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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 But, as the Memoir of this learned botanist is still unpublished, little is known of the manner in which he has treated his subject: the best account of it is given in the sixth edition of Achille Richard's Nouveaux E'lémens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 distinet 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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Another 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 observations 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 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 horizontal parenchymatous system may, however, go on growing, and so form new layers.

Dutrochet mentions some cases of extraordinary longevity in the stock of Pinus Picea, after the trunk had been felled, and which he supposes fatal to the theory of wood being formed by the descent of organised matter. He says that, in the year 1836, a stock of Pinus Picea, felled in 1821, was still alive, and had formed 14 thin new layers of wood, that is, one layer each year; and another, felled in 1743, was still in full vegetation, having formed 92 thin layers of wood, or one each year. But, in reality, these cases prove nothing more than that the stock of Pinus Picea is singularly tenacious of life; for, although M. Dutrochet does not say so, there can be little doubt that these layers of wood were a parenchymatous developement of the horizontal system. (See Comptes rendus, iii. 748.)

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Those who object to the theory of wood being generated by the action of leaves either suppose, 1st, that liber is developed by alburnum, and wood by liber; or, 2dly, that "the woody and cortical layers originate laterally in the cambium furnished by pre-existing layers, and nourished by the descending sap. The first of these opinions appears to be that of Turpin, as far as can be collected from a long memoir upon

the grafting of plants. The second is the opinion commonly entertained in France, and adopted by De Candolle in his latest published work.

The objections to the views of Turpin need hardly be stated. Those which especially bear upon the view taken by De Candolle are, that his theory is not applicable to all parts of the vegetable kingdom, but to exogenous plants only; and, that endogens and cryptogamic plants, in which there is no secretion of cambium, nevertheless have wood.

Such is the state of this subject at the time I am writing. To use the words of De Candolle, "The whole question may be reduced to this Either there descend from the top of a tree the rudiments of fibres, which are nourished and developed by the juices springing laterally from the body of wood and bark; or new layers are developed by pre-existing layers, which are nourished by the descending juices formed in the leaves."

I would only add, that, after attentively considering the various arguments adduced in connection with this difficult question, it appears to me that the two greatest objections to the theory of Du Petit Thouars are, 1st, the existence of Dutrochet's embryo buds, already described, p.79.; and, 2dly, M. Decaisne's statement (Comptes rendus, vii. 944.), that in the Beet-root, where new vascular tissue is produced, it, in the beginning, is distinct from the previously formed vascular tissue. These two points deserve to be carefully considered and re-examined.

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