356 Bower . — The comparative examination of the 
more robust Osmundaceae, and it is only as the filmy cha- 
racter becomes pronounced that the segmentation as in the 
Hymenophyllaceae appears. The points of most importance 
are (1) that no clearly marked series of marginal cells is 
present from the first ; (2) that both the contour of the wing 
as seen in section, and its internal structure are variable ; and 
(3) that though the segmentation is finally that seen in the 
Hymenophyllaceae , still where the whole structure when mature 
is but one layer of cells in thickness, no other mode of seg- 
mentation is possible but that by transverse walls. 
Todea hymenophylloides ( = T. pelhicidd) occupies an inter- 
mediate position between T. barbara and T. superba as regards 
the texture of the leaf: the thin expanded wings of the 
pinnae and pinnules have no stomata or intercellular spaces, 
and consist of three to four layers of cells, of which the 
superficial cells are relatively small, while the central are 
large, the margins do not run out to a thin edge as in 
T. superba : in fact, the structure of the wings resembles that 
of Hymenophyllum dilatatum , and accordingly the question of 
their mode of development is an interesting one for comparison. 
Transverse sections of the pinnules show that the wings do 
bear marginal cells, but that these are not large or prominent, 
nor is their segmentation very regular : in the large majority 
of sections the segments appear to be cut off alternately from 
the marginal cell, according to the polypodiaceous type, but the 
segments divide up according to no fixed rule (Figs. 68, 69) : 
the cells which constitute the central layer are formed by peri- 
clinal walls, which apparently do not appear in all the segments. 
Thus it is seen that, though certain species of Todea assume in 
the wings of the upper parts of their leaves the £ filmy ’ structure 
which is characteristic especially of the Hymenophyllaceae, 
they do not at the same time assume the stereotyped mode of 
segmentation of marginal cells of the latter ; and indeed a 
marginal series of cells seems not to be formed till a compara- 
tively late period. When it is also remembered that the 
conformation of the base of the leaf and of its apical meristem 
corresponds to that of the robust T. barbara , the conclusion 
