274 CORDAITBAE [CH. 



with bordered pits or the inner rows of the xylem-cyhnder may be 

 of the scalariform or spiral type. DifEerences shown in transverse 

 sections of the inner portion of the xylem are due to the circum- 

 stance 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-waUed 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, It, 

 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, It, 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 

 surroimded by a zone of secondary xylem with broad medullary 

 rays. 



Among other species of Mesoxylon mention may be made of 

 M. Lomaxi and M. poroxyloides. M. Lomaxi Scott and Maslen^ 

 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 



1 Soott (12) p. 1012, Pis. Lxxxvn., Lxxxix. 



