288 THE BRITISH FERNS. 
cellular tissue, of a delicate semitransparent character, and differing 
in appearance from that of most other Ferns. All the lesser divisions 
of the ribs are margined in this way, and the flat leafy parts thus 
formed are more or less confluent downwards ; the margin being con- 
tinued in some cases down nearly to the base of the stipes itself, 
and generally as far down as the top of the stipites, though some- 
times the main rachis is not bordered. The degree in which this 
confluence of the cellular portions takes place, represents the degree 
of division in the frond, and in most cases gives the peculiar form it 
assumes. So that the fronds in these two genera may be said to con- 
sist of branched wiry ribs, margined with a thin lamina of cellular 
tissue, these margins being flattened out and more or less confluent ; 
while the ends of a portion of the ribs as they reach the margin, or 
become projected beyond it, are surrounded by urn-shaped expan- 
sions of similar tissue within which the fructification is formed. 
On this genus, Sir W. J. Hooker observes* :—“ Closely allied as 
are the genera Hymenophyllum and. Trichomanes, it is rare that one 
has a difficulty in recognising them ; and yet it is not easy to point 
out the characters in few words. In Trichomanes, the involucres are 
mostly subeylindrieal narrow-urceolate, the mouth spreading, entire 
or cut into two short usually spreading lips, which when a little 
elongated afford the character of Didymoglossum of Desvaux ; their 
texture is firm and subcoriaceous, yet cellular; they are often quite 
sunk or immersed in the segment of the frond. The receptacles are 
filiform and not only exserted, but sometimes very much protruded, 
so as to be several times longer than the involucre, and either often 
varying on the same plant, or by their great fragility easily broken 
away, and then apparently short. The fronds are more generally 
erect, as far as can be judged from the dried specimens, and I am 
not aware that except in a very few instances, the margins of the 
segments are ever toothed or serrated, as is common in Hymeno- 
phyllum; but the characters now mentioned are not invariably 
constant. The species are I think more remarkable for beauty of 
form and delicacy of texture, than even those of Hymenophyllum.” 
The distinguishing feature of the genus Trichomanes may be taken 
to consist in the peculiar form of the involucre or urn-like cup in 
` * Hooker, Species Filicum, ï. 114. 

