111 
The Hymenophyllacece. 
system or stomata, and usually consisting of but a single layer of 
cells. At one time the view was extensively held that this filmy 
habit of leaf is primitive, a relic of incomplete adaptation to aerial 
life, and comparisons were made with the leaves of Bryophyta. 
There is a good deal that can be urged against this view, though it 
can hardly he said to have been conclusively disproved, and on the 
whole we may regard it as more probable that the filmy fronds have 
been derived, as they undoubtedly have in Todea and Asplenium, 
from fronds of the normal type, with stomata and intercellular 
spaces. The sori and sporangia of the Hymenophyllaceae belong 
to a relatively primitive, though not the most primitive type, as 
Bower has admirably shown. 
The vascular system (a knowledge of which we owe mainly to 
the work of Prantl and especially of Boodle, from whose excellent 
paper (’00) the data here employed are mainly taken) shows many 
interesting features. Though there is a fairly wide range of 
structure, it is on the whole strikingly simple, and while the 
characters of the simplest forms are undoubtedly due to reduction, 
there is, I think, a great deal to be said for the belief that the 
vascular structure of what may be called the “central” types shows 
really primitive features which are shared by the Botryopterideae. 
The genus Trichomanes has a greater variety of vascular 
structure than Hymenopliyllum, but one species, T. reniforme, has a 
type of stem-stele very closely resembling that of Hymenopliyllum, 
Fig. 23. Trichomanes renifonne. Stele of rhizome. The thick pericycle 
is shown round part of the stele. The smallest of the central thick-walled 
elements represent protoxylem ; two distinct groups are visible, one to the 
right, the other to the left, x 75. From Boodle. 
particularly of H. dilatation, H. demission and H. scabrum. From 
such a type Boodle is inclined to derive the forms of stele found in 
the other species of both genera. This type, which I call the central 
type of the family, is illustrated in Fig. 23. The stele, somewhat 
oval in section, contains a more or less interrupted ring of tracheids. 
