Includes the external covering of all herbaceous growths, viz., the epidermis, stomata, hairs, glands, cuticle, etc., organs which in older stems give place to bark.
676. The epidermis (skin) consists of a layer of united, empty cells, mostly tabular, forming a superficial membrane. It invests all plants higher than mosses, and all parts save the extremities, the stigma and rootlets. Its office is to check evaporation.
5S2, Cells of epidermis with a stoma from leaf of Helleborus foetidus. 583, Vertical section of a stoma of Narcissus; a, cuticle. 584, Epidermis cells with stomata of Tradescantia Vir-ginica.
677. Example. That delicate membrane which may be easily stripped off from the leaf of the houseleek or the garden iris is the epidermis. It is transparent, colorless, and under the microscope reveals its cellular structure.
678. Stomata. The epidermis does not entirely exclude the tissues beneath it from the external air, but is cleft here and there by little chinks called stomata (mouths). Each stoma is guarded by a pair of reniform cells, of such mechanism (not well understood) as to open in a moist atmosphere and close in a dry.
679. Position of stomata. The stomata are always placed over and communicate with the intercellular passages. They are found only on the green surfaces of parts exposed to the air, most abundant on the under surface of the leaves. Their numbers are immense. On the leaf of garden rhubarb 5,000 were counted in the space of a square inch; in the garden iris, 12,000; in the pink, 36,000; in Hydrangea, 160,000.
885, Cells and stomata of the epidermis of Oxalis violacea; and 5SG, of Convallaria racemosa.
680. Cuticle. The surface of the epidermis at length becomes itself coated with a delicate, transparent pellicle, not cellular, called the cuticle. It varies in consistency, being thicker and stronger in evergreen and succulent plants. It seems to be merely the outer cell wall of the epidermis thickened and separated from the newly-formed wall beneath it
681. The hairs which clothe the epidermis are mere expansions of its tissue. They may each consist of a single elongated cell, or of a row of cells. They may also he simple, or branched, or stellate, or otherwise diversified.
682. Glands are cellular structures serving to elaborate and contain the peculiar secretions of the plant, such as aromatic oils, resins, honey, poisons, etc. A gland may be merely an expanded cell at the summit of a hair, or at its base, and hence called a glandular hair (Labiatae). Or it may be a peculiar cell under the epidermis, giving to the organ a punctate appearance, as in the leaf of lemon. Other glands are compound and either external (sundew), or internal reservoirs of secretion (rind of orange).
683. Stings are stiff-pointed, 1-celled hairs expanded at base into a gland containing poisonous secretion. An elastic ring of epidermal cells presses upon the gland so as to inject the poison into the wound made by its broken point (nettle).
684. Prickles are hardened hairs connected with the epidermis alone, thus differing from spines, which have a deeper origin. Examples in the rose.
5S7, Rootlet of Madder, showing cells expanded into fibrillae. 588, Glandular hair of Fraxi-nella, section. 5S9. Hair of Bryonia, of several cells. 590, Hair of several cells, surmounted by a gland, of Antirrhinum majus. 591, Sting of Urtica dioica. 592, Jointed hair of the stamens of Tradescantia. 593, Stellate hair from the petiole of Nuphar advena (magnified 200 diameters, Henfrey). 594, Branched hair, one cell, of Arabis.