96 Gwynne- Vaughan . — Observations on the 
original design has become more or less masked by the split- 
ting up of the horseshoe into a number of separate portions, 
which portions may subsequently fuse or anastomose with 
each other in different ways ; yet it is rarely very difficult 
to trace back such arrangements to the typical scheme. 
I lay some stress upon this point because, in the Ferns, 
where such a continuity of design is apparent, I am inclined 
to regard the modifications in the structure of the leaf-trace 
as being in a manner complementary to those of the stelic 
system of the stem, and taken together as presenting material 
which deserves very careful consideration in discussing the 
relationships of the plants themselves. I believe that the 
simplest form of this general design (an undivided horseshoe 
such as in L ox soma) may be regarded with tolerable safety 
as being relatively primitive, and that therefore its occurrence 
together with a primitive stem-anatomy may be used as 
additional evidence of much weight regarding the relative 
position of the plant as a whole. 
Finally, I cannot refrain from referring to one other point 
which the study of Loxsoma forcibly accentuates, and that is, 
the general similarity that exists between the anatomy of 
the solenostelic Ferns and that of Marsilea. Allowing for 
a certain amount of simplification due to a water-habitat, 
Marsilea differs from them in no essential feature, neither 
in the structure of the solenostele, the departure of the 
leaf-trace, nor in the form and structure of the latter. As 
regards the leaf-trace, it is almost a facsimile of the petiolar 
meristele of Loxsoma when reduced in size far up the rachis 
(Fig. no, d). Cavity-parenchyma is present, and the sieve- 
tubes in the deep bay opposite the median protoxylem, 
although they are not exactly sclerosed, are exceptionally 
thick-walled. A similar parallelism is found in other details, 
such as the splitting of the scalariform tracheides along a spiral 
line when macerated, &c. The leaf-trace protoxylems do not 
appear to be prolonged down into the stem. In the stems 
of species with a tolerably stout xylem-ring there do not 
seem to be any localized protoxylems, although the smaller 
