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cular system, until it is either much reduced in diameter or wholly disappears; and in proof of this assertion, the Elder has been referred to, in which the pith is very large in the young shoots, and very small in the old trunks. Those, however, who entertain this opinion, seem not to consider that the diameter of the pith of all trees is different in different shoots, according to the age of those shoots'; that in the first that arises after germination, the pith is a mere thread, or at least of very small dimensions - that in the shoots of the succeeding year it becomes larger- and that its dimensions. increase in proportion to the general rapidity of developement of the vegetable system: the pith, therefore, in the firstformed shoots, in which it is so small compared with that in the branches of subsequent years, is not small because of the pressure of surrounding parts; it never was any larger.

The pith is always, when first forming, a uniform compact mass, connected without interruption in any part; but the vascular system sometimes developing more rapidly than itself, it occasionally happens that it is either torn or divided into irregular cavities, as in the Horse Chestnut, the Rice-paper plant, and many others; or that it is so much lacerated as to lose all resemblance to its original state, and to remain in the shape of ragged fragments adhering to the inside of the vascular system: this is what happens in Umbelliferous and other fistular-stemmed plants.

Sometimes the pith is much more compact at the nodes than in the internodes, as in the Ash; whence an idea has arisen that it is actually interrupted at those places: this is, however, a mistake; for in general there is no interruption of continuity, but a mere alteration in compactness. It does however sometimes happen, that the pith takes a large developement at the nodes, so as to cut off the vascular system of the internodes into almost distinct parts. This occurs in what are called articulated stems, as in Piper, Viscum, &c., and in the Vine when young. Dutrochet regards such cases as evidence that each internode is an independent creation in the beginning, and that it is only after having been growing for a period of time, varying in different cases, that the internodes become connected by woody formations.

It seldom happens that any part of the vascular system intermixes with the pith, which is usually composed of cellular tissue exclusively; but in Ferula and the Marvel of Peru, it has been proved by Mirbel and De Candolle, that bundles of woody fibre are intermixed; in Nepenthes there is a considerable quantity of spiral vessels scattered among the cellular tissue of the same part; and many other cases of a similar kind are now known. In Nyctaginaceæ generally, in Piperaceæ, Cycadaceæ, Chloranthaceæ, &c., this occurs, and has been made by Professor Schultz the character of a large division in his Natural System of Botany, called by him Synorgana dichorganoidea; but such cases may be found in Loranthus, and are not uniform in the orders quoted: in Boerhaavia repanda, for example, the pith contains no bundles of vascular tissue, but is filled with fistula containing very soft, lax, spheroidal, cellular tissue, surrounded by smaller, harder, and more cubical tissue, which passes into the medullary rays; a most curious organisation.,

The Bark is the coating of the stem immediately above the wood, to which it forms a sort of sheath, and from which it is separable without difficulty at certain seasons. But, although it appears as an independent formation, it is, in reality, organically connected with the wood by the processes of cellular tissue, which, under the name of medullary rays, pass through the wood, and lose themselves in the thickness of the bark. Formerly bark was distinguished into cortical or cellular integument, under which name was comprehended the whole of the external parenchymatous part, and liber or inner bark, a name used to denominate the fibrous woody portion lying next the alburnum. But it is necessary to look at the organisation of the bark with more precision, if we are to understand all the peculiarities found in its many modifications. It appears to me that the observations of Mohl are the best and most complete which have hitherto been made upon this very important subject: they, and many more of considerable value, by Dutrochet, Link, and others, render a peculiar nomenclature for the parts of the bark indispensable; so many false or indefinite ideas are there which attach to the older terms. Bark may be described anato

mically as composed of four separate parts:-1.The Epidermis, which is continuous with that of the leaves, resembling what is found upon their veins, like it composed of cells a little lengthened, and rarely furnished with stomates; it often bears hairs. 2. The Epiphlæum of Link, Phloum or Peridermis of Mohl, consisting of several layers of thin-sided tubular cells, rarely coloured green. 3. The Mesophlæum of Link, or cellular integument of others, composed of cells usually green, and placed in a different direction from those of the epiphlœum; sometimes, as in Quercus Suber, containing cellular concretions. 4. The Endophlæum or Liber, of which a part is cellular and a part composed of woody tubes. These are modified differently in different trees; and the appearances of Cork in many plants, of thin white lamellæ or hard plates in others, are so produced. Usually each stratum has a separate growth, which takes place by the addition of new matter to its interior; thus the endophloum, or liber, grows next the alburnum, the mesophlæum next the endophlæum, and the epiphlæum next the mesophloum; the epidermis does not grow at all. Such growth is often indicated by concentric circles, which correspond in each layer with the zones of wood.*

When the substance called Cork is formed, the epiphloum consists of polyedral cells, which multiply with unusual rapidity and in great quantities. It does not appear to have any communication by lateral passages with the interior of the plant; although Dutrochet represents them to exist in Ulmus suberosa, where I cannot find them. After a certain age, it exfoliates in the Cork Tree, but in such plants as Acer campestre, Ulmus, &c., it is simply rent and thrown off piecemeal. In the Birch, the Cherry, and similar trees, it forms annually only a few layers of tabular cellular tissue, arranged in transverse rows, which separate at a certain age into thin silvery lamella: these have been improperly confounded with the epidermis. The cause of the separation of the lamellæ of the epiphloeum of the Birch is found in the developement, between the lamellæ, of a layer of thin-sided

• But, according to Decaisne (Comptes rendus, v. 393.), in Menispermaceæ the liber is only formed for the first year, and is afterwards covered over by new wood; and consequently is found near the centre round the pith, and not at the circumference.

cells, less compactly arranged, and easily separating into a fine powder when disturbed.

As strata of cellular tissue, in a peculiar state, may form between the lamellæ of the Birch and other such trees, so may it in other parts of the bark. This causes the sloughing of hard thin plates from the bark of the Plane tree; which Mohl explains thus:-Up to its eighth or tenth year, the bark of the Plane tree is like that of the Beech; at that period there forms in different parts of the liber a stratum of tabular cells, in all respects analogous to those of the epiphloeum. This new epiphloeum is not exactly parallel with that of older date, which exists at the surface of the bark, and cuts off an exterior portion, which then dies and drops off in the manner with which we are all familiar. The scales produced by this formation of epiphlæum inside the liber or mesophlæum Mohl calls Rhytidoma, from puris, a wrinkle. (Ann. des Sciences, N. S. IX. 290.)

In some plants the epiphlæum forms regular strata, parallel with the axis of the stem, and afterwards separates into strips analogous to those of the liber, as in the Juniper, Callistemon lophanthus, &c. In others, a portion of the liber is really thrown off annually, as in the Vine, the Honeysuckle, &c.

Hence in exogenous trees, the thickness of the bark is annually diminished by one of two causes; either by an exfoliation of the external and dead portions of the epiphloum only, or by a formation of a second epiphlæum, or false cork, among the liber, the result of which is the throwing off the parts of the bark lying over it as soon as they die.

So long as the parts of the bark remain alive, they give way to the expansion of the wood within it, by adding new tissue to themselves, as has been already stated: but when they die, they are necessarily torn into clefts, rents, or ribands, as we find in the trunks of trees.

It will have been seen that the only part of the bark in which woody tissue occurs is the endophlæum. Here it is often very abundant, and exceedingly tough and thick-sided; in consequence of which it is of great value for many useful purposes. When freed from the cellular tissue adhering to

it, it is often manufactured into cordage, especially in trees and shrubs of the natural order Malvaceæ. The Russia mats of commerce are manufactured from the thin lamina into which the endophlæum of Tilia europæa readily separates. The Lace bark of Jamaica, remarkable for its beautiful lace-like appearance when gently pulled laterally, and for its great toughness, whence it is often twisted into whiplashes, is the laminated liber of Lagetta lintearia.

When stems are old, the bark usually bears but a small proportion in thickness to the wood; yet in some plants its dimensions are of a magnitude that is very remarkable. For instance, specimens of Abies Douglasii have been brought to Europe twelve inches thick, and these are said not to be of the largest size.

Air cells and Vasa propria are exceedingly common in the bark, but there is no authenticated instance of any spiral or other vessels having been found in it; except in Nepenthes, in which they occur in almost every part, and exist in no inconsiderable numbers in the bark.

Beneath the bark, and above the wood, is interposed in the spring a mucous viscid layer, which, when highly magnified, is found to contain numerous minute transparent granules, and to exhibit faint traces of a delicate cellular organisation. This secretion is named the CAMBIUM, and appears to be exuded both by the bark and wood, certainly by the latter; but Dutrochet says only by the former, founding his opinion upon the presence of cambium in bark nodules, which, he says, have no communication with the wood of the parent tree; see page 80.

The cellular system of the pith and that of the bark are, in the embryo and youngest shoots, in contact; but the woody system, as it forms, gradually interposes between them, till after a few weeks they are distinctly separated, and in very aged trunks are sometimes divided by a space of several feet; that is to say, by half the diameter of the wood. But whatever may be the distance between them, a horizontal communication of the most perfect kind continues to be maintained. When the woody system is first insinuated into the cellular system, dividing the pith and cortical integument, it does not

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