mer is terns of Ferns as a Phylogenetic Study. 379 
specially located in the superficial tissues, and much less 
plentiful within 1 . The chief differences of external condition 
between submerged and subaerial plants are, that transpiration 
is in the former impossible, and that they are less exposed to 
direct sunlight. Now these are also the conditions of plants 
which grow in shade, where the air is constantly near to the 
point of saturation with water ; in them transpiration is at 
a minimum, while they receive only diffused sunlight. Ac- 
cordingly a similarity of construction to that of water-plants 
may be expected, and it is clear that the leaves of the 
Hymenophyllaceae show more or less distinctly all the cha- 
racters above noted, some of them even in a more marked 
degree than those of submerged Phanerogams. Further, 
while intercellular spaces are often large in submerged Phane- 
rogams, they are entirely absent from the wings even of those 
filmy Ferns which are several layers in thickness. The 
c filmy 5 texture is then to be regarded as an adaptive charac- 
ter, suited to life in a moist and shaded habitat 
But it is not found exclusively in the Hymenophyllaceae; 
Asplenium resectum and the Leptopteris section of the genus 
Todea show a similar texture of the frond, with absence of 
stomata and intercellular spaces, and with special location of 
the chlorophyll near the surface. I have shown that in the 
development of these filmy wings the segmentation of Asple- 
nium resectum is that of the Polypodiaceae ; also that that of 
the ‘filmy 5 Todeas (which in every other respect correspond 
to other Osmundaceae) is at first like the latter, and that it is 
only in the later stages that a mode of segmentation is as- 
sumed which corresponds, and even then imperfectly, to that 
characteristic of the Hymenophyllaceae. Taking all the facts 
together, the general conclusion may be drawn that the 
‘ filmy ’ texture is an adaptive character assumed by plants 
which live in shady and damp situations, and that it is not 
a safe indication of affinity ; that while its most characteristic 
representatives are found among the Hymenophyllaceae, it 
has probably originated independently in at least three dis- 
1 Schenk, Vergl. Anat. d. subm. Gewachse, p. 3 , and various other writers. 
