36 Maidenhair 



leaflet occurred on the part of the rachis o j in Fig. 8, no leaflet 

 is seen on the corresponding section o' j' in Fig. 9. This ex- 

 plains why, in mature leaves of A. pedatum, a leaflet is seen on 

 one side of the dichotomous rachis below the first (central) pinna 

 and not on the other. 



The dichotomous leaf-blade (Fig. 9) having been formed, the 

 dichotomous rachis is prolonged and pinnae and pinnules are 

 added to its upper side by the development, repeated in successive 

 leaves, of the outer basal leaflet of the outermost pinna at each 

 end of the dichotomous rachis into a branch. For instance, by 

 the development of the outer basal leaflet h of the outermost 

 pinna i in Fig. 9 into a branch, Fig. 9 becomes, essentially, Fig. 

 10. The leaflet h in Fig. 9 becomes the outermost pinna h f in 

 Fig. 10, the part of the pinna i above the leaflet h in Fig. 9 be- 

 comes the pinna i' in Fig. 10, the part of the pinna i below the 

 leaflet h in Fig. 9 becomes the part /' /' of the dichotomous rachis 

 in Fig. 10, and the inner basal leaflet k of the pinna i in Fig. 9 

 becomes the leaflet k', on the dichotomous rachis in Fig. 10. It 

 is evident that if in Fig. 9 the inner basal leaflet k of the pinna i 

 had been higher on the stalk of that pinna than the outer basal 

 leaflet h, instead of lower, the corresponding leaflet in Fig. 10 

 would appear on the pinna i' instead of on the dichotomous 

 rachis. Since there seems to be no reason why the outer rather 

 than the inner basal leaflets of the outermost pinnae of the di- 

 chotomous leaves, or of the basal pinnae (b f , c f ) of the leaf shown 

 in Fig. 8, should be the higher, we should expect to find in 

 some leaves of this plant, and do find, leaflets missing from the 

 dichotomous rachis that are present in others. 



Fig. 1 1 represents a leaf in a later stage of development than 

 Fig. 10. 



