342 Bower . — The comparative examination of the 
results in the midrib 1 , — a structure which is necessary in order 
to strengthen and support the thin expansion : this formation 
of the midrib, which in the Hymenophyllaceae begins below 
the extreme apex, extends in the higher Ferns to the extreme 
tip ; in other words, the leaf in early stages (Polypodiaceae), or 
throughout its whole length (Marattiaceae) may be of massive 
construction ; but though this is the case, the wings which 
are borne laterally upon it are still to be recognised as corre- 
sponding to those of the Hymenophyllaceae, and represent the 
true margins of the bilateral leaf. On this ground, and also 
on account of the constant presence of the wings on the 
leaves of Ferns (and as far as I am aware Pilularia is the only 
case of their entire absence), they are parts worthy of more 
exact study than has yet been conceded to them. Since 
morphological importance of phenomena is measured chiefly 
by their constancy of occurrence in long series of organisms, 
these wings which are so constant and prominent in Ferns, as 
also in Cycads and many Angiosperms, must rank as parts 
which cannot be ignored. In studying the development of 
these wings, a similar series of forms to those above quoted 
in reference to the apices of root, stem, and leaf will be 
examined, and I shall show that, as we rise in the series, in 
point of structure of the young wing, an increase of complexity 
accompanied by a lowering of the centre of construction is to 
be traced, similar to that which has been demonstrated in the 
apices of root, stem, and leaf. 
In describing the development of the leaf in the Hymeno- 
phyllaceae Prantl has already drawn attention to the early 
differentiation of marginal cells 2 , and both he and Mettenius 
have figured and described the wings as consisting of a single 
layer of cells : this is typically the case for these plants, but 
as noted by both writers 3 , it is by no means without ex- 
ception, an increase in the number of layers being seen near 
the margin or near the nerves in various species, while in 
1 Compare Prantl, Hymenophyllaceae, Plate III, Figs. 38 A, B, C, and 39. 
2 Hymenophyllaceae, Plate I, Fig. 1. 
3 Mettenius, Hymenophyllaceae, pp. 457-8. 
