Development of the Fern Leaf 13 
lowest primary branches on that side, and each successive inci- 
sion, except the first, occurring on either side, normally occurs 
between the two primary branches next above the last incision 
previously formed on that side.* Each new segment thus con- 
a 

Fi 
Fic. 1. Fic. 2. 
tains one of the midvein’s primary branches, including its ram- 
ifications, if any, and this branch, unless its development be 
arrested first, develops into the segment’s midvein, bearing 
branches on both sides. This will be readily understood from 
figures 1-4. 
In figures 1-3 the partial subdivision of a primary segment, 

of a leaf with free pinnate venation, into secondary segments of 
the leaf, is shown. In Fig. 4, which represents a section of this 
primary segment, the ending of this subdivision and the begin- 
} * In some foreign species the incisions, while occurring between the primary branches 
of the midveins in such a way that each segment formed by them contains one of these 
primary branches, occur at wider intervals than as above described, some primary branches 
without incisions between them intervening between the new segments. 
