THEIR STRUCTURE AND FORMS. 



93 



classes into which they divide are the pinnately veined and the 

 palmately veined. 



177. Pinnately or Feather- reined (or Penninerved) leaves 

 are accordingly those of which the veins and their subdivisions 

 are side branches of a single central rib (midrib), which traverses 

 the blade from base to apex ; the veins thus being disposed in 

 the manner of the plume on the shaft of a feather. (Fig. 142, 

 152, &c.) Sometimes these continue straight and undiminished 

 from midrib to margins (straight- veined, as in Beech and Chest- 

 nut, Fig. 152), sending off only small lateral veinlets ; some- 

 times they ramify in their course into secondary or tertiary veins, 

 and these into veinlets. Pinnate venation in reticulated leaves 

 naturally belongs to leaves which are decidedly longer than wide. 



178. Some of the primary veins, commonly among the lower, 

 may be stronger than the rest, and thus take on the character of 

 ribs, or by gradations pass into such. The leaf of the common 

 annual Sunflower (Fig. 155) becomes in this way triple-ribbed or 

 tripli-nerved. The appearance of a second pair of such strength 

 ened veins makes the venation quintupli-ribbed or quintupli-nerved. 



Through the approximation of such strong veins to the base of 

 the blade, this venation may pass into the 



179. Palmately, Digitately, or Radiately Veined (or Palmi- 

 nerved) class, of which leaves of common Maples and the Vine 

 are familiar examples. (Also Fig. 158-160, &c.) In these 



FIG. 144-157. Various forms of simple leaves, explained in the text and in the 

 Glossary. 



