Sinnott. — The Evolution of the Filicinean Leaf -trace. 185 
arose from the early diarch Zygopterideae and perhaps gave off as a side 
branch the highly developed tree-fern flora of the Carboniferous, but which 
has persisted to the present day in a relatively simple state. 
The question now arises as to the origin of the leaf-trace in that great 
body of living ferns which we have called the primitively triarch group. 
There is no clear indication in any of the Botryopterideae of a bundle with 
three protoxylems. Asterochlaena , a genus closely related to Clepsydropsis , 
presents a suggestive structure, however, for here the two terminal immersed 
parenchyma islands, each possessing two protoxylem groups, become elon- 
gated and oblique in the upper part of the petiole, as shown in Text-fig. 11, 
though the bundle resembles that of Clepsydropsis in the lower part of its 
course. This general flattening of the bundle may possibly be an adapta- 
tion to the recently assumed dorsiventrality of the leaf. If such a trace as 
this should become somewhat reduced, and if the two abaxial or median 
groups of protoxylem should tend to approach each other and eventually 
to fuse, a triarch mesarch trace rather triangular in shape would result. 
Some such process as this seems the most probable one by which the leaf- 
strand of Lygodium or Gleichenia might have arisen. This explanation is 
further strengthened by the fact that in living ferns it is always the median 
group of protoxylem which is the first to divide in the course of the higher 
development of the trace. This is perhaps shown most clearly by the 
series from Plagiogyria to the Onoclea type. Indeed, from the great readi- 
ness of the median protoxylem to bifurcate, it may perhaps be inferred that 
complete fusion has been attained only in certain cases, and that the con- 
dition of two closely approximated median groups may be the ancestral 
one. Against this should be placed the evidence from Lygodium palmatum , 
the only species of the genus with but a single median protoxylem. Here 
it is mesarch instead of exarch, and thus in a distinctly primitive position. 
The evidence from the leaf-trace, then, as far as it goes, seems to point 
to the triarch Filicales as originally perhaps a somewhat reduced branch 
from the primitive Zygopterideae, that group which apparently stands so 
close to the common ancestry of all the ferns. Lygodium is the most 
ancient of all, for its solid bundle, with no sign of endarchy, runs through 
the whole leaf. In all the rest centripetal xylem is present only at the base 
of the trace, which becomes successively a double bundle and finally a com- 
plex series of separate bundles. The fact that Lygodiitm , which has held 
ancestral characters so well, is a climbing fern, and that the Botryopterideae 
may have clambered among their larger neighbours, leads to the suggestion 
that the ancestors of our living ferns may once have possessed the climbing 
habit. 
It is worthy of note that in Lyginodendron % one of the most ancient and 
primitive of the Cycadofilices, the bundle at its very base is a monarch and 
mesarch strand, but that this soon becomes diarch, and finally divides into 
