12 Development of the Fern Leaf 



mostly appear, but in some plants development of the segments 

 is arrested before incisions appear. A case in point is afforded 

 by Camptosorus rhizophyllus. Lateral prolongations are some- 

 times seen at the base of the leaf-blade of this plant. These 

 have never, so far as I know, been found separated from the 

 main part of the leaf by incisions, yet their position and 

 character show that they are to be regarded as partly formed 

 segments. 



The position of each incision that divides each leaf or seg- 

 ment of a leaf into segments depends largely upon the character 

 of the leaf's venation ; in what way will be seen from the follow- 

 ing description of the different kinds of venation and their modes 

 of development as seen in the leaves of the species of the north- 

 eastern United States. 



These kinds of venation are: 



( i. Pinnate. f 



FreeK 3. Flabellate. Anastomosed 2. Pinnate. 

 ( 4. Unilateral. ( 



In the leaf with free pinnate venation the leaf -blade, and 

 each of the leaf's segments, if there are any, are entered at base by 

 a vein that extends longitudinally through it in the form of a mid- 

 vein which bears branches on both sides. These branches, each 

 of which has its base attached to the midvein, are the midvein's 

 "primary branches." They are usually either alternate or oppo- 

 site, and are either simple or bear branches. 



When the leaf or segment subdivides into segments, the 

 incisions, by means of which the segments are formed, occur be- 

 tween the primary branches of its midvein. In our north- 

 eastern species the first incision between these branches that oc- 

 curs on either side of the midvein usually occurs between the two 



