Development of the Fern Leaf 1 3 



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- 



b 

 Fig. I. 



Fig. 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, 



Fig. 3. 



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. 



