VEGETABLE PHYSIOLOGY. 
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By ARTHUR HEXFREY, Esq., F. L. S., Lecturer ox Botaxy at St. George’s Hospital. 
THE EPIDERMUS, OR SKIN. 
I'J 7E have now examined sufficiently, for our purpose, the general character of the structures 
JA forming the chief part of the substantial and permanent organs of plants. The peculiarities of 
leaves and of the green herbaceous tissues forming the surface of young parts in general, has been hut 
lightly dwelt on, since they possess a special kind of structure which has not yet been spoken of. 
All parts of a plant are closed up and protected from the direct action of the atmosphere, by a 
layer of cellular tissue, having its own peculiar organization, and called the epidermis, or skin. The 
very lowest plants, indeed, those composed of mere layers or filaments of cellular tissue have, of course, 
no distinct epidermis; and even the leaves of the Mosses and allied tribes have none, since they are 
simply single or double layers of flattened cells, closely joined together, and may thus ahnost he 
compared with the epidermis of the more highly organized leaves, which consists of a similar layer of 
flattened cells, applied together closely, like the flag-stones of a pavement, and forming a coat or skin 
completely enveloping and protecting the loose, open, and irregular cellular tissue within. 
This epidermis may he readily stripped off the leaves or green stalks of many plants, especially of 
the Monocotyledons, as for example, from the leaf of the Hyacinth or Iris, and then appears as an 
extremely thin, transparent, and colourless membrane, 
like a piece of very fine gold-beater’s skin. When placed 
beneath a microscope this exhibits the tesselated appear¬ 
ance resulting from its cellular nature, and is found to * 
be composed of a single layer or plate of very much 
flattened cells. The microscopic examination, however, 
reveals something more than this; we find numerous 
bodies of peculiar form interspersed among the cells, and 
a careful examination shows that these consist of pairs 
of somewhat kidney-shaped cells, much smaller than the 
rest, with a slit-like opening passing down between 
them, right through the membrane. These are the sto- 
a b c 
A and B, stomates of the epidermis of the Iris. A, 
a perpendicular slice through a stomate, showing also 
a few of the loose cells, b b, beneath ; a a are the two 
kidney-shaped cells bordering the slit c. B, a view of a 
stomate from above; the epidermis is composed of ob¬ 
long cells, much larger than the stomate-cells, and only 
part of their walls in view. C, stomate and epidermis 
cells of an apple leaf; a a, cells of the stomate; b b, 
cells of the epidermis; c, the slit-like opening or breath¬ 
ing pore of the stomate. 
mates or breathing pores, and are pairs of cells forming 
the borders of a mouth-like orifice, through which the 
ah’ or gases may pass in or out, as may be required in 
the chemical changes going on in the spongy cells below. 
A leaf of the kind just mentioned is composed of a quantity of irregularly formed cells, but loosely 
packed so as to leave large spaces between them, and containing green colouring matter and other 
substances hi their interior. This spongy tissue is traversed by fibrous cells, here of the spiral vessel 
character, as may be seen by carefully breaking a Hyacinth leaf across and drawing the pieces 
asunder, when the elastic spiral fibres will be drawn out between the fractured ends. The fibrous 
bundles form the veins of the leaf. The whole is covered up by a complete sheath of epidermis or 
skin, giving the smooth even surface to the leaf. The stomates exist here both on the upper and under 
surface ; generally they are most abundant beneath, excepting in leaves floating upon water, in which 
they are naturally all upon the upper exposed face. 
Leaves of firmer texture, such as those of the Laurel or the Iw, differ little in the essential nature 
of then* structure; the veins are stronger, containing a certain amount of true wood-cells, but the 
principal difference in the consistence of the leaves depends upon the condition of the epidermis. The 
hard, firm, and often shining surface is produced by the thickening of the inside of the walls of the 
cells of the epidermis, which become thus partially filled up by deposits, not applied like those 
thickening the wood-cells, on all sides equally, but almost exclusively on the free side, next the 
external air; then, as the cells are close together, these outer walls combine to form a continuous, 
thickened, often leathery or horny covering over the outside of the leaf, which is now usually 
distinguished by the special name of the cuticle. The outside of the cuticle, exposed to the air, 
frequently undergoes a chemical change so as to be converted into a wax-like substance, or, as some 
suppose, tiffs waxy matter is secreted through the cuticle, and solidifies as it exudes upon the outer surface. 
In the Grasses and Horse-tails, it becomes strengthened by a deposit of silica or flint. 
In the thick fleshy leaves, such as those of the Ice-plants, the spongy substance of the leaf is much 
more abundant than in the flattened kinds ; and according as the epidermis is more or less thickened 
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