Mas ten. — The Structure of Mesoxylon Sutcliffii (Scott). 397 
On PI. XXXIII, Fig. 6, a longitudinal section passing through the 
two bundles of a leaf-trace at the margin of the pith is shown. The cells of 
the outer pith are shown at p " ., and the inner part of the secondary xylem- 
ring at s.x. The latter consists mainly of closely packed, straight, pitted 
tracheides with narrow medullary rays shown at a y but on its inner margin, b, 
the tracheides are spiral or scalariform, and follow a more undulating course 
and have broader medullary rays : the latter part, in fact, presents a similar 
appearance to that of the centrifugal xylem of the leaf-traces described 
below. In this more or less tangential section (PI. XXXIII, Fig. 6) the leaf- 
trace bundles, l.t ., l.t., are seen in the outer part of the pith. In the upper 
part of the photograph (PL XXXIII, Fig. 6) two distinct bundles are 
shown with medullary elements between them, while below they have fused 
into one broad bundle exactly as we have seen to take place from a study 
of the transverse sections. 
The long straight course of the bundles is well seen in this slide, where 
the same two bundles can be traced for a distance of o*6 cm., and they are 
of course only intercepted by the plane of section for a short part of their 
course. The very slow convergence of the bundles of a trace is also 
well illustrated by this slide, since there is but little diminution in the 
distance between the two bundles in the distance (o*6 cm.) shown in the 
photograph. 
The centrifugal tracheides of the bundles follow a somewhat undulating 
course, as is seen in the photograph (PL XXXIII, Fig. 6, s.b.), and between 
them are relatively broad medullary rays. The appearance of this tissue in 
longitudinal sections is quite different from that of the secondary xylem 
zone with its closely packed, straight tracheides and narrow medullary rays. 
Some of the centrifugal xylem elements from one of the bundles shown on 
Pl. XXXIII, Fig. 6 , are also shown on a larger scale on PL XXXV, Fig. 14. 
Here the thickening of the walls of the tracheides appears to be of the spiral 
kind, and between the tracheides the relatively broad medullary rays are 
shown. The scalariform centrifugal tracheides of the bundle pass gradually, 
by transitional forms, into the pitted tracheides which form the ring of 
secondary wood. In the centrifugal xylem of the bundle the spiral or 
scalariform tracheides are thickened on all sides, while in the ring of 
secondary xylem outside the bundles the pitting is confined to the radial 
walls, as is usually the case in secondary xylem elements. The centripetal 
wood of the bundles is shown on PL XXXIII, Fig. 6,/.£., cut obliquely as 
the plane of the section passes into the pith. It forms a denser kind of 
wood than the centrifugal xylem, as it consists entirely of spiral or scalari- 
form tracheides. Between the centrifugal and centripetal wood the slide 
shows some traces of narrow elements with loose spiral thickening which 
doubtless belong to the protoxylem. The fact that all the tracheides of the 
leaf-trace in Mesoxylon Sutcliffii (as well as in the other species of the 
