the stem-wood; the latter may consist exclusively of tracheids with bordered pits or the inner rows of the xylem-cylinder may be of the scalariform or spiral type. Differences shown in transverse sections of the inner portion of the xylem are due to the circumstance that in certain parts of the inner face of the secondary wood leaf-traces are unrepresented, while in other places the dwindled remains of the outer, centrifugal, portions of a trace are still recognisable. As each double leaf-trace passes down the pith the bundles fuse and the single strand retains for a time some centripetal xylem; this gradually disappears and at a lower level the centripetal xylem also dies out. The space enclosing the obtuse apices of the bundles shown in fig. 483, B, was originally occupied by thin-walled tissue which accompanied the trace in its outward course. In _Mesoxylon Sutcliffii_ the leaf-strands pass almost horizontally through the secondary wood, bend outwards in the phloem and follow a steeply ascending course to the leaves. In fig. 483, B, a double leaf-trace is seen at the inner edge of the secondary wood with the centrifugal xylem, _cf_, continuous with that of the stele: fig. 483, D, _lt_, shows a leaf-trace in the pericycle where one of the bundles has divided and the other is tangentially extended and partially divided. The branching is carried further in the cortex, as seen in fig. 483, C, where the trace is represented by a curved row of six bundles, _lt_, and at a higher level further subdivision may occur. The leaf-bundles are collateral and in the leaf retain both centripetal and centrifugal tracheids. In the section shown in fig. 483, C, the oval stele of an axillary shoot is seen at _s_ subtended by the row of collateral bundles: the stele has a fairly large pith surrounded by a zone of secondary xylem with broad medullary rays. [Illustration: Fig. 483. _Mesoxylon Sutcliffii_. Transverse sections of stem. A. _a_, outer pith; _b_, axillary bud; _x_² secondary xylem. B. _cp_, _cf_, centripetal and centrifugal xylem; _px_, protoxylem. C. _lt_, leaf-traces in the cortex; _s_, stele of axillary branch; _ph_², secondary phloem. D. Leaf-trace bundles, _lt_, external to the phloem, _ph_². (A, C, D, sections in the Manchester Collection, 717 _b_; B, Cambridge Botany School, 530.)] * * * * * Among other species of _Mesoxylon_ mention may be made of _M. Lomaxi_ and _M. poroxyloides_. _M. Lomaxi_ Scott and Maslen[718] generally resembles _M. Sutcliffii_ but shows the following distinctive features: the leaves are more scattered and less crowded; the twin-bundles of the leaf-traces fuse immediately on entering the pith, thus appearing for the most part as single and not double strands in the perimedullary zone; the centripetal xylem is well developed, the medullary rays are deeper and the outer cortex has shorter bands of mechanical tissue.