36 Maidenhair 
leaflet occurred on the part of the rachis 0 7 in Fig. 8, no leaflet 
is seen on the corresponding section 0’ 7’ 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 pinne 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 i of the outermost 
pinna 7 in Fig. g into a branch, Fig. 9 becomes, essentially, Fig. 
10. The leaflet / in Fig. 9 becomes the outermost pinna h’ in 
Fig. 10, the part of the pinna z above the leaflet / in Fig. 9 be- 
comes the pinna 7’ in Fig. 10, the part of the pinna 7 below the 
leaflet i in Fig. 9 becomes the part /’ 7’ of the dichotomous rachis 
in Fig. 10, and the inner basal leaflet & of the pinna z 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 7 
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 7’ 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 pinnz of the di- 
chotomous leaves, or of the basal pinne (0’,c’) 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. 11 represents a leaf in a later stage of development than 
Fig. ro. 
