38 Maidenhair 
A curious analogy exists between the development of the 
venation and the development of the form of the leaf-blade. As, in 
the course of development, each successive outermost pinna of 
each half of the dichotomous rachis becomes forked (by the trans- 
formation of its outer basal leaflet into a branch), the part below 
the fork becoming a continuation of the dichotomous rachis, one 
of the two parts above the fork a pinna upon that rachis, and the 
other part the new outermost pinna; so each successive outermost 
veinlet of each unilateral midvein becomes forked, the part 
below the fork becoming a continuation of the unilateral midvein, 
one of the two parts above the fork a primary branch upon that 
midvein, and the other part the new outermost veinlet. In this 
way the incipient unilateral midveins are developed and primary 
branches are added to their upper sides. 
In some leaflets, notably the apical and some of the lower, 
the basal primary branch of the unilateral midveins of each is 
developed in a similar way into a unilateral midvein, and the 
two unilateral midveins taken together may be said to constitute 
a dichotomous midvein with primary branches springing from 
its upper side. The analogy between this and the plant’s di- 
chotomous rachis with branches springing from its upper side is 
apparent. In those leaflets that possess only one unilateral mid- 
vein, or one half of a dichotomous midvein, the latter may be 
likened to one half of the dichotomous rachis. But here the 
analogy ceases, for the branches that spring from the upper sides 
of the midveins, unlike the branches that spring from the upper 
side of the dichotomous rachis, fork repeatedly. They are less 
complex in the early stages of leaf-development than in the 
later. 
Incisions, cutting the leaflets into shallow lobes, occur between 
