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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