no colouring matter enters them. Considering, however, the thinness of the epidermis of many plants, and the great permeability of vegetable membrane in general, it can hardly be doubted that they do possess the power of absorption which Bonnet contends for. This seems to be proved by the effects obviously produced by a shower of rain in the summer, or by syringing the fading plants in a hothouse.

In their growth, leaves usually increase in length by addition to the base, the apex altering but little after it is originally formed. Upon this subject there is a paper by Steinheil which deserves to be consulted (Ann. Sc., n. s., viii. 257.). This is only what theory would necessarily lead to, when it is considered that a leaf is an expansion of the epiphloum (see p. 89.) and mesophloum, its apex representing the external or ungrowing part of those cortical layers, and its base their interior or growing part.

Leaves usually are so placed upon the stem that their upper surface is turned towards the heavens, their lower towards the earth; but this position varies occasionally. In some plants they are imbricated, so as to be almost parallel with the stem; in others they are deflexed till the lower surface becomes almost parallel with the stem, and the upper surface is far removed from opposition to the heavens. A few plants, moreover, invert the usual position of the leaves by twisting the petiole half round, so that either the two margins become opposed to earth and sky, or the lower surface becomes uppermost: the former is especially the case with plants bearing phyllodia, or spurious leaves.

At night a phenomenon occurs in plants which is called their sleep it consists in the leaves folding up and drooping, as those of the Sensitive Plant when touched. This scarcely happens perceptibly except in compound leaves, in which the leaflets are articulated with the petiole, and the petiole with the stem: it is supposed to be caused by the absence of light, and will be farther spoken of under the head of Irritability.

After the leaves have performed their functions, they fall off: this happens at extremely unequal periods in different species. In some they all wither and fall off by the end of a single season; in others, as the Beech and Hornbeam, they

wither in the autumn, but do not fall off till the succeeding spring; and, in a third class, they neither wither nor fall off the first season, but retain their verdure during the winter, and till long after the commencement of another year's growth: these last are our evergreens. Mirbel distinguishes leaves into three kinds, as characterised by their periods of falling:

1. Fugacious, or caducous, which fall shortly after their appearance; as in Cactus.

2. Deciduous, or annual, which fall off in the autumn; as the Apple.

3. Persistent, evergreen, or perennial, which remain perfect upon the plant beyond a single season; as Holly, common Laurel, &c.

With regard to the cause of the fall of the leaf a number of explanations have been given, which may be found in Willdenow's Principles of Botany, p. 336. There are, however, only two much worth recording; those of Du Petit Thouars and De Candolle.

If you watch the progress of a tree, of the Elder, for example, says the former writer, you will perceive that the lowest leaves upon the branches fall long before those at the extremities. The cause of this may be, perhaps, explained upon the following principle:- In the first instance, the base of every leaf reposes upon the pith of the branch, to the sheath of which it is attached. But, as the branch increases in diameter by the acquisition of new wood, the space between the base of the leaf and the pith becomes sensibly augmented. It has, therefore, been necessary that the fibres by which the leaf is connected with the pith should lengthen, in order to admit the deposition of wood between the bark and the pith. Now how does this elongation take place? As the bundles of fibres which run from the pith into the leaf-stalk are at first composed only of spiral vessels, it is easy to conceive that they may be susceptible of elongation by unrolling. And in this seems to lie the mystery of the fall of the leaf; for the moment will come when the spiral vessels are entirely unrolled, and incapable of any further elongation: they will, therefore, by the force of vegetation, be stretched until they snap, when

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.

CHAPTER VII.

OF THE BRACTS AND FLORAL ENVELOPES.-DISENGAGEMENT OF CALORIC.

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. 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