« PreviousContinue »
the necessary communication between the branch and the leaf is destroyed, and the latter falls off.
De Candolle explains the matter otherwise and better. The increase of leaves, he says, whether in length or in breadth, generally attains its term with sufficient rapidity; the leaf exercises its functions for a while, and enjoys the plenitude of its existence; but, by degrees, in consequence of exhaling pure water, and preserving in the tissue the earthy matters which the sap had carried there, the vessels harden and their pores are obstructed. This time in general arrives the more rapidly as evaporation is more active: thus we find the leaves of herbaceous plants, or of trees which evaporate a great deal, fall before the end of the year in which they were born; while those of succulent plants, or of trees with a hard and leathery texture, which, for one cause or another, evaporate but little, often last several years. We may, therefore, in general say that the duration of life in leaves is in inverse proportion to the force of their evaporation. When this time has arrived, the leaf gradually dries up, and finishes by dying: but the death of the leaf ought not to be confounded with its fall; for these two phenomena, although frequently confounded, are in reality very different. All leaves die some time or other; but some are gradually destroyed by exterior accidents, without falling; while others fall, separating from the stem at their base, and fall at once, either already dead, or dying, or simply unhealthy.
It is probable that both these explanations are required to understand the phenomena of the fall of the leaf; and that it is neither the rupture of the spiral vessels, nor the choking up of other kinds of tissue, separately, which produce it, but the two combined; the one acting principally in some cases, and the other in others.
OF THE BRACTS AND FLORAL ENVELOPES.-DISENGAGEMENT
The bracts, 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 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 fertilisation : 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 these beautiful ornaments have given birth is one, that the petals and disk are the agents of a secretion which is destined to the nutrition of the anthers and young ovules. 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 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 its full colour and velvety surface; and 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 shrivel 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.
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 those parts which in certain plants contain a large quantity of fæcula, as the tubers of the Potato, Cyperus esculentus, &c. This led Dunal to suspect that the young disks also contained fæcula: which 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 ovules; 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, Dunal remarks that the conditions requisite for germination are analogous to those which cause the expansion of a flower. The latter opens only in a temperature above 32° Fahr., that of 10° to 30° centig. (50° to 86° Fahr.) being the most favourable; it requires a considerable supply of ascending sap, without the watery parts of which it 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 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. Dunal is of opinion that the sugar of the nectary and petals is, in like manner, conveyed to the anthers and young ovules; and that the free liquid honey, which exists in such abundance in many flowers, is a secretion of superabundant fluid; 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. 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 spadixes stood at 111°, and in the centre of twelve at 121°, although the temperature of the external air was only 66o. The greatest degree of heat in these experiments was at sunrise. The same observer found that the male parts of six spadixes, deprived of their glandular part, raised the temperature only to 105°; and the same number of female spadixes only to 86°; and, finally, that the heat was wholly destroyed by preventing the spadix from coming in contact with the air.
Similar observations were made by others, with corresponding results; but, nevertheless, as many persons attempted in vain to witness the phenomenon, it began to be doubted, especially after Treviranus added his authority to that of those who doubted the existence of any disengagement of heat. The truth of the statement of Saussure and others has lately, however, been placed beyond all further doubt, by the experiments of Adolphe Brongniart upon Colocasia odora. (Nouv. Ann. du Muséum, vol. iii.) From the period of the expansion of the spathe, he applied to the middle of the spadix a very delicate and small thermometer, which he fixed to its place by a piece of flannel rolled several times round it and the spadix, so that the bulb of the thermometer touched the spadix on one side ; and on all others was protected by the flannel from contact with the air. All this little apparatus covered so small a portion of the spadix, that it was left in its place without interfering with the functions of that part. On the 13th of March, the spathe not being open, the flower diffused, notwithstanding, a fragrant smell. On the 14th it was open, and the odour was much increased. The emission of pollen took place on the 16th, between 8 and 10 A.M., and continued till the 18th. On the 19th the