222 PHARMACEUTICAL BOTANY 
is the largest, most prominent and essential part. Other parts 
may be wanting. It consists of upper and lower surfaces 
provided with a layer of protective epidermal cells which may give 
rise to outgrowths in the form of hairs or papilla. Either or 
both of these surfaces show openings called stomata (sing. stoma) 
which are protected by guard cells that regulate their opening 
and closing. ‘The stomata serve for the exchange of carbon 
: U 
Palisade parench. tia 
> y F) 3 _ 
Fic. 151.—Stereogram of leaf structure. Part of a veinlet is shown on the right. 
Intercellular spaces are shaded. (From Stevens.) 
dioxide and oxygen and the escape of water vapor. Between 
the epidermal layers, in the region called the mesophyll, is a soft 
green tissue called chlorenchyma which is made up of leaf paren- 
chyma cells containing chloroplasts and intercellular-air-spaces. 
Coursing through the mesophyll are the veins or fibro-vascular 
bundles which are in continuity with the fibro-vascular bundles 
of the stem, These veins branch and rebranch and so make up 
the frame-work of the lamina. The outer walls of the epidermal 
cells are provided with a cuticle which is generally thicker on 
the upper than on the lower surface. In most leaves the meso- 
phyll is differentiated into upper palisade parenchyma, lower 
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