62 HISTOLOGY OF MEDICINAL PLANTS 
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; _bella- 
donna 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 
