There are exceptions to this rule, as, for example, in Cinchona calisaya, which shows no trace of epidermis, this being replaced by a corky layer of tubular cells. Where this occurs in a plant to any extent, the whole of the outer tissues are displaced, and the bark consists exclusively of phloem tissues. This, although of constant occurrence in C. calisaya, is not so common in other species, as C. succirubia, the middle structure of which consists of parenchyma in which appear more or less numerous isolated store-cells, and when these are absent there is a formation of rhytidoma and displacement of the tissues containing the store-cells and ducts. The chlorophyll of C. succirubia is very marked, and its spectrum presents seven distinct absorption bands.
The epidermal system of plants in general includes other tissues than those already named, as the guard-cells of the stomata, the water-pores, plant-hairs or trichomes, and the external or epidermal glands, all of which are but modifications of ordinary epidermal tissue.
The Stomata or Breathing Pores are apertures in the epidermal which lie over large intercellular spaces (Fig 312, 2, b). These are usually bordered by two modified epidermal cells, called guard-cells. Stomata are formed in the following manner: A young epidermal cell divides into two equal portions by the formation of a septum across its middle, each half developing into a guard-cell; the septum now splits lengthwise and separates the guard-cells, leaving an aperture or stoma between them.
In the higher plants the guard-cells of the stomata are crescent-shaped and occur in pairs, the concave sides of the cells facing each other with the aperture between, while in mosses the stomata possesses but a single annular guard-cell which surrounds the aperture. The guard-cells of stomata usually contain chlorophyll-bodies in addition to the ordinary protoplasm. They have the power of increasing or diminishing the size of the aperture under the influence of light and moisture, thus regulating the amount of evaporation from the internal tissues of the plant.
Water Pores or Water Stomata are apertures in the epidermis, similar in structure to ordinary stomata, but differ from them both in function and distribution. Water-pores excrete water in the form of drops, and have their guard-cells fixed and immovable. They always occur at the ends of vasal bundles, and are found on the margin and at the apex of leaves.
Plant Hairs or Trichomes are modified epidermal cells prolonged externally, and may be either unicellular or multicellular. Each hair consists of a basal portion, or foot, which is embedded among the ordinary epidermal cells, and an apical portion or body, which is prolonged externally. Ordinary epidermal hairs are usually thin-walled, the inner layers of the wall being composed of cellulose, while the outer layer is more or less strongly cutinised. The walls may become hardened by deposits of lime-salts or silica. Sometimes the cells become glandular and secrete oily, resinous, or irritating matters, as in stinging-nettle hairs ([Plate XIII]., No. 19), when they are known as glandular hairs. The development of resin-passages may be observed in transverse sections of the stem of the ivy (Hedera helix) cut from a young succulent stem, and mounted in glycerine. The resin is seen scattered through the cortex and pith, and in the soft bast which lies outside the cambium in various stages of development, starting from a group of four cells without intercellular spaces.
Root hairs spring from the epiblema and are never cutinised, but are frequently more or less mucilaginous. The root-hairs are the principal absorbing organs of the plant, and are confined to the younger roots, occurring just above their tips. Root-hairs are never present in aquatic plants, and are absent from the roots of certain of the Coniferæ. It is a curious fact with regard to bell-heather growing in higher latitudes, that the plants possess a peculiar root structure as a protection against droughts. In most of them the sustentation of life depends upon the formation of a number of long thin filaments on their roots resembling root-hairs, which penetrate the root, forming nodular masses within it. These filaments belong to a fungus entirely parasitic to the root, and yet different from a common parasite, inasmuch as the plant in this way obtains so much of its nourishment, and when the fungus is not present, or is removed, the plant can no longer live on a peaty soil. The leaf-blade of the coarse moorland grass Nardus is likewise endowed with a singular property—that of rolling up cylindrically and spreading out again to adapt itself to the dry and wet weather of the moorlands of Scotland.
Fig. 313.
a. Section of the testa of Gourd Seed, showing communicating cells filled with colouring matter; b. Section of stem of Clematis, three pores separated and more highly magnified; c. Transverse section of same, showing medullary rays.