THEIR VENATION. 



155 



277. Parallel-veined or nerved leaves are characteristic of En- 

 dogenous plants ; while reticulated leaves are almost universal in 



Exogenous plants. We are thus furnished with a very obvious, al- 

 though by no means absolute, distinction between these two great 

 classes of plants, independently of the structure of their stems (198). 

 278. In reticulated leaves, the coarse primary veins (one or 

 more in number), which proceed immediately from the apex of the 

 petiole, are called ribs ; the branches are termed veins, and their 

 subordinate ramifications, veinlets. Very frequently, a single strong 

 rib (called the midrib), forming a continuation of the petiole, runs 

 directly through the middle of the blade to the apex (Fig. 229, 238, 

 &c), and from it the lateral veins all diverge. Such leaves are 

 termed feather-veined or pinnately veined; and are subject to vari- 

 ous modifications, according to the arrangement of the veins and vein- 

 lets ; the primary veins sometimes passing straight from the midrib 

 to the margin, as in the Beech and Chestnut (Fig. 238) ; while 

 in other cases they are divided into veinlets long before they reach 

 the margin. When the midrib gives off a very strong primary vein 

 or branch on each side above the base, the leaf is said to be triple- 

 ribbed, or often tripli-nerved, as in the common Sunflower (Fig. 



FIG. 229 A leaf of the Q 

 p, petiole or leaf-stalk : st, stipules. 



I'lG 230. Parallel- veined leaf of the Lily of the Valley 



of the netted-veined or reticulated sort : b, blade : 



