24 Development of the Fern Leaf 
veins (by means of which segments are formed), have not quite 
reached the midveins. 
(2) When in the flabellate-veined leaf one of the two segments 
formed by the subdividing of a simple leaf or segment subdivides 
into segments coincidently with the occurrence of the incision by 
means of which it is formed, and before the lower part of it has 
become transformed by attenuation into a rachis, the same effect 
may be produced as when, in the pinnate-veined leaf, two of the 
incisions between the primary branches of the midvein of a 
simple leaf-blade or segment occur above the two lowest primary 
branches (one on each side of the midvein), before other such 
incisions occur and while the midvein is in an incipient state. 
(3) When, in the pinnate-veined leaf, one of the incisions 
between the primary branches of the midvein of the simple leaf- 
blade or segment occurs above the lowest primary branch on 
one side of the midvein before any other such incision occurs, and 
while the midvein is in an incipient state, this leaf-blade or seg- 
ment may appear to be cut more or less vertically in two, and 
thus like the simple leaf-blade or segment of the flabellate-veined 
leaf which is subdividing into two segments. 
In the leaf with unilateral venation, each segment of the leaf 
is entered at base by a vein which extends through it at one side, 
in the form of a unilateral midvein bearing branches on one side 
only. These branches, each of which has its base attached to the 
midvein, are the midvein’s primary branches, and may be simple 
or bear branches. 
In Fig. 9 is shown the segment of such a leaf. 
I have been able to study the development of the leaf with 
unilateral venation in one plant only, namely, Adzantum pedatum. 
The development of the venation of the leaf of this plant, and 
