CHAPTER II 
THE EPIDERMIS AND PERIDERM 
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 char- 
acter 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 pow- 
ders—the side walls are more conspicuous than the end wall 
(Plates 2 and 3). This is so because the light is considerably re- 
tarded 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 
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