172 Sinnott . — The Evolution of the Filicinean Leaf -trace. 
arm. The resemblance of this bundle to that of certain species of Lygodium 
is increased by the fact that the most outward elements, those on the end of 
the projection, are the first of the metaxylem tracheides to lignify, a condi- 
tion which Prantl considers the beginning of the third protoxylem cluster 
found in the other members of the genus, but which instead seems rather to 
be its persistence in a reduced form. The only other type of bundle ob- 
served was in S.fistulosa , by Boodle, where certain of the traces in the cortex 
showed an arched structure somewhat resembling that found in Aneimia. 
S. pusilla is a very delicate form, and never seems to possess more than one 
small protoxylem cluster. 
To summarize conditions in the family, then, Lygodium palmatuni 
approaches most closely the roughly triangular mesarch trace which we 
have considered primitive, and has departed from it only in the position 
of its two lateral protoxylem groups, which are exarch instead of mesarch. 
The other species of the genus have progressed still further, the median 
group having bifurcated and become exarch also. The leaf-trace in 
Schizaea seems clearly to be reduced from the Lygodium condition by the 
loss of the adaxial phloem and the disappearance of one or more of the 
protoxylem clusters. Aneimia and Mohria , which are rather far advanced 
along other lines, show a leaf-bundle much modified from the hypothetical 
prototype. At the very base, however, the only difference is the band- 
shaped lengthening of the trace. The first-formed elements are still 
peripheral and the phloem surrounds the xylem, though these characters 
are soon lost at a higher level. 
Another important family of the Simplices is the Gleicheniaceae, 
comprising the genera Gleichenia and Platyzoma. These are all ferns 
of simple structure, possessing for the most part a protostelic central 
cylinder, near the periphery of which occur a number of mesarch protoxylem 
groups. Platyzoma and several species of Gleichenia have clearly attained 
a siphonostelic structure. 
The leaf-trace is always a single strand. The simplest method of its 
departure from the stele was observed in G. Speluncae , where a concentric 
triangular mass of xylem, with three mesarch protoxylem groups at its 
corners, is constricted off from the central cylinder (Text-fig. 3 and PI. XI, 
Fig. 15). Before this is fairly started on its way through the cortex, how- 
ever, a wedge of phloem and parenchyma enters it from the adaxial face. 
This soon comes in contact with the median protoxylem, and though the 
two lateral groups long retain their mesarch position, they, too, soon lie next 
the parenchyma and the typical endarch, arched petiolar bundle of the 
family is formed. The nodal structure of this species rather closely 
resembles that described for G. dicarpa by Boodle ( 5 ). 
In G. circinata v. macrophylla , and G. dicarpa v. longipinnata, a nodal 
island of parenchyma forms in the cylinder before the departure of the 
