296 PLANT STRUCTURES 



155. Leaves. Leaves usually develop from an apical 

 region in the- same general way as do stems and roots, 

 modified by their common dorsiventral character. Com- 

 paring the leaf of an ordinary seed-plant with its stem, it 

 will be noted that the three regions are represented (Fig. 

 278) : (1) the epidermis ; (2) the cortex, represented by 

 the mesophyll ; (3) the stele, represented by the veins. 



In the case of collateral bundles, where in the stem the 

 xylem is always toward the center and the phloem is toward 

 the circumference, in the leaves the xylem is toward the 

 upper and the phloem toward the lower surface. 



