396 Mcislen.- — The Structure of Mesoxylon Sutcliffii (Scott), 
traces in Mesoxylon Sutcliffii is mesarch also. As the whole of the elements 
of the trace, centrifugal as well as centripetal, consists of tracheides with 
similar spiral and scalariform thickenings, the longitudinal section affords 
little help in determining this point. 
Tracing the twin-bundles of a leaf-trace of Mesoxylon Sutcliffii down- 
wards from the level at which they have just reached the margin of the pith, 
after coming in from a leaf, the following changes are seen to take place. 
PI. XXXIII, Fig. 3, shows a leaf-trace shortly after it has entered from 
a leaf, magnified about 40 diameters. The two bundles of the trace are, at 
this level, separated by tissue, s.x., about equal in width to that of one of 
the bundles, and the distance between the bundles (measured from the centre 
of each) is about 0-9 mm. The intermediate tissue consists partly of ordinary 
secondary xylem elements, s.x., arranged in radial rows, and partly of larger 
cells which belong to the pith. 
There is considerable difference between the five species of Mesoxylon 
in the distance between the bundles of a trace, and in the rapidity with 
which they fuse together when followed down the stem : l in M. Lomaxii 
the twin-bundles fuse immediately on reaching the pith, while M. platypodium 
is characterized by the extreme separation (as much as % mm.) of the bundles. 
Mesoxylon Sutcliffii occupies an intermediate position in this respect, the 
separation being about 0*9 mm. 
At this level (PI. XXXIII, Fig. 3) the centripetal wood, p.b., is quite 
well developed, and the arc-like form is more pronounced than in sections 
of the trace at lower levels. Each bundle is partly surrounded by the 
space, j., above mentioned. 
On PI. XXXIII, Fig. 4, a leaf-trace is shown after it has passed down 
through, probably, several internodes. Here the bundles have become 
approximated to one another, and the distance between the two bundles is 
now about 0-55 mm. Otherwise there is but little difference between the 
trace at the two levels represented in Figs. 3 and 4, excepting that at the 
lower level the centripetal wood is perhaps slightly smaller in amount, and 
its arc-like form is less evident. 
PI. XXXIII, Fig. 5, shows a leaf-trace at a still lower level. Here the 
two original bundles have fused laterally into one broad bundle, although 
the two masses of centripetal xylem ,p.b., are still distinct. The distance 
between the centres of the individual bundles is now only 0*4 mm. The 
amount of centripetal wood is distinctly less and the arc form is lost. 
Examination of the traces at even lower levels shows that the previously 
distinct inner wood of the two bundles fuses into one straight line and 
finally disappears altogether, leaving only the radially arranged centrifugal 
elements abutting on the pith, and these gradually lose their identity as 
distinct bundles altogether. 
1 Scott and Maslen, Preliminary Note, loc. cit. 
