16 



A TEXT-BOOK OF BOTANY 



by a wall, and in the epidermis these cells fit closely to- 

 gether, sometimes dovetailing with one another. 



Characteristic openings in the epidermis also will be dis- 

 covered, sometimes in very great numbers. Guarding each 

 slit-like opening are two crescent-shaped epidermal cells, 

 called guard-cells (Fig. 11). The whole apparatus is known 

 as a stoma (plural stomata), which really means "mouth," 

 of which the guard-cells might be thought of as the lips. 

 One important fact about stomata is that the guard-cells 

 can change their shape, and so vary the size of the opening. 

 These numerous openings are passageways into the interior 

 of the leaf, putting the internal cells into communication 



with the air out- 

 side, and so fa- 

 cilitating the in- 

 terchange of gases 

 that will be de- 

 scribed later in 

 connection with 

 the work of the 

 leaf. In horizon- 

 tal leaves the sto- 

 mata are chiefly 

 and sometimes ex- 

 clusively on the 

 lower surface, a 

 fair average number being about 100 to each square milli- 

 meter of surface (about 62,500 to the square inch) ; although 

 in some cases the number may reach 700 to the square milli- 

 meter (almost 450,000 to the square inch). In leaves 

 exposed alike on both sides to the light, as in the erect 

 leaves of the common flag, the stomata are equally dis- 

 tributed on both surfaces. In floating leaves, as those of 

 water-lilies, the stomata are all on the upper surface; and in 

 submerged leaves there are no stomata. From this dis- 



FIG. 11. Surface view of the epidermis of a hyacinth 

 leaf: A, epidermal cells and four stomata with their 

 guard-cells ; B, enlarged view of a single stoma. 



