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.
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