Stomata are often absent, absorption and excretion of gases in solution being carried on through the epidermal layer.
In a few cases there is a special surface or epidermal layer, but usually all the outer layers of the stem are composed of brown, thick-walled, lignified, prosenchymatous, fibre-like cells forming a peripheral stereom (mechanical or supporting tissue) which forms the outer cortex.
Mention may be made here of a class of epidermal organ, the hydaihodea, the wide distribution and variety of which have been revealed by recent research.
In one type they may take the form of specially-modified single epidermal cells or multicellular hairs without any direct connection with the vascular system.
The epidermal, conducting and strengthening tissues show on the other hand considerable modifications both in form and structure.
The uppermost is a purely muscular cell from the sub-umbrella; the two lower are epidermo-muscular cells from the base of a tentacle; the upstanding nucleated portion forms part of the epidermal mosaic on the free surface of the body.
U.e., Upper epidermal cells, with (c) cuticle.
The latter ultimately reaches the external air by diffusion through the stomata, whose dimensions vary in proportion as the amount of water in the epidermal cells becomes greater or less.
Chloroplastids are frequently present in the epidermal cells, as in some shade plants.
Similarly, the small amount of cuticular and of epidermal protection, and of lignification in succulent halophytes may also be related to the same circumstance.
Their stomata are frequently not limited to the underside of the leaves, but may occur scattered all over the epidermal surface.
The epidermal cells may contain chlorophyll.
Organs which respond to the mechanical stimulus of contact are found to possess special contrivances in certain of their cells(I) sensitive spots, consisting of places here and there on the epidermal cells where the wall is thin and in close contact with protoplasmic projections.
A, Epidermal cells of Saxifraga hirsulum.
5, B, shows a photograph taken from life through the epidermal cells of Tradescantiafluminenus.
Each fibre is formed by the outgrowth of a single epidermal cell of the testa or outer coat of the seed.
The nervous system is thus essentially epidermal in position and diffuse in distribution; but an interesting concentration of nerve-cells and fibres has taken place in the collar-region, where a medullary tube, closed in from the outside, opens in front and behind by anterior and posterior neuropores.
The dorsal epidermal nerve-tract is continued in front into the ventral wall of the collar nerve-tube, and at the point of junction there is a circular commissural thickening following the posterior rim of the collar and affording a special connexion between the dorsal and ventral nervetracts.
The crural glands, which occur in many terrestrial Arthropods, are epidermal in origin and totally distinct from the coxal glands.
Posteriorly beneath the posterior adductors, and covered only by a thin layer of elongated epidermal cells, are the visceral ganglia.
They form a superficial mycelium on the surface of the plant, the hyphae not usually penetrating the tissues but merely sending haustoria into the epidermal cells.
Such obligate parasites may be epiphytic (Erysipheae), the mycelium remaining on the outside and at most merely sending haustoria into the epidermal cells, or endophytic (Uredineae, Ustilagineae, &c.), when the mycelium is entirely inside the organs of the host.
The exterior of the culms is more or less concealed by the leaf-sheaths; it is usually smooth and often highly polished, the epidermal cells containing an amount of silica sufficient to leave after burning a distinct skeleton of their structure.
Epidermal appendages are rare, the most frequent being marginal, saw-like, cartilaginous teeth, usually minute, but occasionally (Danthonia scabra, Panicum serratum) so large as to give the margin a serrate appearance.
These differences arise mainly from the different arrangement of the constituent elements into which the epidermal cells are modified.
The leaves have a single main bundle, and in the mesophyll are four longitudinal series of large intercellular spaces separated by transverse diaphragms. The sporangia, which are situated singly on the adaxial surface of the leaves, between their insertion on the stem and the ligule, arise from a considerable number of epidermal cells.
As pairs in each ring-like segment or somite of the body, and some of these are in all cases retained as gonoducts and often as renal excretory organs (green glands, coxal glands of Arachnida, not crural glands, which are epidermal in origin); but true nephridia, genetically identical with the nephridia of earthworms, do not occur (on the subject of coelom, coelomoducts and nephridia, see the introductory chapter of part ii.
The black represents the cuticular product of the epidermal cells of the ocular area, taking the form either of))...,r,, f lens, cl, of crystalline body, cry, or of rhabdom, rhab; hy, hypodermis or epidermal cells; corn', laterallyplaced cells in the simpler stage, A, which like the nerve-end cells, vit' and ret', are corneagens or lens-producing; corn, specialized corneagen or lens-producing cells; vie, potential vitrella cells with cry', potential crystalline body now indistinguishable from retinula cells and rhabdomeres; vit, vitrella cell with cry, its contained cuticular product, the crystalline cone or body; ret', rhab', retinula cells and rhabdom of scorpion undifferentiated from adjacent cells, vit'; ret, retinula cell; rhab, rhabdom; nf, optic nerve-fibres.
The ovary adheres firmly to the seed in the interior, so that on examining a longitudinal section of the grain by the microscope the outer layer is seen to consist of epidermal cells, of which the uppermost are prolonged into short hairs to cover the apex of the grain.
These seeds are richer A, Epidermal cells.
It has been found useful in some cases to examine microscopically the thin film of coal that often covers the pinnae of fossil fronds, in order to determine the form of the epidermal cells which may be preserved in the carbonized cuticle; rectilinear epidermal cell-walls are usually considered characteristic of Cycads, while cells with undulating walls are more likely to belong to Ferns.
This distinction does not, however, afford a safe guide; the epidermal cells of some ferns, e.g.
The guard-cells contain chlorophyll, which is absent from typical epidermal cells, the latter acting as a tissue for water storage.
They possess (save for certain Archiannelida, most Hirudinea, and other very rare exceptions) setae or chaetae implanted in epidermal pits.
The central mass soon becomes differentiated into an outer epidermal and a dermal layer of flat-cells.
B, Sub-epidermal fibres.