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 
ean 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- 
A 
cilitating the in- 
terchange of gases 
Fic. 11.—Surface view of the epidermis of a hyacinth and sometimesex- 
leaf: A, epidermal cells and four stomata with their clusively on the 
that will be de- 
scribed later in 
guard-cells; B, enlarged view of a single stoma. 
connection with 
the work of the 
leaf. In horizon- 
tal leaves the sto- 
mata are chiefly 
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- 
