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: 
| 1. Pinnate. | 
Free 3. Flabellate. Anastomose < 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 
