Fig. 311.
a. Epidermis, reticulated ducts, and conjunctive palisade cells; b. Vertical section of alder root, woody layer, and boundary ducts.
In stems the external layer of cells, whatever its origin, is known as the epidermis, while in roots it is called the epiblema. The epidermis usually consists of a single layer of cells, but in some cases it is two or three-layered, as in the leaves of figs and begonias.
In land plants the epidermis is usually strongly cutinised, while in submerged plants it is never cutinised. The epidermis of land plants is also often waxy, the wax occurring on the surface as minute grains, rods or flakes, constituting the so-called bloom of leaves and fruits, and giving to them their glaucous appearance. Chlorophyll bodies are usually absent from the ordinary epidermal cells of land plants, while they commonly occur in the epidermal cells of aquatic plants.
Ordinary epidermal cells are usually thin-walled and transparent, and contain a nucleus and colourless watery protoplasm, but are destitute of both chlorophyll-bodies and starch-grains.
The external layers of the outer walls constitute the cuticle of the plant, while the internal layers and the radial and inner walls are composed of cellulose. The cells of the epidermis are always very compactly arranged, having their walls so closely adherent that the intercellular spaces are entirely obliterated except at the stomata and water-pores.
Fig. 312.
1. Vertical section of leaf of Iris germanica; a, a. Elongated cells of the epiderm; b. Stomata cut through longitudinally; c, c. Green cells of the parenchyma; d, d. Colourless tissue of the interior of the leaf. 2. Portion of leaf torn from its surface; a. Elongated cells of the cuticle; b. Cells of the stomata; c. Cells of the parenchyma; d. Limiting wall of the epidermic cell; e. Lacunæ or openings in the parenchyma corresponding to the stomata.