Page images

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, Phlæum 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 epiphloum; 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 endophlæum, or liber, grows next the alburnum, the mesophloum next the endophlæum, and the epiphloum next the mesophloeum; 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 lamellæ : these have been improperly confounded with the epidermis. The cause of the separation of the lamella of the epiphlæum 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 epiphloum 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 laminæ into which the endophloeum 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

completely separate them, but pushes aside a quantity of cellular tissue, pressing it tightly into thin vertical radiating plates as the woody system extends, these plates increase outwardly, continuing to maintain the connection between the centre and the circumference. Botanists call them medullary rays (or plates); and carpenters, the silver grain. They are composed of muriform cellular tissue (Plate I. fig. 7.), often not consisting of more than a single layer of cellules; but sometimes, as in Aristolochias, the number of layers is very considerable. In horizontal sections of an Exogenous stem, they are seen as fine lines radiating from the centre to the circumference; in longitudional sections they produce that glancing satiny lustre which is in all discoverable, and which gives to some, such as the Plane and the Sycamore, a character of remarkable beauty.

No vascular tissue is ever found in the medullary rays, unless those curious plates described by Griffith in the wood of Phytocrene gigantea, in which vessels exist, should prove to belong to the medullary system.

The vascular system in an Exogenous stem is confined to the space between the pith and the bark, where it chiefly consists of ducts, and pitted or woody tissue collected into compact wedge-shaped vertical plates, fig. 34. the edges of which rest on the pith and bark, and the sides of which are in contact with the medullary rays.

That portion of the vascular system which is first generated is in immediate contact with the pith, to which it forms a complete sheath, interrupted only by the passage of the medullary rays through it. It consists of spiral vessels and woody tissue intermixed, and forms an exceedingly thin layer, called the medullary sheath. This is the only part of the vascular system of the stem in which spiral vessels are ordinarily found; the whole of the vessels subsequently deposited over the medullary sheath being bothrenchymatous tissue, with a few exceptions. The medullary sheath establishes a connection between the axis and all its appendages, the veins of leaves, flowers, and fruits, being in all cases prolongations of it. It has been remarked by Senebier, and since by De Candolle, that it preserves a green colour even in old

trunks, which proves that it still continues to retain its vitality when that of the surrounding parts has ceased.

The vascular system of a stem one year old consists of a zone of wood lying between the pith and the bark, lined in the inside by the medullary sheath, and separated into wedgeshaped vertical plates by the medullary rays that pass through it. All that part of the first zone which is on the outside of the medullary sheath is composed of woody tissue and vessels intermixed in no apparent order; but the vessels are generally either in greater abundance next the medullary sheath, or confined to that side of the zone, and the woody tissue alone forms a compact mass on the outside. The second year another zone is formed on the outside of the first, with which it agrees exactly in structure, except that there is no medullary sheath; the third year a third zone is formed on the outside of the second, in all respects like it; and so on, one zone being deposited every year as long as the plant continues to live. As each new zone is formed over that of the previous year, the latter undergoes no alteration of structure when once formed: wood is not subject to distension by a force beneath it, as the bark is; but, whatever the first arrangement or direction of its tissue may be, such they remain to the end of its life. The formation of the wood is, therefore, the reverse of that of the bark; the latter increasing by addition to the inside of its strata, the former by successive deposits upon its outside. It is for this reason that stems of this kind are called Exogenous (from two Greek words, signifying to grow outwardly). According to Dutrochet, each zone of wood is in these plants separated from its neighbour by a layer of cellular tissue, forming part of the system of the pith and bark; but although this is true in certain plants, such as arborescent nettles and others, it is by no means a general law.

After wood has arrived at the age of a few years, or sometimes even sooner, it acquires a colour different from that which it possessed when first deposited, becoming what is called heart-wood, or duramen. For instance, in the beech it becomes light brown, in the oak deep brown, in Brazil wood and Guaiacum green, and in ebony black. In all these it

« PreviousContinue »