THE LEAF 75 
sade and spongy mesophyll runs the vascular bundle 
containing xylem towards the upper part and phloem 
towards the lower. Endodermis and pericycle can be 
recognized in the larger bundles. 
44, Stomata.—If the epidermal layer be torn off a 
leaf and mounted and examined under the microscope 
it presents the appearance shown in fig. 45 a. Dotted 
Fic. 45.—Stomata. 
A. As seen in surface view. 3B. Transverse section. ©. Stoma of 
Tradescantia. g. Guard cell. e. Epidermis. 
about the epidermis one can notice a number of small 
pores, each pore being surrounded by a pair of small 
green cells. Each of these structures is called a Sroma 
(Greek—Stoma, a mouth). The two cells surrounding 
the opening are called guard cells. The pore communi- 
cates with a large intercellular space called the Resprra- 
Tory CAVITY or air chambers. These stomata are 
scattered about the surface of all the aerial parts of 
a plant, and their number may reach as high as 
700 per square millimetre, although the average is 
