The Vegetable World: Being a History of Plants, with Their Botanical Descriptions and Peculiar Properties

Fig. 44.—Wood-vessels (punctuated and striated vessels of the Melon).

is marked with small dots, furrows, and streaks, they are called dotted vessels and striated vessels.

There is a particular part of the wood, however, in which the vessels are of a very different nature from that just indicated. We find them round the pith, in the innermost portion of the woody circle, and never anywhere else. These vessels, with the slender fibres which accompany them, have received, and most improperly retained, the name of the medullary sheath. We say improperly, for here there is only a combination of vessels, and no sheath. The image which this word recalls is not of a nature to enable us to understand the important modification of structure which belongs to the innermost vessels of the woody circle, which e2 are the air-vessels, or trachee, of the system.

Fig. 45 represents the central part of @

| piece of tree as seen in the microscope, with

a very strong magnifying power. In this central portion, which has been most improperly called the medullary sheath, we see — the trachea, that is to say the air-vessels, of | which we are speaking, on one side touching the pith at the centre of the stem, on the other side in contact with the woody re.

i

an! \ The structure of these tracheii is very sin-

& gular. They form masses of elongated fibre, \ \ x

| which is still more slender at the extremities.

= Any one would believe at the first glance that t_i" these vessels were very finely streaked diagonally, and that their external coat is con-erraend tinuous;but if the slightest pulling is used aeere towards ‘thin, they unroll like a spiral spring. These vessels are, then, formed of a spiral, ailing — thread, twisting round a cylinder with con-_

Fig. 45.—Trachee of plants 4°. A mene surrounded by pith and woody tiguous spiral turns, which are joined together by a membrane, which is so extremely thin, that it is difficult to find the traces of it when the spiral tube has _ been unrolled. There is one final peculiarity which marks the section of the |

stem of one of our forest trees. It is that assemblage of diverging lines which bear the name of medullary rays. In a transverse section of the stem of a tree, the mass of wood is traversed by a great number of radiating lines, all of which start from the back and converge towards the pith, or medulla. But they do not all reach it; there is a certain number which stop short in some of the layers, more or less deep in the trunk, without reaching the pith. These radiating lines result from the transverse section of the cellular laminee, the edge of which we thus see, and the length and thickness of which are variable.

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Fig. 45.—Medullary rays of the Cork-tree Fig. 47.—Medullary rays of the Maple. (horizontal section).

Fig. 46 represents the medullary rays of a trunk of Cork-tree (Quercus suber), in a transverse section. Fig. 47 shows the same organs in a similar section of the stem of the Mapie, magnified by means of the microscope. In this last, Rm are the medullary rays, which go from the centre to the circumference of the stem. The trachew, the ligneous fibres, and the vessels, are represented by the

letters T, FL, v, and the pith by the letter m. To make this more clear to the reader, we avail ourselves of an illustration borrowed from Mr. Christopher Dresser’s “ Rudiments of Botany ” (Fig. 48), in which a is a horizontal section of an exogenous stem, that: 1s, of a stem consisting of pith, wood, and bark, which is enlarged by external additions. a 4c are the cellular mass of the stem, in which ' bundles of woody fibre, 4, have been deposited. By the disposition of these bundles, the cellular matter has » become divided inte 4 ’ central portion, @, which 1s the pith, an outer portion, ¢, which is the bark, or corticle, and the plates, 6, intervening between the bundles of wood- Fig. (Sting of an exogenous stem. fibre which connect the bark and the pith; these are the medullary rays or plates. 8 is the section of an older stem, 1 which a bundle of woody fibre has been deposited between each of the older bundles. a, pith, 6, medullary rays, ¢c, cortex, or bark, 4, bundles of wood-fibre.; The vertical section c represents a portion of a stem with the bark removed, showing that the bundles of fibre d do not descend vertically through the cellular matter 6, but in waved lines, dividing the cellular into lenticular masses, which form the medullary rays or plates 4. Most trees are provided with medullary rays of only one kind. A few only present thick and thin rays together. Thus in the Oak or the Hornbeam we find both thick and thin rays, whilst in Willow and Maple the rays are visibly equal. Whilst the fibres and vessels never contain solid nutritive matter, the cells, which, by their junction, form the medullary rays, are the seat of an abundant production of small granules of starch.

The dark of trees is essentially composed of fibrous and cellular tissue;but it is easy to understand how varied are the forms, disposition, and structure of these substances, when we consider the variety in the appearance of the bark of trees, andextraordinary the diversity of their products. To explain everything which relates to the structure of the bark, would lead us into details which our space does not permit. We must therefore limit our remarks, and content ourselves with pointing out the principal characteristics of bark, considering generally the trees of our own climate. Briefly, the young stem is invariably covered with a thin cuticle, the epidermis. As the stem increases, new bundles of woody fibres are deposited in regular annular layers one in each year, the new layers being deposited outside those already formed. The new layers of bark and wood are thus formed almost in contact, being juxtaposed. The epidermis covers the bark, as it does every other part of the vegetable, but its existence is altogether ephemeral. It is destroyed at an early stage as much by the growth of the vegetable as by the action of external agents. It is otherwise with the suber, which forms the next layer, the cells of which are of a cubical form, and are closely united to each other with thin walls or partitions, without colour at first, but afterwards they acquire a brownish colour.

In many trees the suber is very slightly developed. But this is not the case with the Cork-oak (Quercus suber). In this beautiful tree, which furnishes man with one of his most useful commercial products, the swberous layer acquires an extraordinary thickness; it is, in short, the substance known as cork, in Latin swder, whence the specific name of the tree. When about five years old the suber, which constitutes the greater part of the bark in the cork-tree, begins to make a remarkably quick growth; then all the energy of its vegetation seems to concentrate itself on this part of the tree. New cells appear on the internal face of the primitive zone, pushing the exterior cells which preceded them. Independently of these cells, the successive accumulation of which constitute the mass of cork, others are formed which are shorter, darker in colour, of a flat or plate-like form, which divide the mass of cork into successive zones of growth. This mass attains by degrees to a considerable thickness. If left to itself, it would crack so deeply as to become

unfit for the uses to which cork is destined. It is necessary, therefore, to strip it off before it acquires this hard and fissured appearance. Barking or peeling off the suber of the Cork-oak dose no injury to the tree, it is so managed as to avoid injury to the newly formed suberous layer, and consequently to the living and underlying layers of the bark. The operation is usually performed when the trees have attained the circumference of ten or twelve inches. The process is performed during the summer months, by cutting a longitudinal notch in the trunk of the tree, intersecting it with several transverse incisions distant about forty inches from one another. The bark is then beat in order to break away the adhesion of the cork to the living layers, and separate the underlying tissues. The bark is then detached in the shape of cylindrical pieces, by means of the handle of an axe, made crooked and ~ thin at the end, as represented in Plate ITI.

The Cork-tr£E is peculiar to hot countries. Algeria possesses several forests of this tree in course of working. Spain has long been celebrated for its produce. The crops of cork are generally gathered, in each forest, once every eight years. The suber lies immediately over a cellular mass of a very different nature. The cells which constitute this layer are polyhedral, they are thicker and more loosely joined, and of a greenish colour. This colouring is owing to the presence of Chlorop/y/l, a matter peculiar the green organs of vegetables, which is applied to the internal face of the cellular walls. Chlorophyll presents itself, in a mature state, under the form of very small rounded globules, formed of albuminous and fatty matter, sometimes enclosing small kernels of starch in their interior, and appearing to be super- — ticially penetrated by the green colouring matter.

To these three cortical formations a fourth must be added which a | bears the name of /ider, and generally appears formed of rows composed of substances with thick and thin walls alternately. — The first are fibres of a brilliant white, longer and more slender than the ligneous fibres; their walls are very thick and are often © 4 dotted and extremely tough.

The fibres of /ier render an important service to human industeyy 4 = since they furnish the materials for ropes, thread, and the strongest a

as well as the most delicate cloths. In Fig. 49 the fibres of Hemp (Cannabis sativa) are shown as a common example of the vegetable substance known under the name of /ider. These fibres are joined in bundles. The bundles are arranged in concentric circles, frequently joined to one another by anostomose at the extremities, and constitute | layers, which appearvery thin super-imposed like a sort of tissue, of a more or less loose texture. The whole of these layers together were formerly compared to a book, every leaf of which would represent a layer. Thence the rather unsuitable name of lider.

The stratum having thin walls is formed of cells, which, in spring, enclose fecula and some very remarkable fibres, the very thin walls of which are extensively punctured and covered by wonderfully delicate meshes of network, with interstices often not more than suiooth part of a line in diameter. These fibres, whose physiological functions appear to be considerable, are called reticulated fibres. We cannot conclude our examination of bark without noticing the existence of a product which latterly has singularly occupied © the attention of botanists. We mean the — is-4—tiberine filres of latex and the /aticiferous vessels.