VEGETABLE.] HISTOLOGY 17 may be considered physiologically to belong to the epidermal system. These will be described as hypoderma and colleuchyma under the ground tissue. Epidermis. The cells of the single layer forming the epidermis mis. vary in shape, but usually the form is determined by the shape of the part on which they are developed, being elongated on long leaves, broad with straight or wavy margins on broad leaves. Usually the cells of the epidermis, although parenchymatous, have no intercellular spaces, except in Osmunda and Todca, and a few other rare instances. The only openings are those in the stomata,schizogenous intercellular spaces, between the special cells (guard cells) of the stoma. In many plants, as monocotyledons and needle-leaved conifers, the epidermal cells contain no chlorophyll ; but in ferns and in many dicotyledons, as has been shown by Stoe hr (Bot. ZeiL, 1879, p. 581), chlorophyll is present. Not unfrequently authocyan fills the epidermal cells, and completely obscures the green colour of the chlorophyll-bearing cells below. The outer wall of the epidermal cell is usually greatly thickened and corky, forming the cuticle, which generally forms a continuous sheet separable by the action of caustic potash from the rest of the wall below. In applying Schultz s solution to a thin section of an epidermal cell, the outer layers become brown, while the inner give the reaction of cellulose. The outer layers are soluble in boiling caustic potash and in nitric acid and chlorate of potash, but insoluble in sulphuric acid and in ammoniacal solution of cupric oxide. Many of the cells have a marked deposit of mineral matter, more particularly silica (Equiseturri), in their walls. See Nageli and Schwendener, Das Mikroskop (2d ed.), p. 489. x. Wax is frequently produced : either it is on the surface of the cuticle forming a variously constructed coating, or minute particles are embedded in its texture. The chief modiiications are described by De Bary (Vcrglcichcnde Anatomic, p. 86): (1) a layer or crust, cither thin, homogeneous, and transparent, or thick and striated, the former seen in Sempervivum, the latter in the waxpalm (Klopstockio.) ; (2) a coating of rod-like particles placed perpendicularly to the sur face, eitherclosely placed or somewhat loose and irregular (Saccharum, Musa, and Scitaminecc) ; (3) a layer of granular particles, close or widely separated, and not placed one over the other (Allium, Acer, Vitis), &c. ; and (4) irregular granules piled up one over the other in several layers, as in Eucalyptus, Ricinus, Abies pectinata, &c. nata. Stomata (De Bary, Vergleich. Anat., p. 36 sq.) are the openings in the epidermis which permit the entrance and escape of gases. They are formed by two semilunar cells, the guard cells, with the pore or intercellular space between them, the pore opening into a large air-space in the tissue below, and in communication, by means of the small intercellular spaces of the parenchyma, with most of the tissues of the plant. The stomata are found on those parts above ground exposed to air and light, hence chiefly on the leaves and tender green stems of plants. On leaves they are most abundant on the under side, and are generally absent from the upper surface. In many leaves, however, especially of monocotyledons, they are equally distributed on both sides, and in water-plants with floating leaves they are abundant on the upper side but absent from the lower. They rarely occur on submerged water-plants and never on roots. As a rule the stomata are irregularly scattered, but in some plants, as in Equisetum, they occur in tolerably regular longitudinal rows on the stem. Usually the stomata consist of only two cells, the guard cells, or of two pairs of guard cells (Equisetum) one over the other, or there are many, as in the peculiar stomata of Marchantia. In some plants two or more additional cells, the accessory cells, are formed. These accessory cells differ from those of the epidermis on the one hand, and from the guard cells on the other. The posi tion of the stoma varies. It is sometimes at the end of the long epidermal cells, as in the hyacinth, or at the side, in a few cases free in the centre of the epidermal cells (Anemia, &c. ). The guard cells may be on a level with the epidermis ; rarely they project slightly ; but frequently they are depressed below the surface. The guard cells often contain chlorophyll and starch, the outer wall is often thick ened, and occasionally even wax forms on their surface ; but as a gene ral rule no wax forms, and thus, when a thick coat of wax is developed, narrow canals through it indicate the position of the stomata. Development of Stomata. In long epidermal cells (hyacinth) a portion is cut off at one end by cell-division, and forms the mother- cell of the stoma. It then divides into two daughter-cells, each form ing one of the guard cells. The lamella between the two splits, either from without inwards or within outwards, and forms a schizogenous intercellular space. "When the epidermal cells are not elongated ((Enothcra, Silene, &c. ), a portion of the epidermal cell is cut off at one part by a bent wall. This is the mother-cell of the stoma, and either forms the daughter-cells immediately, or may divide by segments cut off at one side and then at the other side, either one, two, or more times before the central cell divides to form the daughter-cells which form the guard-cells of the stoma. The other cells cut off on each side are the accessory cells. In other cases the accessory cells have a different origin, being cut off from the neighbouring epidermal cells after the guard-cells are formed. In stances of the former may be seen in Crassulaccce, Crucifcrce, and Papilionacece ; of the latter in Juncaccce, Cyperacca?, and Gramincix. In Anemia and some other ferns a cell is formed inside the epidermal cell, cutting a cylindrical piece out of it. This divides and forms the guard-cells of the stoma. Two kinds of stomata exist in many plants. The one kind, already Water fully described, are the air-stomata, to distinguish them from the stomata second kind, the water-stomata. The latter occur in many plants on the leaves, immediately over the ends of the nbro-vascular bundles, near the margin on the upper surface, and often on the serrations of the margin itself. They give off water, during a portion of the life of the leaf, which appears on the surface in the form of drops, under the action of root-pressure. They are at once distinguished by their large she, and by their not opening aud closing like air-stomata. Hairs (De Bary, Vergleich. Anat., p. 58) are usually out-growths Hairs, of single epidermal cells ; but occasionally some of the cells below the epidermis assist in the construction of large massive hairs or emergences, as they are called by Sachs. Hairs vary very much in construction, size, and appearance, and not unfreqnently different kinds of hairs occur mixed together on the same part of the plant, although in many instances only a single characteristic variety of hair may be developed on the epidermis. De Bary distinguishes several typical varieties of hairs: (1) hairs proper ; (2) papula?, short rounded sac-like structures ; (3) scales ; (4) villi ; and (5) warts or prickles. The simplest hairs are outgrowths of single epidermal cells, having the cavity either continuous with that of the epidermal cell, or cut off by a wall. Long cylindrical unicellular hairs occur in cotton ; and on most roots root-hairs, with thin or sometimes with peculiarly and irregularly thickened walls ( Viola tricolor}. The cells may divide and form a moniliform hair, as in Tradescantia, or much more complex branched ( Vcrbascum thapsus) or club-shaped and glandular hairs may be produced. Flat, dry scales, either unicellular or multicellular, are seen in Deutzia, Ehcagmis, and in many ferns. Papulae are mentioned by De Bary as occurring on Jiochea, Begonia, Piper, Ampclopsis, and others. The villi or colleteres occur on bud-scales and buds, while spiny hairs or warts occur frequently as the prickles of the rose and bramble, and in Dipsacns, Smilax, &c. The walls of hairs are often thin, and composed of nearly pure cellulose, or thickened and stratified in various ways, with an outer cuticular layer. The thickening is either general or local, and may assume the forrr of pores or spiral striation (hairs on stamen of Bulbine aloidcs), or may form peculiar warts or nodules. Silicious hairs (Deutzia), or hairs containing lime, sometimes occur. In some cases the hairs (nettle) are supported on cellular, elevations of the epidermis. These may be distinguished as the accessory cells of the hair. Glandular hairs are of frequent occurrence, the end cell or cells secreting some ethereal oil or resin ; the secretion collects below the cuticle, and either it remains there, causing the absorption of the secreting cells, or the cuticle ruptures. The villi or colleteres are peculiar many-celled glandular hairs on young leaves, stipules, or bud-scales (llibes, Viola, Polygoimm, ^Esculus), and secreting a gum or resin. Frequently the secretion of these colleteres is sup plemented by the formation of a resin from below the cuticle of the epidermis, forming the gelatinous secretion covering buds, termed blastocolla (horse-chestnut). In some plants, as Pop/iilus, the blastocolla is formed by the epidermal cells alone, in others both by the colleteres and epidermis. Beneath the epidermis the cells are often peculiarly modified to Second- form the hypoderma and collenchyma ; but as these belong to the ary ground system of tissues they are described below. The secondary epidern: epidermal tissues, or the covering that replaces the epidermis on the tissue, perennial parts, consists largely of cork, either in the form of a thin Periderr layer or in repeated layers developing deeper and deeper in the tissues and of the stem or root and forming the massive bark or rhy tidome. Cork rhy ti- tells arise usually from the cortical cells, i.e., those of the ground dome, tissue placed a short distance below the epidermis (Popuhis, Sambucus). In other cases the cork forms still deeper, among the green chlorophyll -bearing cells of the cortex, as in Rubus Idceus, Ribcs, &c. Rarely the cork-cells arise from the epidermis itself (Salix). In all cases cork is formed by the division of the cells of the cortex or epidermis by a tangential wall, separating the mother- cell into two daughter-cells. The outer cell becomes corky, rapidly losing its contents and becoming lilled with air ; while the inner one retains its protoplasm and forms new cork-cells by division. The formation of cork does not necessarily begin at all parts of the circumference simultaneously, but sooner or later a complete layer of cork is formed. When the layer has become a few cells thick, it is known as the periderm ; while the active cells from which it arises are distinguished as the cork cambium or phellogen. Inside the cork cambium new cells are often formed, which contain chloro phyll, and are known as the phellodenna (Fagus, Salix), such cells being also formed by division of the cork cambium. After the forma tion of the periderm, as is easily seen in the stem of the black currant, the whole of the epidermis and of the ground tissue immediately below becomes withered, and is thrown off. In the formation of bark, the layers of cork form repeatedly in the cortical tissue of the stem, and even in the bast portion of the fibro-vascular bundles. The layers of cells between the plates of cork, being cut off from a
XII. - 3
