Maidenhair 39 



the primary branches of the midveins* By a deepening of the 

 one next the basal primary branch, which brings its base between 

 this branch and the midvein itself, the incision described above, 

 that divides the leaflets into two in the development of the leaf, 

 is formed; since that incision occurs between the vein-branches 

 formed by the first forking of the vein that enters the leaflet, and 

 since the basal primary branch of a unilateral midvein and the 

 midvein itself represent those branches. This applies both to 

 those leaflets possessing only one midvein and to those possessing 

 two midveins, since in the latter case either of the two may be 

 regarded as the basal primary branch of the other. 



The incisions between the primary branches of the midveins 

 suggest the lines along which future development, normal or 

 monstrous, of this plant's leaf may follow. The lobes which the 

 incisions form can be regarded as partly formed leaflets, and it 

 would not be surprising if at any time the incisions should deepen 

 sufficiently to render these leaflets distinct. It has been seen that 

 the basal incision already deepens at times (in the development 

 of the leaf's form), sufficiently to render the basal one of these 

 leaflets distinct. 



* This is best seen in the sterile leaves. In the sporophylls an indusium occasionally 

 extends over the tops of several primary branches, so that no incisions occur between those 

 branches. 



