CHAPTER III. ORGANS OF PLANTS. 



195 



median bundle is present, or two bundles enclosed in a common 

 sheath but unconnected by cross-veins, run parallel to each 

 other from base to apex of the leaf. In Welwitschia and most 

 Monocotyledons the bundles run nearly parallel to each other 

 from base to apex or margin, or from a median group of bundles, 

 forming a midrib, to the margin. Adjacent bundles are con- 

 nected laterally by minute branches which remind one of the 

 rounds of a ladder. The ends of the veins seldom terminate 

 free, but curve toward each other and become united near the 



Fig. 455. — Apical portion of leaf of Smilacina stellata. Magnified about 30 diameters 

 to show arrangement of ribro -vascular bundles. 



margin. See Fig. 455. A few exceptional Monocotyledons 

 belonging mainly to the Yam, Arum and Smilax families, have 

 the bundles reticulately arranged, as in most Dicotyledons. 



In the reticulate type the bundles that enter the blade branch 

 freely through it, running in all directions and anastomosing to 

 form a net-work. The net- work may be fine or relatively coarse, 

 but the meshes are seldom very regular, except in some Ferns 

 which exhibit this type of venation. In many cases there are 

 bundles which end free in the interior of the meshes, as in Fig. 

 456 ; in others these are wanting, but some of the veins terminate 

 free in the margin ; in still other cases both modes occur in the 



