THE DIFFERENTIATION OF THE PLANT-BODY 33 
other and form a system extending throughout the plant. 
They can be detected in the root, in the cortex of which 
they are conspicuous (fig. 35); they may be traced through 
Fic. 37.—Part or Lower Suprace or A Lear, sHowinc THREE SToMATA 
IN DIFFERENT STAGES OF OPENING. x 300, 
all the ramifications of the stem, and are seen to form a 
very prominent feature of the mesophyll of the leaves (fig. 
36). They communicate with the exterior in all the green 
parts of the plant, 
especially the leaves. 
In the epidermis of 
all such parts are 
small openings known 
as stomata (figs. 37, 
88), which are pro- 
vided with two guard- 
: Fie, 38.—Srction or LowER EpPipERMIS oF 
cells by the behaviour A LEAF, SHOWING ASToMA. x 3800. 
of which the aper- 
tures can be opened or closed. In those regions of the 
axis where corky layers cut off the metabolic tissue of the 
cortex from the exterior, certain other special apertures, 
3 
