170 Sinnott . — The Evolution of the Filicinean Leaf -trace, 
investigated. Fig. 14, Plate XI, shows a typical double bundle, in both parts 
of which the protoxylem occupies a central position. 
It would seem reasonable to conclude, then, that the primitive con- 
dition of the leaf-trace in this group of ferns consisted of two concentric 
bundles, each possessing a single protoxylem cluster which was possibly, 
and as we shall later see, probably, in a mesarch position (Text- fig. 2). 
The third main group of the Filicales, which has been characterized as 
primitively triarch, comprises all the remaining families of the order. Many 
of its members have reached a high degree of development, and in them the 
structure of the base of the leaf-bundle is very complex. In the opinion of 
the writer, however, the primitive condition from which they have all been 
derived was one where the leaf-trace consisted of a single roughly triangular 
Fig. 1. The primitive monarch type, as represented by the base of the leaf-trace of Thamnopteris . 
Fig. 2. The primitive diarch type, as represented by the base of the leaf-trace of Danaea. 
Fig. 3. The primitive triarch type, as represented by the base of the leaf-trace of Gleiclienia 
Speluncae. 
and concentric strand, near each corner of which was embedded a mesarch 
protoxylem group (Text-fig. 3). In support of this view, evidence derived 
from a study of the leaf-trace in the various fern families will be brought 
forward. 
The Filicales have recently been divided by Bower (7) into three main 
groups, the Simplices, the Gradatae, and the Mixtae, according to the order 
of development of the sporangia in the sorus. This classification has 
received considerable support along anatomical and other lines, and is now 
accepted by most botanists as a rather close approach to a natural arrange- 
ment, the three groups representing an ascending series in evolutionary 
history, with the Simplices as unquestionably the most primitive. Aside 
from the ferns which we have already discussed, this group includes the 
simplest of the triarch Filicales, the Schizaeaceae, Gleicheniaceae, and 
Matonineae. 
The Schizaeaceae include four genera : Lygodium , Aneinna, Mohna, 
and Schizaea. Lygodium is protostelic, and for this reason, as well as on 
other evidence connected with its reproduction, is well considered the most 
primitive member of the family. Aneimia and Mohma are more advanced 
in their stelar anatomy, spore output, and gametophytic characters, while 
Schizaea is in all probability a reduced form. 
