Maslen . — The Structure of Mesoxylon Sutclijfi (Scott). 389 
known to be characteristic of Cordaites and of those specimens of wood 
which have been described under the names of Dadoxylon and Araucari- 
oxylon , some of which have been identified as belonging to the Cordaiteae. 
Moreover, the secondary wood of Mesoxylon is practically indistinguishable 
from that of a Conifer of the family Araucarieae. Pairs of leaf-trace bundles 
are sometimes seen passing out through the zone of secondary wood, 
although, owing to the patchy preservation of this tissue and to the nearly 
horizontal course taken by the bundles, they are not as commonly met 
with in the transverse sections as might have been expected. 
Outside the secondary xylem a cambium occurs, and the stem thus 
had secondary growth in thickness of a normal character. 
The cambium is followed by a continuous zone of phloem, which is 
shown on PL XXXIII, Fig. 7,/., and PL XXXV, Fig. 1 7,/. The secondary 
phloem has a thickness of about o«6-o*8 mm., and consists of elements 
arranged in radial rows corresponding to those of the secondary wood 
separated by narrow medullary rays. In addition to long empty-looking 
cells (? sieve tubes) the phloem includes many long tubular elements con- 
taining dark contents. The structure of the phloem is much more like that 
of Cordaites than that of Poroxylon Edwardsii (see p. 383). 
Surrounding the phloem there is a fairly thick band of tissue which may 
be interpreted as the pericycle (see PL XXXIII, Fig. 7 ,/<?., and Pl. XXXV, 
Fig. 1 7,/*.), although it does not appear to be sharply marked off from the 
cortical tissue outside. Longitudinal sections show that this zone consists 
mainly of rather large short parenchymatous cells, most of which possess 
brown contents (PL XXXIV, Fig. 13, /<?.), and which are essentially similar 
to those which form the greater part of the outer persistent pith. 
The paired leaf-trace bundles, after their escape from the zone of 
secondary xylem (which they traverse in a nearly horizontal direction), 
ascend steeply (almost vertically) through the phloem and pericycle, so 
that in transverse sections of the stem the bundles are cut across nearly 
transversely, as is shown on PL XXXIV, Figs. 8 and 9. When once free 
from the xylem cylinder the leaf-trace bundles possess a distinct collateral 
structure with external phloem, and this collateral structure is preserved as 
far out as the bundles can be traced into the leaves. 
On reaching the phloem and pericycle the leaf-trace bundles, which 
still possess centrifugal and centripetal wood, experience a marked tan- 
gential dilatation which is preparatory to the division of each bundle into 
two which takes place in the inner part of the pericycle (PL XXXIV, Fig. 8). 
A similar division to that described in Mesoxylon Sutcliffii takes place in 
the other species of Mesoxylon ; in one species, however — M. platypodium — 
each of the bundles had already divided, as regards its primary xylem, even 
before leaving the wood. 1 
1 Scott and Maslen, Mesoxylon. Preliminary Note. Ann. Bot., vol. xxiv, p. 239. 
D d 
