390 Maslen . — The Structure of Mesoxy Ion Sutclifjii {Scott). 
A similar division of each leaf-trace bundle into two in the pericycle 
also takes place in many (or all ?) species of Cordaites , in which genus the 
leaf- trace is also a double one, while in Poroxylon , which agrees with 
Mesoxylon and differs from Cordaites in the possession of paired leaf-trace 
bundles at the margin of the pith, division of the bundles did not take place 
until the trace had entered the leaf. 
The primary cortex of Mesoxylon Sutcliffii , as well as the leaf-bases by 
which the stem is covered, consists of parenchyma traversed in the outer 
part by a system of strengthening bands of the Dictyoxylon or Spar- 
ganum type similar to that found in so many other Palaeozoic plants (see 
PI. XXXIII, Fig. i, d.). Secondary cortical tissues were formed in abun- 
dance, and a number of successive periderms are seen (PI. XXXIV, Fig. 9, 
p\, and Fig. 10, p'.) } which eventually cut right down to the pericycle, or 
even deeper still. The leaf-trace bundles which, as we have seen, divide in 
passing through the pericycle, continue to do so in traversing the cortex 
before entering the leaf (see PI. XXXIV, Figs. 9 and 10). In some sections 
as many as eight separate bundles belonging to one trace are seen in the 
cortex near the junction of the cortex and an adherent leaf- base, and ten or 
more in the leaf-base itself while it is still adherent to the stem. 
In Mesoxylon Sutcliffii the lower part of the free petiole is sometimes 
preserved, and one of these is shown on PI. XXXV, Fig. 18 ; it is of 
flattened form, and shows ten bundles. As many as sixteen bundles are 
sometimes found in the petiole within a very short distance of its inser- 
tion, showing that division of the trace bundles continues to take place 
after they enter the leaf. 
Throughout their course, as far as they can be traced in our specimens, 
the leaf-trace bundles retain their collateral structure, and in the petiole the 
inner wood of the bundles appears to attain a more distinctly mesarch 
structure. In the possession of mesarch collateral leaf bundles our plant 
agrees with some species of Cordaites (see p. 406) as well as with modern 
Cycads. 
An interesting feature in the morphology of Mesoxylon Sutcliffii is the 
abundance of axillary buds ; indeed, a bud appears to occur in the axil of 
every leaf in our specimens. Axillary branching is a feature of several of 
the species of Mesoxylon (M. midtirame , M. platypodium , see preliminary 
note), as well as of Poroxylon. In Mesoxylon Sutcliffii , owing to the crowd- 
ing of the leaves on the stem, axillary buds are more frequently seen than 
in any other known Carboniferous plant ; a single transverse section such as 
that shown on PL XXXIII, Fig. 1, frequently shows two or three buds, a.b ., 
a.b ., and in addition to these, other bud-steles, a.b.s., embedded in the 
cortex, which pass out to buds which arise in the axils of leaves at a higher 
level. PI. XXXIV, Fig. 10, shows one of these axillary bud-steles in the 
cortex with its subtending leaf-base and a row of six leaf- trace bundles. In 
