viii.] FPIDERMIS. 107 



cells with dense protoplasmic contents, to the appearance 

 of a septum across this cell parallel with the margins of 

 the leaf, and ultimate fission along the median line of 

 this septum, upon which the two cells tend to separate 

 more or less in the middle, remaining in contact at their 

 two ends, when the stomate is complete. In Dicotyledons, 

 especially with broad leaves, the epidermal cells are irre- 

 gular in outline, and so the stomates necessarily vary in 

 direction in relation to the leaf-margin. Generally they 

 are scattered at tolerably equal distances over the epider- 

 mis in which they occur, though they are occasionally 

 grouped in clusters. On the under side of the leaf of the 

 common Lime-tree the stomata are reckoned at about 

 a million. 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 closely applied, and the orifice is closed. 

 Under ordinary conditions of the air as to moisture they 

 are open ; when it is either very dry or very moist, they 

 are generally closed. 



The stomates, therefore, serve to facilitate the absorp- 

 tion of gases, and probably of vapour, from the air. They 

 do not, however, open into cells, but into spaces between 

 the cells of the leaf, called intercellular spaces. These 

 intercellular spaces are widest 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, over the veins or vascular bundles of 

 leaves, nor, usually, on surfaces under water. 



