Development of the Fern Leaf 25 



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 



Fig. 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. 



