184 Sinnott. — The Evolution of the Filicineem Leaf -trace. 
The monarch state was probably still earlier, for it occurs at the attachment 
of the leaf-trace to the stele in Clepsydropsis and Asterochlaena , as shown 
by the observations of Dr. Paul Bertrand. The single bundle with one 
mesarch protoxylem also occurs at the base of the leaf-trace, and very often 
in the foliar bundle among all the main groups of the Lycopsida, Equisetum , 
Lycopodium , Phylloglossum , Selaginella , P silo turn , and Tmesipteris , as well 
as in the fossil forms Lepidodendron and Sigillaria. It is thus probably 
the most primitive type of foliar strand. 
The xylem of the trace in fossil ferns seems to have always been 
surrounded by phloem. This concentric structure, though absent in several 
living families, is clearly the primitive condition of the leaf-bundle in the 
Filicales. 
In the Osmundaceae and Ophioglossaceae, families which stand apart 
from the rest of the ferns on many counts and which have become much 
modified from the ancient type, the base of the leaf-trace shows the primitive 
monarch and mesarch structure. Though this may possibly be a reduction 
from a diarch or triarch state, it seems reasonably clear that in the Osmund- 
aceae, at any rate, it is the original condition, for in ancient members of 
the family from the Jurassic, as described by Kidston and Gwynne- Vaughan 
(19), the base of the leaf-trace is an elliptical mesarch bundle with one 
protoxylem group (Text-fig. 1 ). It is worthy of note that Chrysler (11), who 
has made a careful study of the leaf in the Ophioglossaceae, comes to the 
conclusion that this family approaches the Osmundaceae more closely than 
it does any other group of ferns. It seems reasonable to suppose, therefore, 
that these two families might have left the primitive fern stock before the 
leaf-bundle had become complicated, and that in them its later modifications, 
as well as those of the stele, proceeded independently of the rest of the ferns. 
The occurrence of two bundles at the base of the trace in the Marat- 
tiaceae, a family where the leaf-bundle and the stele are so highly compli- 
cated, is logically explained only by assuming that we have here to deal 
with a persistence of a much simpler condition, which prevailed throughout 
the petiole and rachis of the ancestral leaf. This double bundle, which is 
so clearly mesarch in Danaea , has very possibly arisen by a splitting into 
two of the diarch and mesarch strand of the primitive Zygopterideae, the 
constriction of which is very evident in Clepsydropsis and Metaclepsydropsis . 
The presence of scattered centripetal tracheides in the petiole of Danaea , 
which is almost certainly an ancestral character, seems to indicate that this 
genus is more primitive than the others, in which case the occurrence of 
mesarchy in the base of its trace is all the more significant. That Psaronius 
is near the direct line of descent of the simple living Marattiaceae seems 
very doubtful, and it is perhaps nearest the truth to consider the whole 
family, which, like the Osmundaceae and Ophioglossaceae, stands apart 
from other ferns in many of its characters, to be a very primitive one which 
