20 



Development of the Fern Leaf 



leaf with flabellate free venation into segments, occurs between 

 the two vein-branches formed by the first forking of the vein 

 entering the blade or segment. Each new segment thus con- 

 tains one of these branches, with its ramifications, if any. If no 

 ramifications exist, some may appear afterward, or, if some exist, 

 more. There are also cases in which, coincidently with the fork- 

 ing of the vein that enters each segment, and before either of the 

 two branches formed by this forking fork, an incision subdivid- 

 ing that segment into segments occurs. The latter cases are 

 analogous to those in which the venation of the leaf is pinnate 

 and incisions occur between the primary branches of the leaf's 



midveins coincidently with the 

 formation of these branches 

 and while they are still simple; 



FIG. 7. 



FIG. 8. 



and in both sorts of cases the leaf-blade appears to be made up 

 of lobes containing each a simple vein.* 



In Figs. 7 and 8, segments of a leaf with free flabellate 

 venation are shown in different stages of subdivision. In Fig. 7 a 

 the beginning of the incision by means of which the segment is 

 to subdivide is barely indicated. In Fig. 8 b the incision is pro- 

 nounced. In Fig. jcde the incision has extended nearly far 

 enough to render one (e) of the two new segments distinct, and 



* See page 16. 



