7 1 8 Gwynne- Vaughan. — Observations on the 
ever become of even thickness all round in such a dorsi- 
ventral rhizome. 
Judging from the D aval lias and Lindsayas alone, it would 
seem that a continuous core of phloem was already present 
in the stele before the ground-tissue began to be decurrent at 
all. On the other hand, from Boodle’s 1 description of the 
structure of the node in certain Gleichenias ( G . dichotoma , 
G.flabellata), it appears that both the ground-tissue and the 
phloem are decurrent together and for a short distance only 
into the substance of the xylem. If this may be regarded as 
the first step in another series of modifications similar to 
those described above, it follows that in this case a soleno- 
stele could be reached without passing through a stage with a 
solid core of phloem, because both ground-tissue and phloem 
would be decurrent contemporaneously. 
The whole theory is, of course, open to the inevitable 
criticism that the series of forms in question is perhaps one of 
reduction, and not one of advance. It seems to me, however, 
that the increased thickness of the lower region of the xylem- 
ring forms an insuperable objection to the general application 
of any reduction hypothesis to this series. That the vascular 
system actually has undergone reduction in a number of 
different Ferns is well known, but although it must be admitted 
that in some cases structures have resulted which bear a strong 
superficial resemblance to those described above, it may never- 
theless be shown that there are crucially important differences 
between them. For instance, in Vittaria stipitata , which 
possesses a dorsiventral rhizome and leaves in two rows on its 
dorsal surface, the stele is very small and the xylem-ring 
is only one or two elements thick. Each leaf is supplied 
with two separate traces, one of which departs from each side 
of the leaf-gap, and the ground-tissue is decurrent through 
the leaf-gaps into the stele. So far as the stele itself is con- 
cerned each leaf-gap is closed up again before the next above 
is formed, but the gaps in the xylem-ring remain open long 
enough to overlap, so that in this respect the stele resembles 
1 On the anatomy of the Gleicheniaceae, 1 . c., p. 720. 
