Maslen . — The Structure of Mesoxylon Sutcliffii {Scott). 387 
Passing to the internal structure of Mesoxylon Sutcliffii , we find that 
it presents, of course, all the general characters of the genus which have 
been already briefly described (p. 384). The stem is characterized by the 
possession of a relatively large pith (PI. XXXIII, Fig. 1), which in our 
specimens averages about 1-4 cm. in diameter, or nearly one-half of that of 
the stem as a whole, including the bases of the leaves. 
In the relative dimensions of the pith M. Sutcliffii resembles Cordaites 
and differs from Poroxylon , in which the pith is actually and relatively 
considerably smaller. 
Transverse sections (PI. XXXIII, Fig. 1) show that the pith presents 
a well-marked division into two regions : an outer one, pp ., consisting of 
a narrow continuous zone of parenchymatous cells, many of which possess 
dark contents, and a much larger central region, c.p ., in which the medulla 
has a disorganized appearance. Longitudinal sections, however, show that 
the inner pith has a marked discoid structure (PL XXXIII, Fig. 2, c.p.) 
somewhat resembling the well-known discoid pith found in most species of 
Cordaites. 
The discoid pith is an important character which is probably common 
to all our species of Mesoxylon 1 and to most forms of Cordaites , and it is 
one of the characters in which the new genus differs from Poroxylon , in 
which the pith is throughout a continuous mass of parenchymatous cells. 
Surrounding the pith, and in contact with the zone of secondary wood, 
are a considerable number of ‘ bundles ’ arranged very distinctly in pairs 
(PI. XXXIII, Fig. 1, l.t ., l.t ., Figs. 3, 4). All these bundles represent the 
downward prolongation of the traces which have come in from the leaves, 
and which are also frequently seen traversing the cortex, pericycle, phloem, 
and secondary wood on their way to the inner part of the stele. In the 
transverse section from which PL XXXIII, Fig. 1, has been made, twelve 
or more of these leaf-traces can be seen traversing various regions of 
the stem, and some of these are visible in the photograph at l.t., l.t., I' Y ., 
I'.t' & c. The great number of these traces is correlated with the com- 
plex phyllotaxis and crowding of the leaves in this species. 
Each leaf-trace, as it lies on the margin of the pith, usually consists of 
two distinct bundles which are widely separated in the upper part of its 
course, and gradually approximate and ultimately fuse laterally into one 
bundle as the trace is followed down the stem (compare PL XXXIII, 
Figs. 3, 4, 5, which show a perimedullary leaf- trace at successively lower 
levels). 
Each leaf-trace bundle consists distinctly of two portions, an outer 
part consisting of centrifugally-developed wood, and an inner part com- 
posed of centripetal xylem. These two portions are clearly shown on 
1 This point is not yet demonstrated in the case of M. platypodiu?n in which the inner pith is 
not preserved in our specimens. 
