136 



PLANT LIFE, 



168. Steles. — The steles are numerous and ramify through 

 the blade. Their structure is essentially as described for the 



Fig. 163. — Diagrammatic vertical section of a leaf, e, e, epidermis, with cuticle c, c, 

 and stomata s/>, s/>. Between upper and lower epidermis lies the mesophyll, with cells 

 abundantly supplied with chloroplasts. The upper row of elongated cells is the pali- 

 sade parenchyma ; the rest form the spongy parenchyma, both with many intercellular 

 spaces a, t, i, communicating with outside air through stomata. In the mesophyll lies 

 a small vein, here cut across, composed of a ventral xylem bundle g, a dorsal phloem 

 bundle s, surrounded by the endodermis gs, and the pericycle (between g and gs).— 

 After Sachs. 



stem (% 127). Each of the smaller consists of little more 

 than a single pair of vascular bundles. The xylem bundles 

 alone form the last branches (fig. 164), the phloem disappear- 

 ing earlier. The larger ribs may form one or two strands or 

 a complete sheath of mechanical tissues by the development 

 of the pericycle, and the bundles proper may be increased 

 by the development of secondary wood and bast. (See 

 1 141.) 



