An introduction to physiological and systematical botany/Chapter 3
CHAPTER III.
OF THE CUTICLE OR EPIDERMIS.
Every part of a living plant is covered with a skin or membrane called the cuticle, which same denomination has been given by anatomists to the scarf skin that covers the animal body, protecting it from the injuries of the air, and allowing of due absorption and perspiration through its pores.
There is the most striking analogy between the animal and the vegetable cuticle. In the former, it varies in thickness from the exquisitely delicate film which covers the eye, to the hard skin of the hand or foot, or the far coarser covering of a Tortoise or Rhinoceros; in the latter it is equally delicate on the parts of a flower, and scarcely less hard on the leaves of the Pearly Aloe, or coarse on the trunk of a Plane tree. In the numerous layers of this membrane continually peeling off from the Birch, we see a resemblance to the scales which separate from the shell of a Tortoise. By maceration, boiling, or putrefaction, this part is separable from the plant as well as from the animal, being, if not absolutely incorruptible, much less prone to decomposition than the parts it covers. The vital principle, as far as we can judge, seems to be extinct in it.
The cuticle admits of the passage of fluids from within as well as from without, but in a due and definite proportion in every plant: consequently it must be porous; and the microscope shows, what reason would teach us to expect, that its pores are different in different kinds of plants. In very succulent plants, as Aloes, a leaf of which being cut off will lie for many weeks in the sun without drying entirely, and yet when partly dry will become plump again in a few hours if plunged into water, the cuticle must be very curiously constructed, so as to admit of ready absorption, and very tardy perspiration. Such plants are accordingly designed to inhabit hot sandy countries, where they are long exposed to a burning sun, with very rare supplies of rain.
This part allows also of the passage of air, as is proved by experiments on the functions of leaves. Light probably acts through it, as the cuticle is a colourless membrane. We know the effects of light to be very important in the vegetable œconomy.
But though this fine membrane admits extraneous substances, so as to have their due effect upon the vegetable constitution, according to fixed laws, it no less powerfully excludes all that would be injurious to the plant, either in kind or proportion. Against heat or cold it proves, in general, but a feeble defence; but when clothed with hair or wool, it becomes a very powerful one. Against the undue action of the atmosphere it is so important a guard, that, when any tender growing part is deprived of it, the greatest mischiefs ensue. It forms in the Vegetable, as well as the Animal, a fine but essential barrier between life and destruction.
Some have imagined that the cuticle gave form to the vegetable body, because it sometimes being over tight causes contractions on the stem of a tree, as in the plum or cherry, and because it is found to be cracked wherever an unnatural excrescence is produced on the bark. No doubt the cuticle is formed so as to accommodate itself only to the natural growth of the plant, not to any monstrosities, and those lumps cause it to burst; just as it happens to ripe fruits in very wet seasons. Their cuticle is constructed suitably to their usual size or plumpness, but not to any immoderate increase from too great absorption of wet. If the cuticle be removed from any part, no swelling follows, as it would if this membrane only kept the tree in shape.
The extension of the cuticle is astonishing, if we consider that it is formed, as Grew well observes, on the tenderest embryo, and only extended during the growth of the plant, and that it appears not to have any connexion with the vascular or living part of the vegetable body. But though so accommodating in those parts where it is wanted, on the old trunks of most trees it cracks in every direction, and in many is entirely obliterated, the old dead layers of their bark performing all the requisite offices of a cuticle.
M. Mirbel indeed, though he admits the importance of this part in the several ways above mentioned, contends that it is not a distinct organ like the cuticle of animals, but merely formed of the cellular parts of the plant dilated and multiplied, and changed by their new situation. This is very true; but upon the same principle the human cuticle can scarcely be called a distinct organ. Its texture is continually scaling off externally, and it is supplied with new layers from within. Just so does the cuticle of the Birch peel off in scales, separable, almost without end, into smaller ones.
Examples of different kinds of cuticle may be seen in the following plants.
On the Currant tree it is smooth, and scales off in large entire flakes, both from the young branches and old stem. The same may be observed in the Elder.
The fruit of the Peach and the leaf of the Mullein have a cuticle covered with dense and rather harsh wool, such as is found on many Mexican plants, and on more Cretan ones. The latter we know grow in open places under a burning sun.
The leaf of the White Willow is clothed with a line silky or satiny cuticle.
The cuticle of the Betony, and of many other plants, is extended into rigid hairs or bristles, which in the Nettle are perforated and contain a venomous fluid.
On the fruit of the Plum, and on many leaves, we find a blueish dry powder covering the cuticle, which is a resinous exudation, and it is difficult to wet the surface of these plants. Rain trickles over them in large drops.
In the Cork tree, the Common Maple, and even the Dutch Elm, the cuticle is covered with a fungous substance most extraordinary in its nature, though familiar to us as cork.
In grasses and some other plants the ingenious Mr. Davy has found a flinty substance in the cuticle.
What seems to be the cuticle on the trunk of the Plane, the Fir, and a kind of Willow called Salix triandra, rather consists of scales of bark, which having performed their functions and become dead matter, are rejected by the increasing bark beneath them; and this accords with M. Mirbel's idea of the cuticle. The old layers of bark in the Chesnut, Oak, and many other trees, though not cast off, are of the same nature; and these under the microscope exhibit the same cellular texture as the real cuticle.