116 A TEXTBOOK OF GENERAL BOTANY 
Epidermis. The epidermis of the stem resembles that of the 
leaf very closely in both structure and function. It consists of 
a single layer of cells and is the outermost layer of the stem. 
It contains stomata and produces various types of trichomes. 

Fic. 113. Cross section of a young stem of Aristolochia elegans 
e, epidermis; c, collenchyma of cortex; p, parenchyma; ca, cambium; pi, pith ray; 
ph, phloém; s, sclerenchyma of pericycle ; 2, xylem. (x 55) 
The outer cell walls are greatly thickened and heavily cutinized 
(Figs. 115, 116). The epidermis serves chiefly for restricting 
the rate of transpiration and for protecting the underlying tissues 
from mechanical injury and from disease-producing organisms. 
Cortex. ‘The region that lies next to the epidermis is the cor- 
tex. The innermost layer of the cortex is the starch sheath, 
known also as the endodermis. It consists of a single layer of 

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