mer is terns of Ferns as a Phylogenetic Study. 349 
pellucid, and examination of it when mature shows that 
stomata are absent from the thinner portions of the frond, 
which consist of but two layers of cells, with no intercellular 
spaces between them : thus it has the ordinary c filmy 5 
character. In view of the fact that Trichomanes reniforme 
retains that mode of segmentation of the marginal cells which is 
characteristic of the Hymenophyllaceae notwithstanding that 
its leaf is four to six layers of cells in thickness, it was a matter 
of some interest to ascertain what is the mode of segmentation 
of this ‘ filmy ’ Asplenium, and to see whether it would, not- 
withstanding its ‘ filmy 5 character, retain the segmentation 
characteristic of Asplenium \ or show, in accordance with its 
texture when mature, the simpler type of segmentation of the 
Hymenophyllaceae. The result of the examination was to 
find that the segmentation is according to the more com- 
plicated type, two rows of cells being derived from each 
marginal cell (Fig. 58), and that these remain undivided 
by periclinal walls, except in the vicinity of the vascular 
bundles : thus its leaf represents the polypodiaceous type of 
segmentation reduced to its simplest terms. These observa- 
tions lead towards the following conclusions : — (1) that the 
‘filmy’ character may be assumed in Ferns of various affi- 
nity : this point will be discussed later ; (2) that within the 
circle of affinity of the leptosporangiate Ferns the mode of 
segmentation is not dominated by the absolute size 1 2 or 
external conformation, but that there is in certain Ferns a 
conservation of that type of segmentation which is character- 
istic of the family to which they belong, irrespective of increase 
( Trichomanes reniforme ), or decrease (. Aspleniitm resectum ) of 
bulk as compared with typical members of the same family. 
But the type of segmentation with two rows of segments 
from each marginal cell is not constant throughout the whole 
length of the frond in the Polypodiaceae : just as in Tricho- 
manes reniforme a more complicated structure is found in the 
1 Compare Sadebeck in Schenk’s Handbuch, Vol. I. p. 271, Fig. 36 D. 
2 As regards absolute size compare Fig. 51 of Trichomanes reniforme with Fig. 
58 of Asplenium resectum : they are both drawn to the same scale. 
