are yet to be described; but I have already said, that these are only parts of the stem. I shall attend to the former when commencing the subject of leaves, and to the latter when we enter on that of flowers. 85 CHAPTER III. OF THE SURFACE. THE surface of plants is composed of a thin membrane named the Epidermis, or cuticle. The same terms are applied to the outer layer of the skin of animals, and, I doubt not, that, by taking a slight view of the latter, the nature of the former will be more clearly understood. After a blistering-plaster has been applied to the skin, a pellicle is raised from its surface, containing a limpid fluid. In other words, the skin is blistered; and the thin pellicle which we cut to let the water flow out, is the epidermis, or cuticle. To whatever part of the skin the vesicatory may be applied, a similar pellicle will rise, because the cuticle covers the whole body. It is elastic, generally transparent, insensible, and when destroyed is speedily regenerated: hence, a blistered surface becomes covered with new cuticle in a few days. In all animals it is composed of a substance whose chemical and other properties are the same as horn. It is indeed a modification of horn, and this material affords a remarkable example of the resources and variety of nature. Horn envelops the skin in form of this transparent delicate veil; it composes the shelly covering of the tortoise, the mail of the crocodile, the armour of the porcu pine, and the glassy vesture of the serpent. Modified into hair, wool, feathers, and scales, it forms the external covering of most animals; it composes the hoof of the ox, and the plumage of birds : the same substance which forms the claws of the lion, beams on the neck of the dove, and is fashioned into one of the loveliest ornaments, the tresses of the human head. As the cuticle in all animals is composed of the same substance or material, it is perhaps natural to conclude that the substance, whatever it may be, which forms the cuticle or epidermis of any one plant, forms that of all other plants, however dissimilar their surface may appear. Fourcroy supposed the epidermis of all trees to be formed of a substance the same as cork; but the experiments which led to that conclusion have not been related. Leaving this matter therefore in doubt, we shall attend to other particulars. The thickness of the epidermis in plants varies considerably. On the stem and branches it is tough, and often thick, leathery, and opaque; but on the leaves, flowers, and fruit, it is generally delicate and transparent. The same variety occurs in the animal epidermis; on the lip, for instance, it is extremely fine, on the heel thick and horny. In man, and many animals, it is insensibly renewed, the old cuticle falling off in bran-like or mealy scales; in some constantly, in others only when moulting. A similar process takes place in many plants, but that the detached pieces are CUTICLE OF SERPENTS. 87 larger. Examine a full grown currant-bush, and you will almost constantly find on its stem and branches flakes of old cuticle peeling off from new cuticle underneath. Serpents and lizards are said to change their skins annually, or oftener; but the cuticle only is changed; it is pushed off from the skin by a new one, splits at the head, and the animal escapes by its own exertions. But as plants have no locomotion, it may happen that a stem will be enwrapped by several cuticles at the same time, and of this the birch is a striking and familiar example. It is possible also that an inability to get rid of the old cuticle may in some instances prove injurious to the whole plant, and perhaps this is the case with the cork-tree (Quercus Suber). The cuticle, or at least the surface of the cuticle of that species of oak, is thick and fungous, and of it the corks in common use are inade. When the tree is left to nature, it seldom lives longer than fifty or sixty years; but when the cuticle is stripped off every eighth or tenth year, it will live above a century and a half. If the cuticle be removed from a permanent part of a tree or shrub, as the stem or branches, a new cuticle will form in its place; but if from a leaf, flower, fruit, or any other part not permanent, or from any part of an herbaceous plant, no reproduction follows. In the fir, and some other trees, the surface is that in this instance there is no real cuticle, but that its place is supplied by flakes of dead bark. This opinion, however, I should think very quesționable, and would be inclined to consider this state of the surface as a modification of the epidermis analogous to the scaly appearance often assumed by the cuticle of animals, such as we observe on the tail of the rat and beaver. In plants, as in animals, the cuticle is often naked, but it often also is variously clothed; being in some hairy, in others woolly, in others warty, and so on. We may now attend to these varieties of the surface. Nitidus, polished and shining, as if varnished; as in holly, laurel, and box. Glaber, smooth or bald. Lævis or levis, smooth or even. A surface clothed with any kind of hair, bristles, or down, or rendered uneven by furrows, lines, or risings, cannot, with strict propriety, be called smooth. A freedom from covering, however, and a freedom from inequalities, constitute different species of smoothness, which the present terms express. Glaber means free from any sort of covering; lævis from inequalities; hence CAULIS glaber, means a stem free from hairs, bristles, &c.; CAULIS lævis, a stem, free from furrows, channels, or risings of any description. Scaber, rough. |