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 mid veins, 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, i 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, i 

 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 of 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 



