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 



FIG. 37. PAKT OP LOWER SURFACE OF A LEAF, SHOWING 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, 

 38), which are pro- 

 vided with two guard- 

 cells by the behaviour 

 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 



FIG. 38. SECTION OF LOWER EPIDERMIS OF 

 A LEAF, SHOWING A STOMA. x 800. 



