FOKMS OF SIMPLE LEAVES. 



73 



II. Veins proceeding from base to apex. 



1 . Veins more or less convergent (fig. 1 73), as in Iris, Lilies, Grasses (fig. 194 ). 



2. Veins more or less divergent, as in Fan Palms. 



To this may be added the venation common in Ferns where the veins divide 

 in a forked manner. This venation has been called Furcate (furca, a fork). 



Forms of Leaves. 



145. Leaves have been divided into simple and compound. The 

 former have no articulation beyond the point of their insertion on the 

 stem, or consist of one piece only, which, however, may be variously 

 divided (figs. 136, 137, 138, &c.). The latter have one or more artic- 



ulations beyond the point of their insertion on the stem, or consist of 

 one or more leaflets (foliola) separately attached to the petiole or leaf- 

 stalk (fig. 141). In the earliest stage of growth all leaves are simple 

 and undivided, and it is only during the subsequent development 

 that divisions appear. The forms which the different kinds of simple 

 and compound leaves assume, are traced to the character of the 

 venation, and to the amount of parenchyma produced. 



146. Simple Leaves. When the parenchyma is developed symme- 

 trically on each side of the midrib or stalk, the leaf is equal (fig. 149); 

 if otherwise, the leaf is unequal or oblique (fig. 136), as in Begonia. If 

 the margins are even and present no divisions, the leaf is entire (in- 



Fig. 136. Leaf of Ulmus effusa. Reticulated venation ; primary veins going to the margin, 

 which is serrated, Leaf unequal at the base. 

 Fig. 137. Pinnatifld leaf of Valeriana dioica. 

 Fig. 138. Bipinnatifld leaf of Papaver Argemone. Feather-veined. 



