wood and liber are both formed by the downward descent of bud-roots, at first nourished by the moisture of the cambium, and finally imbedded in the cellular tissue which is the result of the organisation of that secretion. That first tendency of the embryo, when it has disengaged itself from the seed, to send roots downwards and a stem and leaves upwards, and to form buds in the axillæ of the latter, is in like manner possessed by the buds themselves; so that plants increase in size by an endless repetition of the same phenomenon.

Hence a plant is formed of multitudes of buds or fixed embryos, each of which has an independent life and action: by its elongation upwards forming new branches and continuing itself, and by its elongation downwards forming wood and bark; which is therefore, in Du Petit Thouars's opinion, a mass of roots.

A great deal of opposition has been offered to this view, especially among this writer's own countrymen; but it is remarkable that many of his antagonists have been from a class of naturalists of whom it may be said, that they are better known in consequence of the celebrity of the object of their attack than for any reputation of their own. To this, however, there are some exceptions, as, for instance, MM. Mirbel and Desfontaines, two of the most learned botanists of France. This theory, nevertheless, seems the only one that is adapted at once to the explanation of the real cause of the many anomalous forms of exogenous stems which must be familiar to the recollection of all botanists, and that, at the same time is equally applicable to the exogenous and endogenous modes of growth; a condition which, it will be readily admitted, is indispensable to any theory of the formation of wood that may be proposed. It also offers the simplest explanation of the phenomena that are constantly occurring in the operations of gardening.

It has recently been a subject of discussion in the Academy of Sciences at Paris; when M. Poiteau supported the theory, and MM. Mirbel, Cassini, and Desfontaines opposed it. The arguments used by the latter were two, both of which are undoubted fallacies. The first was, that if a large ring of bark be taken from the stem of a sycamore, and be replaced by a similar ring entirely destitute of buds from a red maple, the

new bark will graft itself with the sycamore, and in time red maple wood will be formed beneath it. They said this ligneous production could not be derived from the buds of the red maple, because the ring of bark was devoid of any; nor could it proceed from the buds of the sycamore, because they would produce sycamore wood. But it is obvious that, in this experiment, the character of the red maple wood was derived from its medullary rays, which first formed an adhesion with those of the sycamore, and afterwards an independent horizontal formation, through which the fibres of the sycamore descended without altering its character. The other case was, that if a large ring of bark be taken from the trunk of a vigorous elm or other tree without being replaced with any thing, new beds of wood will be found in the lower as well as upper part of the trunk; while no ligneous production will appear on the ring of wood left exposed by the removal of the bark. Now this is so directly at variance with the observ ations of others, that it is impossible to receive it as an objection until its truth shall have been demonstrated. It is well known that if the least continuous portion of liber be left upon the surface of a wound of this kind, that portion is alone sufficient to establish the communication between the upper and lower lips of the wound; but, without some such slight channel of union, it is directly contrary to experience that the part of a trunk below an annular incision should increase by the addition of new layers of wood until the lips of the wound are united, unless buds exist upon the trunk below the ring.

The secretion called cambium, in the opinion of those who believe wood and bark to be independent simultaneous formations from the surface of the old wood and bark, is the matter which finally becomes organised as such: in Du Petit Thouars's theory it is a matter of organisation only as far as regards the origin of the cellular tissue of the medullary rays, and of the bark; while the superfluity of its moisture is a provision made by nature for the nutriment of the young fibres that descend through it.

CHAPTER V.

OF THE LEAVES.

LEAVES are at once organs of respiration, digestion, and nutrition. They elaborate the crude sap impelled into them from the stem, parting with its water, adding to it carbon, and exposing the whole to the action of air; and while they supply the necessary food to the young fibres that pass downwards from them, and from the buds in the form of alburnum and liber, they also furnish nutriment to all the parts immediately above and beneath them. There are many experiments to show that such is the purpose of the leaves. If a number of rings of bark are separated by spaces without bark, those which have leaves upon them will live much longer than those which are destitute of leaves. If leaves are stripped off a plant before the fruit has commenced ripening, the fruit will fall off and not ripen. If a branch is deprived of leaves for a whole summer, it will either die or not increase in size perceptibly. The presence of cotyledons, or seminal leaves, at a time when no other leaves have been formed for nourishing the young plant, is considered a further proof of the nutritive purposes of leaves: if the cotyledons are cut off, the seed will either not vegetate at all, or slowly and with great difficulty; and if they are injured by old age, or any other circumstance, they produce a languor of habit which only ceases with the life of the plant, if it be an annual. This is the reason why gardeners prefer old melon and cucumber seeds to new ones: in the former the nutritive power of the cotyledons is impaired, the young plant grows slowly, a languid circulation is induced from the beginning; by which excessive luxuriance is checked, and fruit formed rather than leaves or branches.

Various are the secretions of plants that take place through the leaves in those of monocotyledonous tropical plants in our hot-houses, nothing is more common than to see drops of water forming upon them from the effect of perspiration; in

Limnocharis Plumieri there is a large pore terminating the veins of the apex of the leaf, from which water is constantly distilled. The pitchers of Nepenthes, which are only a particular kind of leaves, secrete water enough to fill half their cavity. But, besides this more subtle fluid, secretions of a grosser quality take place in plants. The honey dew, which is so often attributed to insects, is one instance of the perspiration of a viscid, saccharine substance; the manna of the ash is another; and the gum ladanum that exudes from the Cistus ladaniferus is a third instance of this kind of perspiration.

It is believed that absorption takes place indifferently by either the upper or under surface of the leaf, but that some plants absorb more powerfully by one surface than by the other. Bonnet found that while the leaves of Arum, the kidney-bean, the lilac, the cabbage, and others, retained their verdure equally long whichever side was deprived of the power of absorption, the Plantago, some Verbascums, the marvel of Peru, and others, lost their life soonest where the upper surface was prevented from absorbing; and that in a number of trees and shrubs the leaves were killed very quickly by preventing absorption by the lower surface. From this there is only one safe conclusion to be drawn; that the absorbing surface of leaves varies in different species, and depends upon their peculiar organisation.

Leaves usually are so placed upon the stem that their upper urface 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 this 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 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 opuntia.

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

3. Persistent, evergreen, or perennial, which remain perfect. upon the plant beyond a single season; as Ilex aquifolium, Prunus pseudo-cerasus, &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. Sir James Smith was of opinion with Vrolik, that it is evidently a sloughing or casting off diseased or worn out parts, and in this I agree with them; but neither of these authors afford any explanation of the cause of this sloughing; nor do I think that it has been satisfactorily accounted for by any one, except Du Petit Thouars. If you will watch the progress of a tree, — of the elder for example, — says this 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 medulla of the branch to the sheath of which it is attached. But, as the branch increases in diameter by the acquisition of new