meristems of Ferns as a Phylogenetic Study. 343 
Hymenophyllum dilatatum , and Trichomanes reniforme the 
frond consists throughout of several layers of cells, but with- 
out stomata or intercellular spaces. 
I have already pointed out that in the leaf of Trichomanes 
radicans a marginal series of cells is to be distinguished even 
from the very first segmentations (Fig. 30) ; that the segments 
divide so that comparatively small parts are cut oft* from 
their edges, while the middle part (m, in) goes to form the 
marginal series of cells : thus the leaf is from the very first 
a winged structure. If transverse sections be cut from the 
phyllopodium the wings will be seen on either side of it as 
thin projections, which are smaller below than higher up : 
whatever be the extent of their surface they appear to be 
never more than one layer of cells in thickness (Figs. 46, 47, 
48). The marginal cells (in) retain their identity throughout, 
and are the cells from which all the others are ultimately 
derived : the segmentation of these cells is by transverse walls, 
and each of the marginal cells may, as regards its form and 
mode of segmentation when seen in section, be compared to 
the apical cell of certain Algae (e. g. Char a , Polysiphonia , etc.). 
An examination of the further developed wing shows that 
intercalary growth, in addition to the strictly marginal, takes 
place, as is indicated by division of cells at a distance from the 
margin (Fig. 48), but this intercalary growth is not specially 
localised. The development of the wings in the upper parts 
of the branched frond follow the same rule, and as regards 
the apices of the pinnae, etc., I can endorse the observations 
of Prantl on T. speciosum 1 . When seen in surface view the 
marginal cells of a young pinna are easily recognised, and 
their form is then seen to be that of half of a very narrow disc 
(Fig. 49). This form of the marginal cells, and simple mode 
of their segmentation by transverse walls seems to be the rule 
for the Hymenophyllaceae ; it remained however to be seen 
whether those exceptionally thick-leaved forms above quoted 
would conform to the type : it is well known that the frond of 
Trichomanes reniforme is thicker than that of most other Hy- 
1 1 . c. Plate III, Figs. 38, 39. 
