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 cells: 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.