18 



ELEMENTS OF BOTANY. 



lei or slightly divergent course toward the 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, pinnately 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, giving 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 familiar ex- 

 amples of this form. 



Fig. 27.— Simple, palmately veined leaf of castor-oil plant (fiicinus 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 margin of the leaf. There may be 

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



