6

We regret that the length to which this Article has already extended, will not allow us to enter in detail upon the interesting subject of the Second Book, The internal structure of vegetables, or the anatomy of the plant'; considered in connexion with that of the Fourth Book, The process of development.' On the first of these subjects, we must limit ourselves to a single quotation upon the anatomy of the bark. Having previously treated of the Epidermis, or outer membrane, which from its resemblance to that of the animal, has been designated by the same name,' Mr. Keith thus proceeds:

The corticle layers, or interior and concentric layers, constituting the mass of the bark, are situated immediately under the cellular integument, where such integument exists, and where not, immediately under the epidermis...... The outer layers are coarse and loose in their texture, exhibiting individually a conspicuous and considerably indurated but very irregular net-work, composed of bundles of longitudinal fibres......But the intersection of the fibres does not always take place in the same way in all plants, so that the net-work peculiar to one species, is often very different from that of another. This may be seen from comparing together the net-work of the barks of the Oak and Elm......The inner layers are soft, smooth, and flexible, and capable of subdivision till reduced to an absolute film, but not always exhibiting a conspicuous net-work, at least till macerated in water, or exposed to the action of the atmosphere......The innermost of the layers is denominated the Liber, the Latin term for a book, from its having been used by the antients to write on before the invention of paper. It is the finest and most delicate of them all, and often most beautifully reticulated. But the Liber of Daphne Lagetto [the Lacebark Daphne] is remarkable beyond that of all other plants for the beauty and delicacy of its net-work, which is not inferior to that of the finest lace, and at the same time so very soft and flexible, that in countries in which the tree is a native, the lace of the Liber is often made to supply the place of a neckcloth! I. pp. 325-327.

*'

Our other quotation shall be upon The process of development,' which will give a fair specimen of the interesting physiological experiments detailed in these volumes. If a perennial be dissected at the end of a year, it will be found that an augmentation has taken place in diameter by the addition of a new layer of wood between the bark and the wood of the former year. It was long a subject of controversy with physiologists, whether this new ligneous layer was deposited by a secretion from the

*It may not be out of place to mention an admirable method for the exhibition of the different kinds of Barks, at the Museum of Natural History at Berne. It consists of sections of forest trees, which are arranged like books in a Library. The bark forms the back of each ligneous Volume, and is inscribed with the scientific name of the tree. The specimen, when taken from the shelf, exhibits the quality and structure of the wood. Rev.

bark or from the wood, or whether it might not be a conversion of the bark into wood. To ascertain this point, Du Hamel instituted the following beautiful experiment.

In order to ascertain whether the new layer of wood is formed from the former layer of wood or of bark, his first experiment was that of a graft par l'ecusson; which is done by means of detaching a portion of bark from the trunk of a tree, and supplying its place exactly by means of a portion of bark detached from the trunk of another tree, that shall contain a bud. In this way he grafted the Peach on the Prune tree, because the appearance of the wood which they respectively form, is so very different, that it could easily be ascertained whether the new layer was produced from the stock or from the graft. Accordingly, at the end of four or five months after the time of grafting, the tree was cut down, and as the season of the flowing of the sap was past, a portion of the trunk including the graft was now boiled, to make it part more easily with its bark; in the stripping off of which there was found to be formed under the graft a thin plate of the wood of the Peach, united to the Prune by its sides, but not by its inner surface, although it had been applied to the stock as closely as possible: hence Du Hamel concluded that the new layer of wood is formed from the bark, and not from the wood of the pre ceding year. The same experiment was repeated with the same result upon the Willow and Poplar; when it was also found that if a portion of wood is left on the graft, it dies, and the new wood formed by the bark is exterior to it. The above conclusion, therefore, is perfectly legitimate, which Du Hamel also strengthens by the following experiment:-Having detached a portion of bark from its trunk, and covered the wood below it with a thin plate of tin-foil, he then replaced the bark as before, reducing the case to the following dilemma; if the new layer of wood was formed from the old layer of wood, then it was plain that the new layer would be deposited within the tin-foil; and if it was formed from the bark, it was also equally plain that it would be deposited without the tin foil: the result accordingly was, that a new layer of wood was deposited between the bark and the tin-foil, but none between the tin-foil and the einterior layer. This experiment was completely decisive of the point in question; and yet there is an experiment of Dr. Hope's on the same subject, which is, if possible, still more convincing: Having made a longitudinal incision in the trunk of a Willow of three or four years old, so as to penetrate through the bark, he laid bare a portion of the stem, by stripping the bark to the one side, which was, however, still attached to the stem at the upper and lower extremities of the decorticated part; the detached portion of bark was then bent into the form of a hollow cylinder, by uniting its edges as closely as possible, and the whole well secured from the action of the atmos phere. The plant was then allowed to remain undisturbed for several years, when the result of dissection was, that new layers of wood were generated within the lateral cylinder of bark, while the decorticated portion of the stem remained unaugmented; the portions above and below being augmented, as in other ordinary cases of

vegetation. It is evident, therefore, that the additional layer by which the plant increases in diameter is generated from the bark. 11. pp. 218-220.

A further question, of considerable physiological interest, arises: Is the new layer thus formed, merely the indurated Liber of the former year; or is it a perfectly distinct substance, secreted by the bark, but not produced by the conversion of the bark into wood? From some delicate experiments of Du Hamel, it appears certain that the bark acts only as the organ of transmission of the secreted fluid, and that the new layer of wood is formed by the descent of the proper juice (or Cambium as named by Du Hamel) from the leaf through the vessels of the bark. It is highly probable, also, that this gelatinous substance, or fluid of organization, is secreted in two distinct concentric layers; one tending to the centre, and forming the Alburnum, or new wood, the other tending to the circumference, and constituting the Liber, or young bark.

Our limits forbid us to enter, at any length, upon the amusing part of these volumes, which treats of the products of vegetable analysis. In the Section upon' Resins,' we have the following notice, resting on the authority of Mirbel, respecting the Bloom, which is vulgarly attributed to the presence of animalcule.

Upon the epidermis of the leaves and fruit of certain species of plants, there is to be found a soft and glaucous powder. It is par ticularly observable upon cabbage leaves, and upon plums, to which it communicates a peculiar shade. It is known to gardeners by the name of bloom. It is easily rubbed off by the fingers; and when viewed under the microscope seems to be composed of small opaque and unpolished granules, somewhat similar to the powder of starch; but with a high magnifying power it appears transparent. When rubbed off, it is again reproduced, though slowly. It resists the action of dews and rains, and is consequently insoluble in water. But it is soluble in spirits of wine; from which circumstance it has been suspected, with some probability, to be a Resin." Vol. I. P. 438. See also p. 187.

Among the Gum-Resins, (which are obtained from vegetables by expressing and inspissating the juice, not by natural exudation,) we meet with the pigment so familiarly known in every drawing-box, under the name Gamboge. This substance exudes from incisions in the bark of the East Indian tree Mangostana Cambogia. The disagreeable Assafœtida (the concreted milky juice of the Persian Ferula assafætida) is also a Gum-Resin; which, however uninviting to European tastes, from its intolerably fetid odour, is used by the Indians as a seasoning for their food, and is called by them the food of the gods. With this we may contrast a Gum-Resin of more inviting character-Myrrh: it is concreted, in the form of tears, from

some unknown Abyssinian and Arabian plant, which Bruce refers to the Genus Mimosa; it is supposed to be the Acacia vera, or, as it has been called by some Botanists, the Mimosa Nilotica, the Egyptian Thorn.

Many vegetables are known to secrete Wax, in a form very little adulterated by extraneous substances. It exudes from the fruit of Myrica cerifera, Common American Candle-bury Myrtle, a plant which grows abundantly in Louisiana. The beautiful and singular phenomenon exhibited by the Dictamnus albus, or White Fraxinella, is supposed to be owing to a perspiration of Wax, which forms an inflammable atmosphere around the plant.

This plant is fragrant, and the odour which it diffuses around forms a partial and temporary atmosphere which is inflammable; for if a lighted candle, or other ignited body, is brought near to the plant, especially in the time of drought, its atmosphere immediately takes fire. This phenomenon was first observed by the daughter of the celebrated Linnæus, and is explained by supposing the partial and temporary atmosphere to contain a portion of Was exuded from the plant, and afterwards reduced to vapour by the action of the sun.' I. p. 428.

Wax is found in different plants in various states of concretion. When it has the consistency of butter, it is denominated Butter-of-Wax. The Butter of Cacao is expressed from the seeds of Theotroma Cacao, the Smooth-leaved Chocolate Nuttree: it is to this substance that chocolate owes its flavour and ¡ unctuosity.

Our last quotation shall be upon the useful substance Caoutchouc, more familiarly known by the name IndianRubber.

It is obtained chiefly from Havea Caoutchouc and Jatropha elastica [the Elastic Physic Nut], trees indigenous to South America; but it has been obtained also from several trees which grow in the East Indies, such as Ficus Indica [the Banyan-tree], Artocarpus inté grifolia [the Indian Jaca-tree], and Urceola elastica [a native of Prince of Wales' Island, and the coast of Sumatra].

If an incision is made into the bark of any of these plants a milky juice exudes, which, when it is exposed to the air, concretes and forms Caoutchouc. As the object of the natives in collecting lit had been originally to form it into vessels for their own use, it is generally made to concrete in the form of bags or bottles. This is done by applying the juice when fluid, in thin layers, to a mould of dried clay, and then leaving it to concrete in the sun or by the fire. A second layer is added to the first, and others in succession, till the vessel acquires the thickness that is wanted. This mould is then broken, and the vessel fit for use, and in this state it is generally brought into Europe. It has been brought, however, even in its milky state, by being con fined from the action of the air.......

Caoutchouc, when pure, is of a white colour, without taste, and without smell. The black colour of the Caoutchouc of commerce is owing to the method of drying the different layers upon the moulds on which they are spread. They are dried by being exposed to

smoke......

It seems to exist in a great variety of plants combined with other ingredients. It may be separated from resins by alcohol. It may be separated from the berries of the Misletoe by means of water, and from other vegetable substances by other processes. It is said to be contained both in opium and in mastic. But from these substances it cannot be extracted in sufficient quantities to make it worth the labour. It is applied to a great many useful purposes both in medicine and in the arts, to which, from its great pliability and elasticity, it is uncommonly well-adapted. In the countries where it is produced the natives make boots and shoes of it, and often use it by way of candle.' I. pp. 449-451.

Here we must close our extracts from the interesting matter of these volumes. It only remains for us to give our readers some information respecting the general merit of the work; and in doing this, our remarks shall be brief, because, from the general outline, and from the copious quotations which we have brought before them, a tolerably correct idea may be formed of what may be expected in the perusal of the whole. We do not hesitate in declaring our opinion that, in its general execution, it is highly creditable to Mr. Keith, and that it is well adapted both to advance his own reputation in the scientific world, and to promote the extension of knowledge in this department of Natural History. Very few facts with which we were not previously acquainted, are brought forward by Mr. Keith; but bis materials are drawn from the best sources, and he has given a comprehensive statement of the principal physiological phenomena in the vegetable creation, which are scattered throughout many volumes in the works of Linnæus, Grew, Bonnet, Hedwig, Du Hamel, Malpighi, Mirbel, Ellis, Knight, and many other philosophical writers upon the anatomy and functions of plants. It is always a somewhat invidious task to contrast rival works, by different living writers, each of which possesses its peculiar merits; we cannot, however, withhold an opinion, to the declaration of which the readers of our Review are, perhaps, entitled, that Mr. Keith's treatise by no means supersedes the elegant volume upon Physiological Botany, by the President of the Linnæan Society, although the latter is condensed into one half of the bulk of the former. In Dr. Smith's work there is a touching simplicity of style, a delicacy of expression, and an exquisite selection of illustrations, which have rendered his book a classical volume, and which will continue to ensure it a place in the library of every person of taste and science. In Mr. Keith's volumes there is more detail, which is not without its advantages;