128 ESSENTIALS OF BOTANY 
151. Stomata.— A stoma is a microscopic pore or slit 
in the epidermis. It is bounded and opened and shut 
by guard-cells (Fig. 94, g), usually two in number. These 
/s. 
ry 
Fic. 93. Vertical Section of the Leaf of the Beet. (Much magnified.) 
e, epidermis; p, palisade-cells (and similar elongated cells); 7, cells filled 
with red cell-sap; i, intercellular spaces; a, air spaces communicating 
with the stomata; st, stomata, or breathing pores. 
are generally somewhat kidney-shaped and become more 
or less curved as they are fuller or less full of water (see 
Sect. 158). 
In the case of an apple tree, where the epidermis of the 
lower surface of the leaf contains about 24,000 stomata to 
