Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/100

smaller cells overlie them, with their upper walls raised as

ridges running at right angles to the stoma. This gives the stoma a fringed appearance, hence called pectinate. Tho mode of formation of a stoma is an interesting process, result ing from the division of a single epidermal cell. Preparatory to the complete formation of the mother-cell of the stomatic guard-cells, divisions may take place in the epidermal cells by which numerous or few cells are formed surrounding this mother-cell. In the mother-cell a partition wall is developed (fig. 35, s). A thickening of the partition wall occurs, and eventually in the central portion a fissure makes its appearance, which gradually increases in size (fig. 36). It does not, however, pass throughout the whole extent of the partition wall. An opening or stoma is thus formed, bounded on each side by a single guard-cell, leading into an intercellular space in the parenchyma beneath. If we suppose the mother-cell to be divided by numerous vertical septa we should have a stoma surrounded by many guard- cells, as occurs in some Hepaticae ; and in this family, after the formation of the fissure a series of horizontal septa appear in the guard-cell, and thus the stomatic opening becomes a canal. In Ceratopteris thalictroides the stoma is bounded by three cells, two of which, in their open condi tion, are crescentic and concave inwardly, while the third sur rounds them, except for a small space at the end of the long axis of the stoma, and has on this account been called peristomatic. In Urostigma elasticum four cells form the stoma. Subsequent changes in the surrounding epider mal cells may cause alterations in their relations to the stomata. Thus, in the Oleander (fig. 37) the epidermis develops so as to sink the stoma within a ciliated cavity; in other cases the stomata are raised on pillars. The guard- cells have a power of opening or closing the orifice of the stoma; when moistened they swell and curve outward, and leave a free opening ; when dry they collapse and their edges are straightened, and they close the orifice. Stomata are most abundant on the under surface of leaves. Their number varies from 200 to 160,000 or more in a square inch of surface. In the leaves of the White Lily there are 60,000 in a square inch on the iinder surface, and about 3000 on the upper; on the leaves of the Cherry-Laurel there

are 90,000 on the lower surface, and none on the upper.

Fig. 37.

Fig. 38. FIG. 37. Vertical section of lower epidermis of the leaf of Nerium Oleander, e, epidermis composed of several layers of cells ; p, parenchyma of the leaf ; s, cavity filled with hairs, at the bottom of which is a stoma. Fio. 38. Unicellular hair of the Common Cabbage.

Upon the epidermis and as outgrowths from it are found certain appendages. These are hairs, scales, prickles, &c., and all have been embraced under the general name trichome. Hairs (pili or villi) are productions each of an epidermal cell, having typically an elongated or conical form, and covered by the cuticle. They are erect or oblique, or they lie parallel to the surface, and are appresscd. Sometimes they are formed of a single cell, which is simple and undivided (fig. 38), or forked (fig. 39), or branched; at other times they are composed of many cells, either placed end to end, as in mcniliform or necklace-like hairs (fig. 40), or united together laterally, and gradually form ing a cone, as in compound or branched hairs. "When the branches of a hair proceed from a common centre, it is stellate or radiated (fig. 41, a, I), as in hairs of the Mallow tribe, in those of Deutzia scabra, and those on the stem of the Rice-paper plant (Fatsia papyrifera). When stellate hairs are flattened out, so as to form a sort of membranous expansion (fig. 44), a scale or scurf is produced. In Bromeliaceae the scurfiness of the leaves is a marked character. To such expansions of the epidermis the name lepis is applied, and the surface is said to be lepidote. These scales have sometimes a beautiful silvery appearance, as in Eloeagnus and Sea-buckthorn (fig. 44). Surrounding the base of the leaves of Ferns a brown chaffy substance occurs, consisting of elongated cells, to which the name of ramentaceous hairs, or ramenta, has been given. In Palms also a similar substance but of a fibrous texture occurs, called reticulum or mattulla. Setae are bristles or stiff hairs, and the surfaces on which they occur are said to be setose or setaceous. Some hairs, as those of Drosera (Sundew), have one or more spiral fibres in their interior. When the cells of hairs are hardened by thickening of the cell-wall, as in the Rose and Bramble, they are called Prickles (aculei). By some these are not considered as hairs, but are termed emergencies, inasmuch as they arise from a collection of cells, not from one epidermal cell. Various names have been given to the different forms of hairs. They are clavate, or club-shaped, gradually expanding from the base to their apex ; capitate (fig. 42), having a distinct rounded head ; rough or scabrous, with slight projections on their surface ; hooked or undnate, with a hook at their apex pointing downwards and to one side ; larled or glochidiate, with two or more hooks around the apex ; shield-like or peltate, when attached by their middle, and projecting horizontally on either side, as in Malpighia urens (fig. 45), and in many Cruciferous plants ; cilia, when surrounding the margin of leaves. On the pod of the Cowitch (Mucuna pruriens) hairs are produced with projections on their surface, which cause irritation of the skin. In Venus s Fly-trap (Dionsea muscipula) stiff hairs exist on the blades of the leaf (fig. 46), which, when touched, induce their closure.

Fig. 39. Fig. 40.

Fig. 41. /] ^ Fig. 42. Fig. 43. FIG. 39. Forked or bifurcated unicellular hair of Draba or Whitlow-gross. FIG. 40. Moniliform or necklace-like hair of Virginian Spidcnvort (W-adescantia virginica). FIG. 41. Unicellular hair a, of Alyssum, dividing into rays at the apex. The stellate or star-like arrangement is represented in the lower figure b. FIG. 42. Glandular, multicellular or many-celled hair of Frogsmouth (Antirrhi num majus). It is a partitioned capitate hair. Fio. 43. Sting (stimulus) of Nettle (Urtica dioic<f); its base is formed by nu merous cells containing irritating fluid; from these arises a simple unicellular conical hair, which serves as a duct for conveying the fluid.

Hairs occur on various parts of plants, on the stem,

leaves, flowers, seed-vessels, and seeds, and even in the interior of vessels. In the interior of the spathe of some Palms numerous ovate cells, analogous with hairs, occur in clusters, and can, when the spathe is dried, be shaken out in the form of powder. Cotton consists of the hairs surrounding the seeds of Gossypium herbaceum and other species of the genus. These when fresh arc elongated tubular cells; when dried their walls collapse and they appear twisted. Hairs are occasionally developed to a great extent on plants exposed to elevated temperatures, as well as on those growing at high altitudes. When they occur on the organs of reproduction they are connected with fertilization, as the hairs on the style of Goldfussia,

and the retractile hairs on the style of Campanula.