THE VEGETATIVE ORGANS COMPARED 337 



sort of sheath. The framework of the leaf when it reaches 

 the blade divides into a number of main branches, or ribs. 

 These radiate from the top of the petiole and maj'^ divide 

 again into secondary branches, or vei7is, which finally are 

 connected so as to form an irregular net-work by minute 

 branches called reinlets. When a leaf has ribs radiating thus, 

 like the bones in the palm of one's hand, it is said to be pal- 

 mately ribbed, and when the veinlets form an irregular net- 



FiG. 291. — Erect Silky Clematis {Clematis ochroleuca. Crowfoot Family, 

 Ranimculacece) . Flowering branches. Fruit. (Britton and Brown.) — 

 Perennial herb, somewhat woody, 30-60 cm. tall; leaves silky-hairy 

 beneath; flowers yellowLsh; fruit yellowish brown. Native home, 

 Eastern United States. 



work, it is netted-veined . The ribs and veins are also called 

 nerves and their arrangement the nervation of a leaf, the ar- 

 rangement of the veinlets being called the venation. 



On comparing with the leaves of the marsh-marigold those 

 of the ditch crowfoot we find the same general plan of struc- 

 ture l5ut with the difference that the leaf base is narrower, 

 and the l)lade is divided into branches corresponding to the 

 ribs. The branches of the upper leaves are so narrow as to 



