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 whUe 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, Adiantum pedatum. 

 The development of the venation of the leaf of this plant, and 



