to their developement; all indicating the agency of a principle capable of counteracting the laws of chemical affinity; which, at the period of what is usually called the death of the plant, begin also immediately to act, and to give evidence of their action in the incipient symptoms of the putrefaction of the vegetable. Vegetables are therefore obviously endowed with a species of vitality. But admitting the presence and agency of a vital principle inherent in the vegetable subject, what are the peculiar properties by which this principle is characterised?

1624. Excitability. One of the most distinguishable properties of the vital principle of vegetables is that of its excitability or capacity of being acted upon by the application of natural stimuli, impelling it to the exertion of its vegetative powers; the natural stimuli thus impelling it being light and heat. 1625. The stimulating influence of light upon the vital principle of the plant is discoverable, whether in the stem, leaf, or flower. The direction of the stem is influenced by the action of light, as well as the color of its leaves. Distance from direct rays of light or weak light produces etiolation, and its absence blanching. The luxuriance of branches depends on the presence and action of light, as is particularly observable in the case of hot-house plants, the branches of which are not so conspicuously directed, either to the flue in quest of heat, or to the door or open sash in quest of air, as to the sun in quest of light. Hence also the branches of plants are often more luxuriant on the south than on the north side; or at least on the side that is best exposed to light. The position of the leaf is also strongly affected by the action of light, to which it uniformly turns its upper surface. This may be readily perceived in the case of trees trained to a wall, from which the upper surface of the leaf is by consequence always turned; being on a south wall turned to the south, and on a north wall turned to the north. And if the upper surface of the leaf is forcibly turned towards the wall and confined in that position for a length of time, it will soon resume its primitive position upon regaining its liberty, but particularly if the atmosphere is clear. The leaves of the mallow are said to exhibit but slight indications of this susceptibility, as also sword-shaped leaves; and also those of the mistletoe are equally susceptible on both sides. It had been conjectured that these effects are partly attributable to the agency of heat; and to try the value of the conjecture, Bonnet placed some plants of the atriplex in a stove heated to 25° of Reaumur. Yet the stems were not inclined to the side from which the greatest degree of heat came; but to a small opening in the stoves. Heat then does not seem to exert any perceptible influence in the production of the above effects. Does moisture? Bonnet found that the leaves of the vine exhibited the same phenomenon when immersed in water, as when left in the open air. Whence it seems probable that light is the sole agent in the production of the effects in question. But as light produces such effects upon the leaves, so darkness or the absence of light produces an effect quite the contrary; for it is known that the leaves of many plants assume a very different position in the night from what they have in the day. This is particularly the case with winged leaves, which, though fully expanded during the day, begin to droop and bend down about sunset and during the fall of the evening dew, till they meet together on the inferior side of the leaf-stalk, the terminal lobe, if the leaf is furnished with one, folding itself back till it reaches the first pair; or the two side lobes, if the leaf is trifoliate, as in the case of common clover. So also the leaflets of the false acacia and liquorice hang down during the night, and those of mimosa pudica fold themselves up along the common foot-stalk so as to overlap one another. Linnæus has designated the above phenomenon by the appellation of The Sleep of Plants. The expansion of the flower is also effected by the action of light. Many plants do not fully expand their petals except when the sun shines; and hence alternately open them during the day and shut them up during the night. This may be exemplified in the case of papilionaceous flowers in general, which spread out their wings in fine weather to admit the rays of the sun, and again fold them up as the night approaches. It may be exemplified also in the case of compound flowers, as in that of the dandelion and hawkweed. But the most singular case of this kind is perhaps that of the lotus of the Euphrates, as described by Theophrastus, which he represents as rearing and expanding its blossoms by day, closing and sinking down beneath the surface of the water by night so as to be beyond the grasp of the hand, and again rising up in the morning to present its expanded blossom to the sun. The same phenomenon is related also by Pliny. But although many plants open their flowers in the morning and shut them again in the evening, yet all flowers do not open and shut at the same time. Plants of the same species are tolerably regular as to time, other circumstances being the same; and hence the daily opening and shutting of the flower has been denominated by botanists The Horologium Flore. Flowers requiring but a slight application of stimulus open early in the morning, while others requiring more open somewhat later. Some do not open till noon, and some, whose extreme delicacy cannot bear the action of light at all, open only at night, such as the cactus grandiflora, or night-blowing cereus. But it seems somewhat doubtful whether or not light is the sole agent in the present case; for