136 



PLANT LIFE. 



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

 the blade. Their structure is essentiall)- as described for the 



Fig. 163. — Diagrammatic vertical section of a leaf. i\ i\ epidcrims, with cuticle <", c, 

 and stomata J-/, .v/. t'.etween upper and lower epidermis lies llie mesophyl], witli cells 

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

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

 spaces /i, /, ?', commimicating with outside air tlirougii stomata. In the inesophyll lies 

 a small vein, jiere cut across, composed of a ventral xylem bundle i', a dorsal phloem 

 bundle j-, surrounded by the endodermis^'j", and the pericycle (between i' and^'.?). — 

 After .Saclis. 



Stem (^1 127). Each of the suialler consists of httlc moie 

 than a single pair of vascular bundles. The xylem bundles 

 alone form the last branches (lig. 164), the phloem disajipear- 

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

 a complete sheath of mechanical tissues b\- the de\'clopment 

 of the pericycle, and the bundles proper mi\\ be iiKu-eascd 

 by the development of secondary wood and bast. (See 

 1 14'-) 



