at the base of the stem, than near the upper extremity. Nothing can well show more distinctly than this, that the woody bundles of the endogenous stem are a sort of roots emitted by the leaves, plunging down through their whole length into the cellular substance of the stem in ordinary cases; but, in Barbacenia, soon quitting the stem, and continuing their course downwards on the outside. The observation of Du Petit Thouars, that, when Dracenas push forth branches, each of the latter produces from its base a quantity of fibres, which are interposed between the cortical integument and the body of the wood, forming a sort of plaster analogous to what is found in the graft of an Exogen; and that, of the fibres just mentioned, the lowermost have a tendency to descend, while those originating on the upper side of the branch turn downwards, and finally descend also; had already rendered the above-mentioned conclusion probable. The case of Barbacenia can scarcely leave a doubt upon the subject; and leads to the important conclusion, that the theory of the wood of Exogens being also a state of roots belonging to the leaves of the stem is well founded also.

Mirbel, who formerly advocated the doctrine of wood being deposited by bark, has candidly admitted the opinion to be no longer tenable; and he has suggested, in its room, that wood and bark are independent formations, which is no doubt true; but he adds, created out of cambium, in which it is impossible to concur, if by cambium M. Mirbel means the viscid secretion found in the spring between the bark and wood of Exogens; for the following reasons : — All the writers hitherto mentioned have considered the formation of wood exclusively with reference to exogenous trees, and to such only of them as are the common forest plants of Europe. Had they taken into account exotic trees or any endogenous plants, they would have seen that none of their theories could apply to the formation of wood in the latter tribe. In many Exogens of tropical countries, wood is not deposited in regular circles all round the axis, but only on one side of the stem, or along certain lines upon it: were it a deposit from the bark, or a metamorphosis of cambium, it would necessarily be deposited with some kind of uniformity.

In endogenous trees there is no cambium, and yet wood is formed in abundance; and in the centre, not in the circumference so that bark can have, in such cases, nothing to do with the creation of wood.

But, if the word cambium is employed by M. Mirbel as an equivalent for organic mucus (see p. 1.), then the statement of this learned botanist is true, no doubt, but does not affect the question in dispute.

Aware of the difficulties in the way of the common explanations of the formation of wood, Du Petit Thouars, an ingenious French physiologist, who had possessed opportunities of examining the growth of vegetation in tropical countries, proposed a theory, which, although in many points similar to one previously invented by his countryman, De la Hire, is nevertheless, from the facts and illustrations brought by the French philosopher to his aid, to be considered legitimately as his own. The attention of Du Petit Thouars appears to have been first especially called to the real origin of wood by having remarked, in the Isle of France, that the branches emitted by truncheons of Dracæna (with which hedges are formed in that colony) root between the rind and old wood, forming rays, of which the axis of the new shoot is the centre. These rays surround the old stem; the lower ones at once elongate greatly towards the earth, and the upper ones gradually acquire the same direction; so that at last, as they become disentangled from each other, the whole of them pass downwards to the soil. Reflecting upon this curious fact, and upon others which I have not space to detail, he arrived at this conclusion; that it is not merely in the property of increasing the species that buds agree with seeds, but that they emit roots in like manner; and that the wood and liber are both formed by the downward descent of bud-roots, at first nourished by the moisture of the cambium, and finally embedded 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 axils 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 are therefore, in Du Petit Thouars's opinion, a mass of roots.

This opinion would probably have been more generally received, if it had not been too much mixed up with hypothetical statements, to the reception of which there are, in the minds of many persons, strong objections; as, for example, that mentioned in the last paragraph. The theory, nevertheless, seems better adapted than any other to explain the cause of the many anomalous forms of exogenous stems which must be familiar to the recollection of all botanists; and it is equally applicable to the exogenous and endogenous modes of growth, a condition which, it will be readily admitted, is indispensable to every theory of the formation of wood.

In the most recent days, it has had the advantage of being supported by M. Gaudichaud, who has made, it is said, a great number of very important and interesting observations upon the developement of stems. But, as the Memoir of this learned botanist is still unpublished, little is known of the manner in which he has treated his subject: the best account of it is given in the sixth edition of Achille Richard's Nouveaux E'lémens de Botanique, p. 167. So far as I am able to understand the short statement there made, the principal peculiarity in M. Gaudichaud's views consists in his assigning the growth of plants to a sort of polarity produced by the action of two opposite systems, of which the one, or ascending, consists of trachenchyma exclusively, the other, or descending, of bothrenchyma and pleurenchyma. It does not appear to which system the parenchyma is assigned; the line of demarcation between them is called the mesocauleorhiza. The leaf would appear to be regarded as a form of stem divided into three parts, of which the lowest is the internode from which the leaf emanates, the middle the petiole, the upper the lamina. The line of demarcation between the internode and petiole is called the mesophytum ; that between the lamina and petiole the mesophyllum. It is however impossible to form any opinion concerning this theory in the absence of the evidence

to prove his statements, which M. Gaudichaud is said to have produced in the Memoir laid before the Academy of Sciences in 1834.

The most important of the objections which have been taken to the opinion now under consideration are the following: If wood were really organised matter emanating from the leaves, it must necessarily happen that in grafted plants the stock would in time acquire the nature of the scion, because its wood would be formed entirely by the addition of new matter, said to be furnished by the leaves of the scion. So far is this, however, from being the fact, that it is well known that, in the oldest grafted trees, there is no action whatever exercised by the scion upon the stock; but that, on the contrary, a distinct line of organic demarcation separates the wood of one from the other, and the shoots emitted from the stock, by wood said to have been generated by the leaves of the scion, are in all respects of the nature of the stock. Again, if a ring of bark from a redwooded tree is made to grow in the room of a similar ring of bark of a white-wooded tree, as it easily may be made, the trunk will increase in diameter, but all the wood beneath the ring of red bark will be red, although it must have originated in the leaves of the tree which produces white wood. It is further urged, that, in grafted plants, the scion often overgrows the stock, increasing much the more rapidly in diameter; or that the reverse takes place, as when Pavia lutea is grafted upon the common horsechestnut; and that these circumstances are inconsistent with the supposition that wood is organic matter engendered by leaves. To these statements there is nothing to object as mere facts, for they are true; but they certainly do not warrant the conclusions which have been drawn from them. One most important point is overlooked by those who employ such arguments, namely, that in all plants there are two distinct simultaneous systems of growth, the cellular and the fibro-vascular, of which the former is horizontal, and the latter vertical. The cellular gives origin to the pith, the medullary rays, and the principal part of the cortical integument; the fibro-vascular, to the wood and a portion of the bark: so that the axis of a plant

may be not inaptly compared to a piece of linen, the cellular system being the woof, the fibro-vascular the warp. It has also been shown by Knight and De Candolle that buds are exclusively generated by the cellular system, while roots are evolved from the fibro-vascular system. Now, if these facts are rightly considered, they will be found to offer an obvious explanation of the phenomena appealed to by those botanists who think that wood cannot be matter generated in an organic state by the leaves. The character of wood is chiefly owing to the colour, quantity, size, and distortions of the medullary rays, which belong to the horizontal system: it is for this reason that there is so distinct a line drawn between the wood of the graft and stock; for the horizontal systems of each are constantly pressing together with nearly equal force, and uniting as the trunk increases in diameter. As buds from which new branches elongate are generated by cellular tissue, they also belong to the horizontal system: and hence it is that the stock will always produce branches like itself, notwithstanding the long superposition of new wood which has been taking place in it from the scion.

The case of a ring of red bark always forming red wood beneath it, is precisely of the same nature. After the new bark has adhered to the mouths of the medullary rays of the stock, and so identified itself with the horizontal system, it is gradually pushed outwards by the descent of woody matter from above through it; but, in giving way, it is constantly generating red matter from its horizontal system, through which the wood descends, and thus acquires a colour not properly belonging to it. With regard to the instances of grafts overgrowing their stocks, or vice versâ, it seems that these are susceptible of explanation on the same principle. If the horizontal system of both stock and scion has an equal power of lateral extension, the diameter of each will remain the same; but, if one grows more rapidly than the other, the diameters will necessarily be different: where the scion has a horizontal system that developes more rapidly than that of the stock, the latter will be the smaller, and vice versâ. It is, however, to be observed, that in these cases plants are in a morbid state, and will not live for any considerable time.

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