THE LEAF. 



27 



when the petiole is absent), the leaf is said to be Sessile. The 

 blade consists of a frame-work of veins or skeleton of woody 

 tissue, and the soft, green tissue between the veins, called 

 parenchyma. When one central vein surpasses 

 the others in size it is called the midrib; its 

 branches are the veins, and the branches from 

 the veins are the veinlets. When the venation 

 of a large number of different kinds of leaves is 

 examined, three types are found to prevail. 

 In most of our common Fern-leaves the veins 

 are iseparate their entire length and have free 

 ends, though they are often forked one or more 

 times. In the other.two types the smaller vein- 

 lets (and sometimes the veins) anastomose. One 

 is represented by the leaves of most Monocotyls, 

 as the Lily, Flag, Grass, Corn, Wheat, etc. In 

 these a number of conspicuous veins extend 

 from the base to the apex of each leaf, approxi- 

 mately parallel to each other, and this fact has 

 suggested the name parallel-veined (Fig. 10). The term striated 

 may-be used instead. The third type is seen mostly in Dicotyls, 

 a,s the Oak, Elm, Maple, Catnip, Mallow, Dock, etc. ; the veins 



Petiole 



'stipules 

 Fig. 9. 



'Fig. 10. 



Fig. 11. 



and veinlets here form a conspicuous network, and are said to 

 be netted- veined, or reticulated. Of the latter, there are two 

 sorts : the veins may branch from a midrib (Fig. 9), when 

 they are pinnately-veined (Lat. -pinna, feather) ; or they may 

 branch from 3, 5, 7 or more ribs (Fig. 11), in which case they 

 are palmately-veined (Lat. palma, palm). 



