Sinnott . — The Evolution of the Filicinean Leaf -trace. 173 
trace, which at its exit, though triangular in outline, never shows the solid 
character of that of G. Speluncae . Its three protoxylem groups retain their 
mesarch position for some time, and the median one especially seems to 
tend to be shut off from the bay of parenchyma by the coming together 
of the two large lateral masses of metaxylem (Text-fig. 4 and PI. XI, Fig. 16). 
This seems much more pronounced in the former of the two species than in 
the latter. 
The base of the leaf-trace in the sub-genus Mertensia displays a more 
advanced condition than it does in Eugleichenia, to which all the species 
above described belong. In G. dichotoma , a large nodal island is formed, 
dividing the stele from a smoothly rounded arch of xylem, not triangular 
in outline, which departs with its three protoxylems as the leaf-trace. The 
ends of the arch soon unite, completing a continuous ring of xylem. The 
protoxylem is endarch from the very first. 
Fig. 4. The base of the leaf-trace of Gleichenia circinatav.macro1)hyUa. The three protoxylem 
groups are still mesarch. 
Fig. 5 . Leaf-trace and petiolar bundle of the Loxsoma or Dennstaedtia type. 
In the siphonostelic species there is of course no nodal island, but 
a smooth, rounded and rather flattish arch of xylem departs to the leaf with 
apparently three protoxylem groups, all of which are endarch. 
In all but the simplest species, the number of protoxylem groups 
increases in the petiole, usually to six ; but in Mertensia , with its larger 
leaf-bundle, there tend to be more, reaching an extreme in G. longissima , 
where Boodle found twenty distinct clusters. 
Platyzoma is a highly specialized and reduced form. Its leaf-trace 
is very small and possesses but two protoxylems. In the cortex it is 
endarch and collateral, but in the petiole it becomes concentric and in the 
rachis apparently mesarch. 
The structure of the vascular supply of the leaf in the Gleicheniaceae 
represents an advancing series. In G. Speluncae , the base of the trace 
approaches our hypothetically primitive condition even more closely than 
does Lygodium . Unlike the case in the latter genus, however, this structure 
soon changes to a much modified one in the petiole. In the other species 
of Eugleichenia , which is recognized as being the more primitive of the 
