38 VEGETABLE PHYSIOLOGY 
(figs. 87, 88), which are provided with two guard-cells, hy 
the behaviour of which the width of the apertures can be 
Fra. 37.—Part of Lower Surrace oF A LEAF, SHOWING THREE Stomata 
IN DIFFERENT STaGEs OF OPENING. Xx 300. 
enlarged or diminished. In those regions of the axis where 
corky layers cut off the metabolic tissue of the cortex 
Fig. 38.—Srcrion or Lowrr Eripermis or 
4 Lear, sHowrna a Stoma. x 300. 
from the exterior, certain other special apertures, the 
lenticels, ave present (fig. 89). The atmosphere conse- 
quently enters the plant by these orifices and circulates 
