1 82 Sinnott . — The Evolution of the Filicinean Leaf trace. 
which is clearly triarch and apparently exarch, as we should expect. The 
structure of the leaf in young plants of the Polypodiaceae is a subject 
worthy of much further investigation. 
The only family of the Mixtae which has apparently not arisen through 
the general line of ascent which we have outlined above is the Dipterineae. 
These ferns in the adult condition display a very close resemblance to 
Matonia in stelar structure, save that the siphonosteles are mesarch instead 
of endarch. The petiolar bundle is a single arch with much involuted 
ends and very many endarch protoxylems. The number of these, however, 
is much smaller as the trace leaves the stele, though it apparently is never 
as low as three. Young plants were examined by Seward and Dale (22), 
but are not described in sufficient detail to make clear the histology of 
the leaf-trace. From several lines of evidence, however, the family seems to 
have been derived from Matonia , and is therefore without much question 
primitively triarch. 
The heterosporous genus Marsilea was also investigated and seems 
clearly to have come from among the triarch ferns. The leaf-trace, as it 
departs from the stele, is an arch with three endarch protoxylems. The 
median one of these soon bifurcates, and the trace then divides into two 
bundles, each of which has at either end a cluster of protoxylem, though no 
definite hook. This condition, which resembles that among such delicate 
ferns as Asplenium Platyneuron , is doubtless a reduction from the Onoclea 
type. 
From the rapid survey of the Filicales which has just been completed, 
it would seem that our original classification of the order into three groups 
possessing primitively monarch, diarch, or triarch leaf-traces respectively, 
is one which holds for living families of ferns. The conclusions reached, 
though in accord with several writers who consider a relatively simple leaf- 
trace as primitive, do not agree with the opinions of Bertrand and Cornaille, 
who contend that a wide polyarch arch is the ancestral condition and that the 
Onoclea type of bundle represents a reduction from this. The evidence 
which we have cited from conditions in primitive families and from ontogeny 
do not support such a conclusion however. The results derived from our 
comparative study of the leaf-trace in living ferns strengthen most of the 
recent conclusions as to their relationships that have been based on other 
evidence, and it remains in conclusion to see what light is thrown on the 
subject by the structure of the fossil Filicales. 
Anatomical knowledge of Mesozoic and Tertiary ferns is unfortunately 
very slight, and it is the beautifully preserved fossils of the Coal Measures 
which give us our best idea of what primitive members of the order were 
like. These plant remains are comprised in two main groups : the first, of 
large tree-ferns included under the comprehensive genus Psaronius ; and 
the second of much smaller and simpler plants, the Botryopterideae. 
