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, 

 and stomata sp, sfi. Between upper and lower epid 



pidermis, with cuticle c, c, 

 the mesophyll, with cells 



ivii, 1 

 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 

 mmunicating with outside air through stomata. In the mesophyll lies 

 a small vein, here cut across, composed of a ventral xylern bundle v. a dorsal phloem 

 bundle ,v, surrounded by the endodermis .;,*. and the pericyclc (between e and .!,•■»')■ 

 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 

 ■" 141. 



