THEIR STRUCTURE AND FORMS. 89 



that part of the leaf. (Fig. 136, 137.) Hence in good part the 

 deeper green hue of the upper, and the paler of the lower face 

 of leaves. 



170. Epidermis. The whole surface of leaves, as of young 

 stems, is invested with a translucent membrane, composed of 

 one or sometimes two or three layers of empty and rather- 

 thick-walled cells. This is the skin or epidermis, which is so 

 readily separable from the succulent tissue of such leaves as 

 those of Stonecrop and other species of Sedum. It is of a single 

 layer in the Illicium (Fig. 136) and Lily (Fig. 137) ; of as 

 many as three in the firm leaf of the Oleander ; is generally 



137 



hard and thick in such coriaceous leaves as those of Pittosporum 

 and Laurustinus, which thereby the better endure the dry air of 

 rooms in winter. 



171. Stomata or Breathing-pores. 1 The epidermis forms a 

 continuous protective investment of the leaf except where certain 

 organized openings occur, the stomata. They are formed by a 

 transformation of some of the cells of the epidermis ; and consist 

 usually of a pair of cells (called guardian-cells) , with an opening 

 between them, which communicates with an air-chamber within, 

 and thence with the irregular intercellular spaces which permeate 

 the interior of the leaf. Through the stomata, when open, free 

 interchange may take place between the external air and that 



1 The technical name has been anglicized stomatts, singular stomate, which 

 has no advantage over the proper Greek, sing, sfoma, pi. stomata. 



FIG. 137. A magnified section through the thickness of a minute piece of the leaf 

 of the White Lily of the gardens, showing also a portion of the under side with some 

 breathing-pores, stomata. 



