The epidermis and its modifications, the hypodermis and the periderm, form the dermal or protective outer layer or layers of the plant.

The epidermis of most leaves, stems of herbs, seeds, fruits, floral organs, and young woody stems consists of a single layer of cells which form an impervious outer covering, with the exception of the stoma.

Leaf Epidermis

The cells of the epidermis vary in size, in thickness of the side and end walls, in form, in arrangement, in character of outgrowths, in the nature of the surface deposits, in the character of wall - whether smooth or rough - and in size.

In cross-sections of the leaf the character of both the side and end walls is easily studied.

In surface sections - the view most frequently seen in powders - the side walls are more conspicuous than the end wall (Plates 2 and 3). This is so because the light is considerably retarded in passing through the entire length of the side walls, while the light is retarded only slightly in passing through the end wall. The light in this case passes through the width (thickness) of the wall only. The outer walls of epidermal cells are characteristic only when they are striated, rough, pitted, colored, etc. In the majority of leaves the outer wall of the epidermal cells is not diagnostic in powders, or in surface sections.

The thickness of the end and side walls of epidermal cells differs greatly in different plants.

As a rule, leaves of aquatic and shade-loving plants, as well as the leaves of most herbs have thinner walled epidermal cells than have the leaves of plants growing in soil under normal conditions, or than have the leaves of shrubs and trees.

The widest possible range of cell-wall thickness is therefore found in the medicinal leaves, because the medicinal leaves are collected from aquatic plants, herbs, shrubs, trees, etc.

The outer wall is always thicker than the side walls. Even the side walls vary in thickness in some leaves, the wall next to the epidermis being thicker than the lower or innermost portion of the wall. Frequently the outermost part of the side walls is unequally thickened. This is the case in the beaded side walls characteristic of the epidermis of the leaves of laurus, myrcia, boldus, and capsicum seed, etc. The thickness of the side walls of the epidermal cells of most leaves varies in the different leaves.

In most leaves there are five typical forms of arrangement of epidermal calls: First, those over the veins which are elongated in the direction of the length of the leaf; and, secondly, those on other parts of the leaf which are usually several-sided and not elongated in any one direction. If the epidermis of the leaf has stoma, then there is a third type of arrangement of the epidermal cells around the stoma; fourthly, the cells surrounding the base of hairs; and fifthly, outgrowths of the epidermis, non-glandular and glandular hairs, etc.

It should be borne in mind that in each species of plant the five types of arrangement are characteristic for the species.

The character of the outer wall of the epidermal cells differs greatly in different plants. In most cases the wall is smooth; senna is an example of such leaves. In certain other leaves the wall is rough, the roughness being in the, form of striations. In some cases the striations occur in a regular manner; belladonna leaf is typical of such leaves. In other instances the wall is striated in an irregular manner as shown in chirata epidermis. Very often an epidermis is rough, but the roughness is not due to striations. In these cases the epidermis is unevenly thickened, the thin places appearing as slight depressions, the thick places as slight elevations. Boldus has a rough, but not a striated surface.

Surface deposits are not of common occurrence in medicinal plants; waxy deposits occur on the stem of sumac, on a species of raspberry, on the fruit of bayberry, etc. Resinous deposits occur on the leaves and stems of grindelia species, and on yerba santa.

In certain leaves there are two or three layers of cells beneath the epidermis that are similar in structure to the epidermal cells. These are called hypodermal cells, and they function in the same way as the epidermal cells.

Hypodermal cells are very likely to occur on the margin of the leaf. Uva-ursi leaf has a structure typical of leaves with hypodermal marginal cells. Uva-ursi, like other leaves with hypodermal cells has a greater number of hypodermal cells at the leaf margin than at any other part of the leaf surface.

The cutinized walls of epidermal cells are stained red with saffranin.

Plate 2. Leaf Epidermis.

1. Uva-ursi (Arctostaphylos uva-ursi, [L.] Spring).

2. Boldus (Peumus boldus, Molina).

3. Catnip (Nepeta cataria, L.).

4. Digitalis (Digitalis purpurea, L.). 4.A. Origin of hair.

Plate 3. Leaf Epidermis.

1. Upper striated epidermis of chirata leaf (Swertia chirata, [Roxb.] Ham.).

2. Green hellebore leaf (Veratrum viride, Ait.).

3. Bold us leaf (Peumus boldus, Molina).

4. Under epidermis of India senna (Cassia angustifolia, Vahl.).