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. [Illustration: 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.] [Illustration: PLATE 3 LEAF EPIDERMIS 1. Upper striated epidermis of chirata leaf (_Swertia chirata_, [Roxb.] Ham.). 2. Green hellebore leaf (_Veratrum viride_, Ait.). 3. Boldus leaf (_Peumus boldus_, Molina). 4. Under epidermis of India senna (_Cassia angustifolia_, Vahl.).] 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 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.