t8o Sinnott . — The Evolution of the Filicinean Leaf -trace, 
the Onoclea stage from the simple triarch stage which we have considered 
primitive. 
There are also certain members of the Pterideae which show a reduced 
condition, such as Lindsaya and most of the sections Odontolema and Stem - 
lema of Davallia. Here the siphonostele is very much modified, the £ pith ’ 
consisting merely of an eccentric pocket of phloem. The leaf-trace at its 
exit is a small elliptical or slightly curved strand with a protoxylem group 
at each end. In the petiole, however, the typical triarch bundle is present 
although the adaxial hooks are lacking. This condition of the stele, known 
as the ‘ Lindsaya type’, has been much dwelt on by various writers and 
considered of importance as showing the origin of the siphonostele from the 
protostele. However, aside from its occurrence in a family of ferns admitted 
from their soral characters to be highly specialized, the fact that the nodal 
conditions are so widely at variance with what we have seen to be the 
universal rule in families of this series in lacking the triangular triarch 
bundle leads almost inevitably to the conclusion that we are dealing with a 
reduced rather than with a primitive structure. 
In the Pterideae, therefore, we see the development of the leaf-trace 
from a single triarch and mesarch strand at the node of Plagiogyria to the 
endarch and tetrarch condition by the disappearance of the centripetal 
wood ; the further modification of this to the Onoclea type by division into 
two ; and finally the elaboration of this simple double bundle into the 
complex petiolar system of Pteris and its allies. 
Most of the remaining Polypodiaceae probably originated from some- 
where in the vicinity of the Dennstaedtineae and have in general progressed 
further than have the Pterideae. Their simpler forms show the Onoclea 
condition, and in Asplenium Filix-foemina , where the petiole displays this 
type of bundle, numerous instances were observed of the occurrence at the 
base of the leaf-trace of a single strand, of which the two median protoxylem 
groups, the only ones developed at this level, showed a strong tendency to 
be surrounded by metaxylem, and several cases of complete mesarch y were 
noted. One of these is shown in the left-hand group in Fig. 20. Such 
a structure as this seems undoubtedly the primitive one from which the 
typical double and diarch bundle has been developed. 
Another fact noticed in these two-bundled forms was that the traces 
were always close together and sometimes, as we have seen, completely 
fused just as they departed from the stele, but that the sides of the gap 
seemed to pull away from them, and that although the traces themselves 
slowly spread apart, it was not until the base of the petiole was reached 
that they were as widely separated as were the two limbs of the gap from 
which they sprung. This constriction at the base of the leaf-trace seems to 
be present in all ferns. 
In the more advanced Mixtae the petiolar bundle may take on every 
