3 8 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 sprmg 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 



