474 Sinnott. — Some Jurassic Osmundaceae from New Zealand. 
placed upon those who maintain that there have been exceptions to the 
normal course of evolution, and very strong evidence must be brought 
forward before their claim can be considered as established. 
That such evidence is actually at hand, is the contention of many 
eminent anatomists. Facts recently called to notice, particularly by the 
researches of Kidston and Gwynne-Vaughan (1) on the fossil Osmundaceae, 
have led to the establishment of a plausible hypothesis for the origin of the 
family, which assumes that they have been derived from the Zygopterideae. 
These were the dominant Ferrrs of the Carboniferous, and might therefore 
be supposed to be the ancestors of the Mesozoic Osmundaceae. Many 
of the Zygopterideae show a well-marked internal region of shortened 
vascular elements, and a peripheral zone of longer ones, and some species 
show a considerable accumulation of parenchyma in the centre of the stele. 
Considering the anatomy of the stem alone, the transition seems easy, on 
the commonly accepted theory, between such forms and the condition of 
a ‘ medu Hated monostele ' like that of Osmimda regalis. 
A serious objection to this derivation of the Osmundaceae, and one 
which is recognized by the authors just mentioned, is the striking dissimi- 
larity of the two families in the vascular supply of the leaf. The typical 
complex zygopteridean leaf-strand is as widely different from the simple 
arch of the Osmundaceae as could well be imagined, and as it has been 
demonstrated that the anatomy of the leaf is of great importance in 
determining relationships, since it is even more slow to change than that 
of the stem, anything like a close connexion between the Osmundaceae and 
the typical Zygopterideae, at least, is not to be thought of. 
This difficulty is met, however, by assuming that it was the very 
primitive Zygopterideae which gave rise to the Osmundaceae. The leaf- 
trace of such a genus as Clepsydropsis is much less complex than that of 
the other members of the family, and is probably more primitive, but it is 
still typically diarch with two widely separated protoxylem groups. The 
writer (4) has recently called attention to the fact that the structure of the 
leaf-trace, just as it leaves the stele, is remarkably conservative throughout 
the Filicales, and has shown that the Osmundaceae and Ophioglossaceae, 
alone among Ferns, are characterized by a single, mesarch protoxylem at 
the base of the leaf-trace. This monarch condition is a distinctive feature 
of the ancient and modern members of both families. Evidence was at the 
same time presented for the derivation of the leaf-trace of the other Fern 
groups by an amplification of the diarch condition of something like the 
Clepsydropsis type. If the separation of Ferns on the structure of their 
leaf-traces into a primitively monarch and a primitively diarch group is 
a natural one, it is evident that the Osmundaceae fall into one division, 
and the Zygopterideae into the other. Of course one may derive almost 
any type of leaf-trace from the simple condition of Clepsydropsis , and it is 
