3o6 



THE FOLIAGE LEAF 



the outer ones bulging into the adjacent epidermal 

 cells, the pressure of whose cell sap is lower, and the 



Fig. 51. — ^A, diagram of stoma seen in vertical section. The thin 

 lines represent the position of the walls of the guard cells when 

 the stoma is closed, the thick lines when it is open, the outer 

 lateral walls bulging into the adjacent epidermal cells. B, the 

 same in surface view (Half the stoma only shown). C, stoma 

 of Iris leaf in vertical section, showing the guard cells sunk below 

 the surface of the epidermis at the bottom of the vestibule (v). 

 D, the same in surface view : epidermal cells focussed, the 

 outlines of the guard cells seen faintly below. E, the guard 

 cells focussed. F, ending of a fine vein in the mesophyll : one 

 line of tracheids surrounded by sheath cells with fewchloroplasts. 



