94 STRUCTURAL BOTANY. 



283. The Midvein is the principal axis of the venation, or pro- 

 longation of the petiole, running directly through the lamina, 

 from base to apex, as seen in the leaf of the Oak or Birch. If 

 there be several similar divisions of the petiole, radiating from 

 the base of the leaf, they are appropriately termed Veins / and 

 the leaf is said to be three-veined, five-veined, as in Maple. The 

 primary branches sent off from the midvein or the veins we may 

 term the Veinlets, and the secondary branches, or those sent off 

 from the veinlets, are the Veinulets. These also branch and 

 subdivide until they become too small to be seen. 



Varieties of venation. 307, Feather-veined, leaf of Betula populifoUa (White Birch), lying upon n 

 louf of Plum-tree ; same venation with different outlines. 308, Palmate-veined, leaf of White Maple, 

 contrasted with leaf of Cercis Oanadensis. 309, Parallel venation, plant of " three-leaved Solomon's- 

 seal" (Smilacina trifoliata). 310, Forked venation, Climbing Fern iLygodium). 



284. Botanists distinguish three modes of venation, which are 

 in general characteristic of three Grand Divisions of the Vege- 

 table Kingdom viz. : 



Reticulate or Net-veined, as in the DICOTYLEDONS (called also 

 Ext'GExs). This kind of venation is characterized by the fre- 

 quent reunion or inosculation of its numerously branching veins, 

 eo as to form a kind of irregular wet-work. 



Parallel-veined, as in the MONOCOTYLEDONS (called also ENDO- 

 GENS). The veins, whether straight or curved, run parallel, or 



