162 



THE LEAVES. 



though by no means absolute, distinction between these two great 

 classes of plants, independently of the structure of their stems (185). 

 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 peti- 

 ole, runs directly through the middle of the blade to the apex (Fig. 

 196, 197, &c.), and from it the lateral veins all diverge. Such 

 leaves are termed feather-veined or pinnately veined ; and are 

 subject to various modifications, according to the arrangement of 

 the veins and veinlets ; the primary veins sometimes passing 

 straight from the midrib to the margin, as in the Beech and Chest- 

 nut (Fig. 196) ; 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-rilled , or often tripli-nerved, as in the 

 common Sunflower (Fig. 199) ; if two such ribs proceed from each 

 side of the midrib, it is said to be quintuple-ribbed, or quintupli- 

 nerved. 



196 197 198 199 200 201 



279. Not unfrequently the vessels of a reticulated leaf divide at 



FIG. 183-201. Various forms of simple leaves. 



