VENATION. 



89 



LESSON XVII. 



VENATION— LEAF-FOEMS— SIMPLE LEAVES. 



189. 190. Venation. 191. Leaf-forms. 192. Leaf-base. 193. Leaf- 

 apex. 194. Leaf-incisions. 195. Leaf-margins. 196. Simple leaves. 



189. Venation is of three modes, as we hnov/ ,-r-Forked, Parallel, 

 Netted. Forked veins branch by forking simply, and do not reunite. 

 Parallel veins run parallel to one another, or nearly so ; they do not 

 subdivide nor form meshes, except by very short venules. Netted 

 veins (reticulate) branch and subdivide into veins and venules, which 

 meet again and unite, forming a net- work. In Acrogens (Ferns, etc.) 

 the veins are forked ; in Oymnogens they are forked (Ginkgo) or par- 

 allel (C^cas) ; in Endogens they are parallel (Grasses, Banana) ; in 

 Bxogens they are netted (Bryony). 



190. Three typical 

 forms of exogenous vena- 

 tion to be carefully noted 

 are: Ribbed, as in the 

 Gentian (Fig. 116); 

 Feather - veined ( penni- 

 nerved), as in the lAme 

 (Fig. 117); Radiate- 

 veined (palmi-nerved), as 

 in the Sweet- Gum (Fig. 

 118). To one or another 

 of these all other types 

 belong, whether sup- 

 pressed or developed, 

 simple or compound. 

 Compare Gentian with 

 Smilax (Fig. 241) ; Lime 

 with Locust ; Sweet- 

 Gum with Horse-Chest- 

 nut. 



191. Leaf-forms result from Venation, and follow a 

 definite law. 



8* 



Fio. 116.— Yellow Gentian (Oenliana lulea) : a, 

 capsule ; 6, same, out transversely ; c, vert. see. 

 of sd. 



