1 68 Sinnott.—The Evolution of the Filicinean Leaf- trace. 
for much information, especially on the rarer groups of ferns, he is indebted 
to the researches of others, especially of Mr. L. A. Boodle and Professor 
D. T. Gwynne-Vaughan. ‘ The Origin of a Land Flora’ by Professor 
Bower (6) also presents in a condensed form much information regarding 
the morphology of the group, and was found very useful. 
The base of the leaf-trace in living ferns presents three main types of 
differentiation, which may be characterized as the primitively monarch, the 
primitively diarch, and the primitively triarch, possessing one, two, or three 
clusters of protoxylem, respectively. The first type is found in the 
Osmundaceae and Ophioglossaceae, the second in the Marattiaceae, and the 
third in all other families of the Filicales. 
In the Osmundaceae, the first family in the monarch group, three 
species of each of the two genera Osmunda and Todea were examined. 
The structure of the very base of the leaf-trace in all of them is similar. 
It consists of a solid monarch strand of xylem, surrounded by phloem and 
elliptical in cross-section with its long axis at right angles to the stem 
radius, though in T. barbara the bundle tends to be slightly constricted in 
the middle and to be arched from the very first. As the trace passes 
through the cortex it grows larger and becomes steadily more and more 
concave towards the axis till the typical arch-shaped undivided petiolar 
bundle is produced. The protoxylem group, which is single at the base 
but soon begins to divide, shows a very strong tendency to be mesarch, 1 
and numerous instances were observed in every species examined where 
there were one or more centripetal tracheides on the adaxial side of the 
protoxylem (PI. XI, Fig. 12). By the time the trace is well out into the 
cortex, however, it has become clearly endarch, which condition is main- 
tained throughout the leaf. Mesarchy in the leaf-trace of the moder n 
Osmundaceae is interesting in connexion with the recent discovery by 
Kidston and Gwynne-Vaughan (19) of various fossil members of the family 
which show the base of the leaf-trace to be strikingly mesarch (Text-fig. 1). 
In the Ophioglossaceae, the other family in the primitively monarch 
group, the trace at its base seems to be characteristically single and provided 
with one protoxylem group. This is the case in Helminthos tacky s, Botry- 
chium , and nearly all the species of Opkioglossum , though in the sections 
aquilina , L., Adiantum cuneatum , and Pellaea atropurpurea , (L.) Link. ; Cyst op ter is bulbifera, (L.) 
Bernh., C.fragilis , (L.) Bernh., Polypodium vtdgare, L., Phegopteris hexagonopteris, (Michx.) Fee, 
P. Dryopteris , (L.) Fee, P. polypodioides, Fee, Woodwardia virginica, (L.) Sm., W. areolata, (L.) 
Moore, A splenium Filix-foeminct, (L.) Bernh., A. platy neuron , (L.) Oakes, Aspidium Thelypteris , (L.) 
Sw., A. spinulosum , (O. F. Muller) S\v., A. cristatum , (L.) Sw., A. marginale , (L.) Sw., A. Boottii , 
Tuckerm., Polystichum acrostichoides , (Michx.) Schott, and Marsilea quadrifolia , L. 
1 The term * mesarch ’, as used in the present paper, is descriptive of any protoxylem group, 
whether situated at the axis of the stem or not, which is surrounded by metaxylem. ‘ Endarch ’ 
describes any condition where the protoxylem is not at the axis and where there is no metaxylem 
(centripetal wood) formed on its inner face. ‘ Exarch ’ is used in the accepted sense, as the opposite 
of endarch. 
