18 



ELEMEJ^TS OF BOTANY. 



lei or slightly divergent course toward tlie margin or apex (Fig. 15). Such 

 leaves are termed parallel-veined ; and here is found one of the distinc- 

 tions between endogenous and exogenous plants. 



In exogenous plants the venation is extremely varied and gives rise to 

 many different forms. In one particular, however, there is uniformity : 





Fig. 26.— Simple, pinnatcly veined leaf of chestnut. Margin serrate. 



the ultimate ramifications of the veins produce a net-work ; hence such 

 leaves are termed net-veined, and are thereby distinguished from the 

 parallel-veined leaves of endogenous plants. 



One of the commonest forms of exogenous leaves is that in which the 

 mid-vein pursues a direct course from the base to the apex, gi^'ing off, at 

 regular intervals, lateral branches which extend parallel with each other 

 to the margin. Such leaves are denominated pinnately or feather- 

 veined. The leaves of the beech and chestnut (Fig. 26) are famihar ex- 

 amples of this form. 





Fig. 27.— Simple, palmately veined leaf of castor-oil plant (Eicimis communis). 



Another common form of net-veined leaves is that in which there is no 

 ■strongly marked mid-vein, but instead a number of large veins pursue a 

 divergent course from the base to the mai-gin of the leaf. There may be 

 three, four, seven, or nine of these veins, diverging like the fingers of an 



