THE LEAVES. 



Ill 



those near the upper surface are more elongated and close set, form- 

 ing one or two rows, with their ends outward, while cells near the 

 lower surface are irregular in form, with large intercellular spaces 

 (fig. io6). 



The cortex (gs, fig. io6) often develops along the larger steles into 

 one or two strands or a sheath of mechanical tissues. These tissues, to- 



FlG. io6. — Diagrammatic venical section of a leaf, s, e, epidermis, with cuticle c, c, 

 and stomata, s/>, sfi. Between upper and lower epidermis lies the mesophyU, with 

 cells abundantly supplied virith chloroplasts. The upper row of elongated cells is the 

 palisade parenchyma ; the rest form the spongy parenchyma, both with many inter- 

 cellular spaces a, z, r, communicating with outside air through stomata. Inthemeso- 

 phyll lies a small vein, here cut across, composed of a ventral wood bundle £, a 

 dorsal bast bundle s, surrounded by the endodermis gs, and the pericycle (between 

 g- and gs) —After Sachs, 



gether with a stele, constitute the rib or vein, often so massive as to pro- 

 ject beyond the other parts in thin leaves. 



139. Steles. — The steles are numerous and ramify through the blade. 

 Their structure is essentially as described for the stem (^ 107). Each 

 of the smaller consists of little more than a single pair of vascular strands. 

 The wood strands alone form the last branches (fig. 107), the bast disap- 

 pearing earlier. The larger ribs may be accompanied by one or two 

 strands or a complete sheath of mechanical tissues, and the vasCiilar 



