EPIDERMIS. 



If a leaf be examined carefully, it will be found covered 

 with a thin skin or epidermis, which very often (in fleshy 

 leaves) may be torn off in filmy shreds. And a similar 

 epidermis covers nearly all the green and coloured organs 

 which are exposed to the air. If a piece of this epidermis, 

 torn from a leaf with the thumb and a sharp penknife, be 

 placed in a drop of water upon a glass slide, its structure 

 may be easily made out under the microscope. Suppose 

 a shred torn from the leaf of a Lily. It will be found 

 to consist of an excessively thin layer of flattened cells, 

 closely fitting at their angles. Scattered at intervals over 

 the epidermis are pairs of very small cells side by side, 

 with the ends in contact, as shown in the cut. Each pair 

 of cells forms a stomate. When the cells of the stomates are 

 rendered turgid by the absorption of fluid, they separate more 

 or less from each other, leaving a minute 

 opening in the middle between them. When 

 they are flaccid, the guard-cells remain close- 

 ly applied, and the orifice is closed. Under 

 ordinary conditions of the air as to moisture 

 they are open ; when it is eitlier very dry 

 or very moist, they are generally closed. 



The stomates, therefore, serve to facilitate 

 the absorption of gases, and probably of 

 vapour, from the air. They do not how- 

 ever, open into cells, but into spaces be- 

 tween the cells of the leaf, called i7itercellular 

 spaces. These intercellular spaces are wid est 

 between the cells forming the lower layers of the leaf, and 

 we find that stomates are generally much more abundant in 

 the epidermis of the lower than of the upper surface of leaves. 

 There are no stomates on roots, nor usually, on surfaces 

 under water. 



Frc. 92. Fragment of 

 Epidermis with a 

 single stomate. 



