4io Maslen . — The Structure of Mesoxylon Sutclijfii (Scott). 
A summary of the most important characters in which Mesoxylon Sut- 
cliffii resembles, and differs from, Poroxylon and Cordaites will serve to 
make this clear. 
Mesoxylon Stitcliffii RESEMBLES Poroxylon in the possession of centripetal 
xylem strands forming part of the leaf-trace bundles in the perimedullary 
position, as well as in the bundles passing out to the leaves ; in the paired 
leaf-trace bundles, which remain separate for a considerable distance (several 
internodes) after their entry into the inner part of the stele ; in the per- 
sistently collateral structure of the leaf-traces from the perimedullary 
position right out into the leaves ; in the possession of rows of bordered pits 
on the radial walls of the secondary xylem elements ; in the spiral arrange- 
ment of the leaves ; in the mesarch bundles in the leaves ; in the presence 
of numerous axillary buds, &c. 
Mesoxylon Sutcliffii DIFFERS from Poroxylon in the discoid pith ; in the 
fact that all the xylem elements of the leaf-traces, both centripetal and 
centrifugal, consist of spiral or scalariform elements, whereas in Poroxylon 
not only are the centrifugal elements of the leaf-traces provided with rows 
of bordered pits, but the more internal portion of the centripetal xylem is 
pitted also ; in the denser character of the secondary xylem, which consists 
of smaller elements with narrower or shorter medullary rays ; in the struc- 
ture of the phloem ; in the divisions of the leaf-trace bundles in the peri- 
cycle and cortex before they enter the leaf ; in the crowding of the leaves 
on the stem, & c. 
The points of RESEMBLANCE between Mesoxylon Sutcliffii and Cor- 
daites are very numerous, and are generally those in which it differs from 
Poroxylon. They may be summarized as follows: the large size of the 
pith ; the discoid structure of the inner pith, which is found in most species 
of Cordaites and probably in all the forms of Mesoxylon yet recognized ; the 
secondary xylem is practically identical in both genera; the phloem is 
similar; the division of the paired leaf-trace bundles in the pericycle and 
cortex, so that a number of bundles entered the base of the leaf, &c. 
The most important of the DIFFERENCES between Mesoxylon Sutcliffii 
and Cordaites is found in the presence of the strands of centripetal wood 
surrounding the pith in Mesoxylon , whereas in Cordaites the whole of the 
wood of the stele is described as being centrifugal in development. 
All the species of Mesoxylon which were described in our preliminary 
note agree in the possession of similar strands of centripetal xylem, although 
there is some variation in its amount, in the rapidity with which it dies out 
when traced down the stem, as well as in the degree of separation of the twin- 
bundles when they reach the perimedullary position after coming in from 
the leaves (see p. 396). In M. multirame the centripetal xylem dies out 
rather rapidly after the leaf-trace has reached the pith, so making a nearer 
approach to the condition which we get in Cordaites itself, in which the 
