Maslen . — -The Structure of Mesoxyion Sutcliffii ( Scott ). 393 
when present it is not equally developed all round the periphery of the pith, 
being usually absent, or nearly so, opposite to the outgoing leaf-traces, 
especially where the two bundles are widely separated preparatory to their 
exit as on PI. XXXIII, Fig. 3. Where, as shown on PI. XXXIII, Figs. 4 
and 5, the bundles of the trace are more closely approximated (i. e. when 
the trace is cut through at some distance below its entrance into the central 
part of the stele), the zone of cells with very dark contents, p again 
appears. Somewhat similar cells with dark contents (‘ secretory sacs ’) occur 
in the parenchymatous tissues of Pitys and Cordaites . 
IV. The Leaf-traces surrounding the Pith. 
As already briefly described, a considerable number of leaf-trace 
bundles occur around the pith in contact with the zone of secondary 
wood. These bundles occur in pairs, a pair ultimately passing out to each 
leaf. If a leaf-trace is cut through soon after its entry into the interior of 
the stele the twin-bundles are widely separated (PI. XXXIII, Fig. 3), but 
when traced down through the stem the two bundles gradually approximate 
to one another (PI. XXXIII, Fig. 4), and fuse laterally into one bundle 
(PI. XXXIII, Fig. 5), and, finally, when traced still lower, the identity of 
this as a distinct bundle is lost. 
Each leaf-trace bundle on the margin of the pith consists of both 
centrifugally and centripetally developed xylem, the former constituting by 
far the greater part of the bundle and passing gradually outwards into the 
tracheides which form the continuous ring of secondary wood. 
The centripetal primary xylem of a trace bundle is much smaller in 
amount than the centrifugal wood, and consists of a number of non-radially 
arranged elements with spiral and scalariform thickenings, forming an arc, 
convex inwards, abutting at its ends on the rows of centrifugally developed 
tracheides of the bundle (PI. XXXIII, Fig. 3, /A). Between the centrifugal 
outer xylem and the primary centripetal xylem there is a small mass of 
tissue consisting of delicate parenchymatous cells (PI. XXXIII, Figs. 3 and 
4 ,px.). This doubtless consists of conjunctive parenchyma accompanying 
the protoxylem elements, and it thus serves to mark the position of the 
protoxylem. Each bundle is surrounded laterally and internally by an 
empty space, which is shown on PI. XXXIII, Figs. 3 and 4, s . ; it represents 
the position of a very delicate tissue, which is rarely preserved even in part. 
This conjunctive tissue accompanies the bundles out through the wood, and 
is preserved around the bundles in the phloem and pericycle (PL XXXIV, 
Fig. 8, b). 
As far as it has been possible to determine, the development of the 
primary wood in the stem of Mesoxyion Sutcliffii was entirely in the centri- 
petal direction. As can be seen on PI. XXXIII, Figs. 3 and 4, the smallest 
elements of the inner mass of xylem, p.b., occur at or near its outer limit 
