Maslen . — The Structure of Mesoxylon Sutcliffii (Scott). 403 
centripetal xylem of each original bundle divides into a number of small 
patches lying within, but usually separated by a small amount of parenchyma 
from, the wedges of centrifugal wood (PI. XXXIV, Fig. 8, p.b.). The 
enlargement and tangential dilatation of the bundles is preparatory to their 
division into two, which takes place in the pericycle, a strictly collateral 
structure being preserved. 
PI. XXXIV, Fig. 8, shows a pair of leaf-trace bundles in the inner part 
of the pericycle. Each of the bundles shows the above-mentioned charac- 
teristics, although the actual division of the bundles has not yet taken place 
excepting in the xylem portion. Each bundle has a tangential width of 
about 0*9 mm., which is more than double that of a bundle at the margin of 
the pith. Outside the xylem of the bundles is the phloem, ph.b.^ similar to 
and continuous with, that of the stem, and outside this again the arc of pri- 
mary phloem, c ., before mentioned (p. 401). 
Surrounding the xylem portion of the bundles there occurs a delicate 
conjunctive tissue, b, which is doubtless the same as that already mentioned as 
occurring around the trace bundles at the margin of the pith, and which is 
there usually represented by an empty space (PI. XXXIII, Fig. 3, s.). Out- 
side this there is a relatively broad sheath of cells with contents (PI. XXXIV, 
Fig. 8, s.) surrounding the bundles, especially internally and laterally (at the 
level shown on PI. XXXIV, Fig. 8, it is practically absent on the outer side 
of the bundle), and this sheath accompanies the bundles out into the cortex. 
The tissue composing this sheath is probably continuous along the course of 
the leaf-trace with the tissue forming the outer part of the pith, which thus 
becomes continuous with the very similar tissue of the pericycle. Confirma- 
tion of this is found in the tangential section of the wood of the stem 
described on p. 399 ; the slide shows two leaf-trace bundles cut transversely 
in the xylem, and each of these is accompanied by a tissue similar to that 
of the outer pith. 
A similar division of each leaf-trace bundle into two in the pericycle, 
which is here described in Mesoxylon Sutcliffii , also takes place in many 
species of Cordaites , in which the leaf- trace is also a double one. A similar 
division also takes place in the other species of Mesoxylon described in our 
preliminary note ; in one species, however — M. platypodium — each of the 
bundles had already divided, as regards its primary (centripetal) xylem, 
even before leaving the wood. 1 
In Lyginodendron , and in Calamopitys , division also takes place in the 
pericycle, but in these forms there is only one trace bundle to divide, as the 
trace when passing through the secondary wood from the perimedullary 
position is single. In Poroxylon , however, which agrees with Mesoxylon in 
the possession of paired leaf-traces at the margin of the pith, according to 
MM. Bertrand and Renault, each leaf received from the stem a single large 
1 Preliminary Note, loc. cit., p. 239. 
