STRUCTURE OF LEAVES. 



71 



Distribution of the Veins, or Venation of Leaves. 



141. The distribution of the veins has been called Venation, some- 

 times Nervation. In most leaves this can be easily traced, but in 

 the case of succulent plants, as Hoya, Agave, and Mesembryanthemum, 

 the veins are obscure, and the leaves are said to be Hidden-veined 

 (figs. 171, 172). In the lower tribes of plants, as sea- weeds, and in 

 submerged leaves, there are no true veins, but only condensations of 

 elongated cellular tissue, and the term Veinless (avenid) is applied. 

 In an ordinary leaf, as that of Lilac or Chestnut, there is observed a 

 central vein larger than the rest, called the midrib (fig. 133 TO); this 

 gives off veins laterally (primary veins), ns ns ns, which either end in a 

 curvature within the margin, as in Lilac (fig. 133), or go directly to 

 the edge of the leaf, as in Oak and Chestnut (fig. 134). If they are 

 curved, then external veins and marginal veinlets are 

 interspersed through the parenchyma external to the 

 curvature. There are also other veins of less extent 

 (costal veins) given off by the midrib, and these give 

 origin to small veinlets. In some cases, as Sycamore and 

 Cinnamon, in place of there being only a single central 

 rib, there are several which diverge from the part where 



Fig. 133. Leaf of Belladonna, p, Petiole or leaf-stalk, nm, Midrib, nsnsns. Primary 

 veins, ending in curvatures at their extremities. 



Fig. 134. Leaf of Oak, pinnatifld or divided into lobes in a pinnate manner ; feather-veined, 

 the veins going directly to the margin. 



Fig. 135. Leaf of Banana, showing midrib and primary veins running parallel and in a curved 

 manner to the margin. No reticulation. Plant monocotyledonous. 



