"Olefiant gas, in the quantity of four cubic inches and a half, and in the proportion of a hundredth part of the air, had no effect whatever in twenty-four hours.

"The protoxide of nitrogen, or intoxicating gas, the last we shall mention, is the least injurious of all those we have tried; indeed, it appears hardly to injure vegetation at all. Seventytwo cubic inches were placed with a mignonette plant in a jar of the capacity of 509 cubic inches for forty-eight hours; but no perceptible change had taken place at the end of that

time."

Hydrogen seems to act unequally upon vegetation. Saussure found that a plant of Lythrum salicaria, after five weeks, had caused no alteration in a known volume of hydrogen by which it was surrounded, and had not itself experienced any apparent effect. Sir Humphrey Davy, however, states that some plants will grow in an atmosphere of hydrogen, while others quickly perish under such treatment.

In order to enable plants to perform the functions of respiration, and to expose their juices to the action of the atmosphere, leaves are furnished with special organs which are admirably adapted to the end in view. To prevent too rapid an evaporation beneath the solar rays, the leaf is overspread with a cuticle, which, as has been stated, is a hollow plate of membrane enclosing air; and at the same time to furnish them with the means of parting with superfluous moisture, at periods when the cuticle offers too much resistance, the stomata act like valves, and open to permit its passage or when, in dry weather, the stem does not supply fluid in sufficient quantity from the soil for the nourishment of the leaves, these same stomata open themselves at night, and allow the entrance of atmospheric moisture, closing when the cavities of the leaf are full. In submersed leaves, in which no variation can take place in the condition of the medium in which they float, both cuticle and stomata would be useless, and accordingly neither exists. For the purpose of exposing the fluids contained in the leaves to the influence of the air, the cuticle would frequently offer an insufficient degree of surface. In order, therefore, to increase the quantity of surface that is exposed, the tissue of the leaf is cavernous, each

stoma opening into a cavity beneath it, which is connected with multitudes of intercellular passages. Nor are the stomata and the cavernous parenchyma of the leaf the only means provided for the regulation of its functions. Hairs, no doubt, perform no mean office in their economy. In some cases these processes seem destined only for protection against cold, as in those plants in which they only clothe the buds and youngest leaves, falling away as soon as the tender parts have become hardened; but it can hardly be doubted that in many others they are absorbent organs, intended to collect humidity from the atmosphere. In succulent plants, or in such as grow naturally in shady places, where moisture already exists in abundance, they are usually wanting; but in hot, dry, exposed places, where it is necessary that the leaf should avail itself of every means of collecting its food, there they abound, lifting up their points and separating at the approach of the evening dews, but again falling down, and forming a layer of minute cavities above the cuticle, as soon as the heat of the sun begins to be perceived.

CHAPTER VI.

OF THE BRACTEÆ, CALYX, COROLLA, AND DISK.

THE bractea, when but slightly removed from the colour and form of leaves, no doubt perform functions similar to those of the latter organs; and when coloured and petaloid, it may be presumed that they perform the same office as the corolla. Nothing, therefore, need be said of them separately.

With regard to the calyx, corolla, and disk, I shall chiefly follow M. Dunal's statements in his ingenious pamphlet, Sur les Fonctions des Organes floraux colorés et glanduleux. 4to. Paris, 1829. The calyx seems, when green, to perform the functions of leaves, and to serve as a protection to the petals and sexual organs; when coloured its office is undoubtedly the same as that of the corolla.

The common notion of the use of the corolla is, that, independently of its ornamental appearance, it is a protection to the organs of fecundation: but, if it is considered that the stamens and pistils have often acquired consistence enough to be able to dispense with protection before the petals are enough developed to defend them, it will become more probable that the protecting property of the petals, if any, is of secondary importance only.

Among the many speculations to which those interesting ornaments have given birth is one, that the petals and nectaria are the agents of a secretion which is destined to the nutrition of the anthers and young ovula. These parts are formed in the flower-bud long before they are finally called into action: in the almond, for example, they are visible some time before the spring, beneath whose influence they are destined to expand. In that plant, just before the opening of the flower, the petals are folded up; the glandular disk that lines the tube of the calyx is dry and scentless; and its colour is at that time dull, like the petals at the same period. But as soon as the atmospheric air comes in direct, contact with

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these parts, the petals expand and turn out of the calyx, the disk enlarges, and the aspect of both organs is altered. Their compact tissue gradually acquires their full colour and velvety surface; the surface of the disk, which before was dry, becomes lubricated by a thick liquid, exhaling that smell of honey which is so well known. At this time the stamens perform their office. No sooner is that effected than they wither, the petals dry up and fall away, the secretion from the disk gradually dries up, and, in the end, the disk perishes along with the other organs to which it appertained. If the disk of an almond flower be broken before expansion, it will be seen that the fractured surface has the same appearance as that of those parts which in certain plants contain a large quantity of fæcula, as the tubers of the potato, Cyperus esculentus, &c. This led M. Dunal to suspect that the young disks also contained fæcula. This he afterwards ascertained, by experiment, to be the fact in the spadix of Arum italicum before the dehiscence of the anthers; but, subsequently to their bursting, no trace of fæcula could be discovered. Hence he inferred that the action of the air upon the humid fæcula of the disk had the effect of converting it into a saccharine matter fit for the nutrition of the pollen and young ovula; just as the fæcula of the albumen is converted in germination into nutritive matter for the support of the embryo.

In support of this hypothesis M. Dunal remarks, that the conditions requisite for germination are analogous to those which cause the expansion of a flower. The latter open only in a temperature above 32° Fahr., that of 10° to 30° centig. (50° to 86° Fahr.) being the most favourable; they require a considerable supply of ascending sap, without the watery parts of which they cannot open; and, thirdly, flowers, even in aquatic plants, will not develope in media deprived of oxygen.

Thus the conditions required for germination and for flowering are the same: the phenomena are in both cases also very similar.

When a germinating seed has acquired the necessary degree of heat and moisture, it abstracts from the air a portion of its oxygen, and gives out an equal quantity of carbonic acid gas; but, as one volume of the latter gas equals one

volume of oxygen, it is evident that the seed is, in this way, deprived of a part of its carbon. Some changes take place in the albumen and cotyledons; and, finally, the fæcula that they contained is replaced by saccharine matter. In like manner a flower, while expanding, robs the air of oxygen, and gives out an equal volume of carbonic acid; and a sugary matter is also formed, apparently at the expense of the fæcula of the disk or petals.

The quantity of oxygen converted into carbonic acid gas in germination is, cæteris paribus, in proportion to the weight of the seed; but some seeds absorb more than others. Theodore de Saussure has shown that exactly the same phenomenon occurs in flowers.

Heat is a consequence of germination; the temperature is also augmented during flowering, as has been proved by Theodore de Saussure in the Arum, the gourd, the Bignonia radicans, Polyanthes tuberosa, and others.

The greater part of the saccharine matter produced during germination is absorbed by the radicle, and transmitted to the first bud of the young plant. M. Dunal is of opinion that the sugar of the nectary and petals is in like manner conveyed to the anthers and young ovula, and that the free liquid honey which exists in such abundance in many flowers, is a secretion of superabundant fluid, since it can be taken away, as is well known, without injury to the flower.

This opinion will probably be considered the better founded, if it can be shown that the disengagement of caloric and destruction of oxygen are in direct relation to the developement of the glandular disk, and also are most considerable at the time when the functions of the anthers are most actively performed.

In no plants, perhaps, is the glandular disk more developed than in Arums; and it is here that the most remarkable degree of developement of caloric has been observed. Senebier found that the bulb of a thermometer, applied to the surface of the spadix of Arum maculatum, indicated a temperature 7° higher than that of the external air. M. Hubert remarked this in a still more striking degree upon Arum cordifolium at the Isle of France. A thermometer placed in the centre of five