Development of the Fern Leaf fas 
the occurrence of incisions, etc., in the leaf, are discussed in the 
chapter on Adiantum pedatum,* and so need only be touched 
upon here. 
In the leaf of this species, incisions occur between most or all 
of the primary branches of the midveins. These incisions, with 
the exception of the basal one, do not deepen sufficiently to render 
the segments formed by them either distinct or nearly so. The 

Fic. 9. 
basal incision occasionally deepens sufficiently to render the basal 
one of these segments, distinct, and in this way new segments are 
added to the leaf. The basal primary branch of the midvein 
of the segment from which the basal segment is separated is thus 
included in the new segment and develops into its unilateral mid- 
vein. This development may become evident either before or 
after the basal segment becomes separated, and similar develop- 
ment of the midvein’s basal primary branch is sometimes seen 
in segments from which the basal segment never becomes sepa- 
rated. 
It will be seen that the ultimate form of the compound fern 
leaf depends chiefly upon the manner in which the leaf-blade, 
when first formed, and such of its successive segments as do so, 
subdivide into segments, upon which, if any, of the segments 
* See pages 35-37. 
