STRUCTURE OF LEAVES. 



83 



off annually, they are called deciduous ; when they remain for two or 

 more years, they are evergreen. The laminar portion of a leaf is 

 occasionally articulated with the petiole, as in the Orange (fig. 201), 

 and a joint at times exists between the vS.ginal or stipulary portion 

 and the petiole. 



Distribution of the Veins, or Venation of Leaves. 



The distribution of the veins has been called Venation, sometimes 

 Nervation. In most leaves this can be easily traced, but in the case of 

 succulent plants, as Hoya, Agave, Stonecrop, and Mesembryanthemum, 

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

 186, 187). In the fronds of the lower tribes of plants, 

 as seaweeds, and in submerged leaves, there ,are no true 

 veins, but only condensations of elongated cellular 

 tissue, and the term Veinless (avemia) 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. 148 nm) ; this gives off veins 

 laterally {•primary veins) ns ns ns, which either end in a 



Kg. 149. 



Pig. 160. 



curvature within the margin, as in Lilac and Belladonna (fig. 148), or go 

 directly to the edge of the leaf, as in Oak (fig. 149) and Chestnut. If 

 they are curved, then external veins and marginal veinlets are inter- 

 Fig. 148. Leaf of Belladonna, p, Petiole or leaf-stalk. Tjm, Midrib. 715 ns ns, Primary 

 veins, ending in curvatures at their extremities. Fig. 149. Leaf of Oalc, pinnatifid or 



divided into lateral lobes ; feather-veined, the veins going directly to the margin. Fig. 

 150. Leaf of Banana (Musa), showing the midrib, with the primary veins running parallel to 

 each other in a transverse manner, and proceeding to the margin. No reticulation. Plant 

 m onocotyledonous. 



