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 midv^in or the veins we may 

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

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

 subdivide until they become too small to be seen. 



310 



VarictitK of venation. 307, Feather-veined, leaf of Betula populifolia (White Birch\ lyins upon 

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

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

 beal" (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 

 EXOGEXS). 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 net- work. 



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

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



