98 EPIDERMIS AND STOMATA [CH. 



cells are thickened and hardened in a peculiar way so 

 that they do not allow water to pass through in the easy 

 manner in which it diffuses through the walls of the 

 mesophyll-cells. The epidermis is, in effect, a more or 

 less water-tight skin, and we can easily understand its 

 function if we reflect how quickly the thin-walled and 

 pervious mesophyll-cells would dry up in the sunshine if 

 not protected by this covering. Indeed we can prove this 

 by stripping the epidermis from a piece of leaf; or by 

 removing a water-plant, the leaves of which have a very 

 permeable epidermis, from its natural environment and 

 noticing how quickly the leaf shrivels. 



But although it is characteristic of the epidermis to 

 have no ordinary intercellular spaces, it is equally 

 characteristic of aerial organs that the continuity of their 

 epidermis is interrupted at numerous definite points by 

 definite apertures, like minute slits or mouths and there- 

 fore termed Stomata through which communication be- 

 tween the outer air and the intercellular spaces is assured. 

 Each of these apertures is a slit-like opening between a 

 pair of the epidermal cells, usually of special shape, and 

 so far separated from each other by the development of 

 an intercellular space between them, that a free passage 

 is established between the external atmosphere and the 

 intercellular spaces in the sub-epidermal tissues. 



These openings are the stomata, and the paired cells 

 which surround a stoma are termed guard-cells, for they 

 are in most cases capable of so governing the aperture as 

 to widen or narrow it, or even close it altogether, according 

 to circumstances. 



Stomata are found in the epidermis of all ordinary 

 aerial organs, especially leaves, but they are usually (not 

 always) absent from the epidermis of submerged aquatic 

 plants, of most dry fruits and seeds, of subterranean leaves 



