THE GREEN FOLIAGE-LEAF 145 



dicotyledons end blindly among the ground-tissue cells, are 

 formed entirely of tracheids. 



The bast-tissue also undergoes a reduction of elements : as 

 the end of the bundle is approached, the sieve-tubes and com- 

 panion-cells are replaced by single long cells which do not extend 

 so far as the woody elements of the bundle. Surrounding 

 each bundle of the leaf is a sheathing tissue of parenchyma 

 which is continuous with the parenchyma of the vascular 

 cylinder of the stem. Such bundle-sheaths conduct carbohydrates 

 from the leaf to the stem and frequently contain small starch- 

 grains. 



The epidermis covers the whole leaf and, like that of the 

 stem with which it is continuous, consists of a single layer of 

 cells, the outer walls of which have a protective cuticle. 



A surface view (Fig. 74) shows that the cells fit closely 

 together except where the stomata occur. Each stoma consists 

 of two curved sausage-shaped cells (a) termed guard-cells, which 

 are joined together at the ends in such a manner that a narrow 

 slit-like pore or opening is left between them. The pore leads 

 through the epidermis into a 

 somewhat large air-chamber 

 just inside the ground-tissue 

 of the leaf, and this chamber 

 communicates with the air- 

 filled intercellular spaces all 

 through the leaf. 



Changes in the curvature / ^|S^ COci^ \^^§t~^^ 

 of the guard-cells reduce ^Ji'^'C^J / ^V \ 1 



or increase the size of the J ^ p^ f'^^''^/'\j U 

 pores of the stomata ; when 



, , ,, I, J Fig. 74. — Surface view of the epidermis of 



the cells are much curved a bean leaf. »Guard-cersofastonia;*theopen- 

 the pore is widely opened '"«''""='=" "'='"• (Enlarged 3=0 diameters.) 



and when they become straight the slit is closed. 



The stomata are organs specially adapted for the escape of 



