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 



St 



Fig. 03. Vertical Section of the Leaf of the Beet. (Much magnified.) 



e, epidermis ; p, palisade-cells (and similar elongated cells) ; r, 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 



