Development of the Fern Leaf 17 
have examined, although in some species they have appeared 
much more nearly so than in others. Occasionally two on one 
side of the midvein intervene between two on the other, but for 
the most part they appear to vary from opposite to alternate. 
The apparent positions, on the midveins, of the primary branches 
between which the incisions occur have been taken as criteria 
in determining the positions of the segments formed by these 
incisions, instead of the positions of the ends of the incisions, 
since the latter vary according to the degree of development of 
the segments. But the exact positions of these branches on the 
midveins can only be determined by examination of cells of the 
leaf, which I have not attempted. 
It will be noted that in Figs. 1 to 4 the successive segments, 
whether partly formed or distinct, whether entire, cleft, or divided, 
continually elongate at apex, their midveins also lengthening and 
sending out new primary branches in succession above those pre- 
viously formed. This and a similar lengthening of the leaf-blade 
at apex are common occurrences in the development of fern 
leaves. To one or both the long tapering tips of some compound 
leaves are due in many cases. 
It will also be noted that the forms of the segment belonging 
to the primary series of segments of the leaf, shown in Figs. 1 
to 3, are essentially duplicated by the forms of the segments be- 
longing to the secondary series, shown in Fig. 4. Duplication of 
the forms of the segments of one series by those of succeeding 
series is often seen when, as in the section cf leaf here figured, 
formation of the segments of one series upon those of another is 
not particularly evident until the latter segments have become 
fairly large, comparatively speaking. It is also often seen when 
2 
