98 



VEGETABLE PHYSIOLOGY 



slightly and splits so as to form an opening between them, 

 which does not, however, extend the whole length of the 



F IG . 66. SURFACE VIEW OP PART OF THE UNDER SURFACE OF A LEAF, 



SHOWING THREE STOMATA IN DIFFERENT STAGES OF OPENING AND 

 CLOSING. 



wall, so that the two cells remain attached to each other by 

 their ends (fig. 66). The split constitutes the stoma, and the 



two cells are known 

 as the guard - cells. 

 They are commonly 

 of a more or less 

 semilunar form and 

 contain some chloro- 

 plastids, a point in 

 which they differ from 

 the other cells of the 

 epidermis in the 

 higher plants. Their walls become thickened and cuticu- 

 larised, particularly those which abut upon the slit and 

 upon the intercellular space (fig. 67) ; the wall which is 



FIG. 67. SECTION OF LOWER EPIDERMIS OF 

 A LEAF, SHOWING A STOMA. x 300. 



