VEGETABLE PHYSIOLOGY. 
227 
by tbe formation of its cuticular layers, will the leaves be found soft and tender, as in many Mesembryan- 
themums, or tough and resisting as in the Aloes. 
S imil ar differences in the epidermis give to leaves most of their prominent external characteristics. 
I have already said the general texture chiefly depends upon it; and not only is tins the case, but 
what is called the clothing of leaves is a part of the epidermal system. Hans are merely certain of 
the epidermal cells which have grown out into long filaments, and not unfrequently become subdivided 
so as to form jointed strings of cells, simple or branched. All the various gradations of clothing, from the 
slight hairiness to the dense felt-like covering, result from a varying degree of the elongation of the epei- 
dermal cells into hairs; for the felt is shown, like artificial felt, by the microscope, to be composed of a mat¬ 
ted interlacement of hairs. The spiny points or prickles at the tips or on the teeth of leaves are merely 
epidermal cells developed into a woody condition ; and what is called the scabrous condition of the surface 
is in like manner produced by a number of the epidermal cells growing out into little hard woody points. 
The innumerable other modifications that are met with will be readily understood from these few indications. 
The more delicate leaf-like organs of plants, such as the petals, are of similar structure to the green 
leaves, only all is more delicate there. The epidermis is very tender, and often most elegant in its 
conformation ; the sides of the cells being frequently waved, or toothed and dovetailed into one another. 
The glistening surface of many petals arises from the epidermal cells being each slightly swollen and 
protuberant, like a little dome, on the free surface, in a manner which .may be roughly compared to 
the projections on the surface of a pebble pavement; and the velvet-like surface is a result of these 
outer sides growing out as exceedingly delicate hairs. None of the fiat epidermal cells contain any 
colouring matter ; and thus, by their smoothness and transparency, they act like a coat of varnish to the 
surface, giving the colours beneath their beautiful brilliancy and clearness. 
The surface of all green parts, such as that of young shoots, leaf-stalks and the like, of fleshy 
leafless plants like the Cacti, &c., are all clothed by an epidermis like that of the leaves, and continuous 
with it, so as to form a perfect skin all over the green parts of the plants. The cellular tissue beneath is 
spongy, like that forming the parenchyma or internal substance of the leaves, and shares its active 
operations hi the respiration and other processes of life. 
As the stems of woody plants grow older they lose their green colour on the surface, and become 
brown and rough. This arises from the production of a new structure beneath the original epidermal 
cells, the cork-cells, wliick are usually squarish in form, thin-sided but strong, and contain no solid or 
fluid matter in their interior. Layer after layer of these is produced, till by their thickness they 
conceal the still green spongy tissue beneath, and as the plant grows older, first the original epidermal 
cells, then one after another layers of the cork-cells decay from the action of the atmosphere, so as to 
leave a rough surface, which is undergoing continual reparation by the formation of new cork-cells 
below. At least this is mostly the case. Sometimes, however, the decaying cork is not replaced, and 
the liber thus exposed to the ah' becomes affected hi like manner, but is repaired continually below, as 
it wears off on the surface. This kind of growth gives rise to the stringy surface of certain stems, as, 
for example, of the Vine and the wild Clematis. 
Little rough points may be noticed on the sin-face of shoots which are losing their green tint, or 
have already become brown. These are the bodies called lenticels, and consist of little blister-like 
disruptions of the cork-cells ; the spongy layer beneath grows out a little, and the cork-cells give way 
before the pressure, so as to tear irregularly and leave little free spaces, by which ah- can make its 
way to and from the green cells beneath. These bodies are supposed to supply the place of the 
stomates here, as these do not exist in the corky layers. 
The epidermis of the roots of plants differs remarkably from that of the parts exposed to the ah- by 
the absence of stomates, which are unnecessary here, and would only interrupt that continuity which is 
necessary to prevent the passage of any solid matter into the interior. It is very delicate upon all 
growing points, and may be regarded as a kind of filter through which the nutrient fluid passes. The 
tender hair-like fibres clothing the roots of certain plants (not the very fine branched extremities) are 
exactly similar to hairs in their nature; they are epidermal cells, which grow out from the general 
surface, and they decay and fall off as the root grows older and its epidermis tougher. 
It may be mentioned, before concluding this part of the subject, that leaves growing beneath the 
surface of water, always thin and delicate in structure, are frequently filamentous and feathery, but 
even when flat and expanded have no stomates in then- epidermis, which, with its thin outer 
walls, does not form such a barrier to the passage of gases or fluids, as the firmer epidermis of 
aerial organs. 
A peculiar structure is noticed also in the aerial roots of the epiphytic Orchidacese. The cells about 
then- delicate absorbing tips are strengthened and kept tense by slender spiral fibres loosely coiled up 
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