38 



VEGETABLE PHYSIOLOGY 



(figs. 37, 88), which are provided with two guard-cells, by 

 the behaviour of which the width of the apertures can be 



Fro. 37. PART OF LOWER SURFACE OF A LEAF, SHOWING THREE STOMATA 



IN DIFFERENT STAGES OF OPENING. X 300. 



enlarged or diminished. In those regions of the axis where 

 corky layers cut off the metabolic tissue of the cortex 



FIG. 38. SECTION OF LOWER EPIDERMIS OF 

 A LEAF, SHOWING A STOMA. x 300. 



from the exterior, certain other special apertures, the 

 knticels, are present (fig. 39). The atmosphere conse- 

 quently enters the plant by these orifices and circulates 



