THEIR STRUCTURE AND FORMS. 



91 



which have exogenous stems and at least dicotyledonous 

 embiyos. The latter prevails in ordinary plants with exogenous 

 stem and dicotyledonous embryo. 



173. Parallel-veined or Nerved leaves (of which Fig. 143 is 

 an illustration) have a framework of simple ribs (called by the 

 earlier botanists nerves, a name still used in descriptions), which 

 run from the base to tip, or sometimes from a central strong rib 

 to margin of the leaf, in a generally parallel and undivided way, 

 and sending off or connected by minute veinlets only. Grasses, 



Lily of the Valley, and the like, illustrate the commoner mode 

 in which the threads of wood run from base to apex. The 

 Banana and Canna are familiar illustrations of a mode not un- 

 common in tropical or subtropical endogens, in which the threads 

 or "nerves" run from a central rib (midrib} to the margin. 

 Parallel- veined leaves are generally entire, or at least their 

 margins not toothed or indented. The principal exception to 

 this occurs when the ribs or the stronger ones are few in number 

 and radiately divergent, as in the fiabeUiform leaves of Fan- 

 palms, a peculiar modification of the parallel- veined type. 

 Between leaves with nerves wholly of basal origin, and those 

 with nerves all springing from a midrib, there are various grada- 

 tions, and also in respect to curving. But parallel- veined or 

 nerved leaves may be classified into 



FIG. 142. A leaf of the Quince, of the netted-veined or reticulated sort, with 

 blade (6), petiole or leaf-stalk (/), and stipules (st). 



FIG. 143. Parallel-veined leaf of the Lily of the Valley, Convallaria majalis. 



