Mesoxylon Lomaxii and M. poroxyloides. 1015 
The sheath encloses the whole prominent part, both centripetal and centri- 
fugal, of the xylem-strand. Following the strand upwards in its course to 
the point where it is about to leave the pith, the sheath is here a much 
more important structure, consisting of numerous layers of radially arranged 
thick- walled cells, which have evidently increased in number by secondary 
divisions (PI. LXXXVII, Fig. 5 , sh). The double bundle shown in the 
photograph is in the act of fusion, or rather of separation, for we are tracing 
it in the upward direction. Still higher up, where the twin bundles are 
beginning to pass out into the wood, the sheath forms a double arc, 
corresponding to the two strands (PI. LXXXVII, Fig. 4). In this region 
the cells of the sheath have reticulate, tracheoidal markings, which may be 
seen in Fig. 4 on using a lens. As we follow the trace on its outward 
course, we find that the sheath becomes merged in the secondary wood 
which here closes in behind the outgoing strands ; in the transitional region 
the short cells of the sheath are interspersed with the long, sinuous 
tracheides. 
The Wood. 
The structure of the wood, apart from the centripetal xylem of the 
leaf-traces, is of the usual Cordaitean type. In the parts corresponding to 
the primary strands a considerable amount of the inner centrifugal wood 
consists of scalariform tracheides, or in some cases elements with crossed 
spirals ; in the case of an entering bundle, fifteen layers of such elements 
were counted, and in one of the blunter prominences, representing a bundle 
lower down in its course, there were ten such layers. 
In places, however, where the strictly secondary wood abuts on the 
pith, as in the depressions between the bundles, the pitted tracheides may 
extend right up to the inner margin. The scalariform elements are, as 
a rule, sculptured on all sides, while in the pitted tracheides the pits are 
limited to the radial walls. Transitional elements sometimes occur. 
The tracheides of the secondary wood are rather small, ranging from 
30 to 60 ijl, the usual dimensions being 40 to 50 /x. The medullary rays 
are very narrow, 12 to 18 /x in width. As the tangential sections show, they 
may reach a considerable height, up to about twenty-five cells. They range 
from this down to a height of two cells or even one. Only in quite rare cases 
do we find a radial division, making the ray locally biseriate ; otherwise 
the rays are uniseriate throughout (PI. LXXXIX, Fig. 19). In certain 
places the pits are well preserved. They form two or three series on the 
radial wall of the tracheide, and have the somewhat angular outline usual 
in Cordaitales. In a few cases the border of the pit is sufficiently perfect 
to show the inclined, elliptical slit, but usually the pore appears round 
(PI. LXXXIX, Fig. 21). On the walls of medullary ray-cells in contact 
with tracheides the pits are arranged in two or three horizontal rows on the 
