Mas Zen. — -The Structure of Mesoxylon Sutclijfii (Scott). 399 
fact and to the greater thickness of the medullary rays, the wood of Poroxylon 
is of a softer, less compact type, comparable rather with that found in 
Lyginodendron and other Cycadofilices. Indeed, the dense character of the 
wood seems to afford a ready means of distinction between the Cordaiteae 
and the forms more closely related to them, such as Mesoxylon , and the 
more distant Poroxyleae, Cycadofilices and Pteridosperms, in which the 
secondary xylem is of a softer, more Cycadean type with larger tracheides 
and broader medullary rays. 
The radially arranged wood which occurs immediately around the 
pith and between the perimedullary leaf-trace bundles (the c intermediate 
secondary wood ’ of our preliminary note) presents a somewhat different 
appearance in transverse sections to that of the rest of the wood. In many 
places this wood doubtless represents the downward extension of the 
centrifugal portion of leaf-trace bundles which have nearly lost their 
individuality. In other places, however, where there is not the slightest 
trace of bundles, the innermost tracheides of the radial rows are smaller 
than those further out, while the number of radial rows is less and the 
width of the rays greater. In other words, the intermediate wood for a 
short distance in from the pith presents the less compact appearance which 
is characteristic of the outer wood of the traces. 
Radial longitudinal sections show, however, that in some places there 
are only one or two scalariform, spiral or transitional elements between the 
ordinary tracheides with bordered pits and the cells of the pith. In other 
cases, where a greater thickness of such elements is shown, it is probable 
that the prolongation of a leaf-trace is cut through. In the more or less 
tangential section represented on PI. XXXIII, Fig. 6, spiral elements are 
shown at the inner limit of the secondary wood at b ; it appears probable 
that this is really part of a leaf-trace. It thus appears that, with the 
possible exception of a very few elements on its inner edge, the whole of 
the secondary xylem-ring, apart from the leaf-traces, consists of pitted 
tracheides. 
Pairs of leaf-trace bundles are sometimes seen in the transverse sections 
passing out through the secondary wood. When cut across in the inner 
part of the secondary xylem the bundles are cut nearly transversely, but 
when in the outer part they appear in obliquely longitudinal section, from 
which it is inferred that their course through the wood is at first highly 
inclined (i. e. nearly vertical), and then, having fairly entered the secondary 
wood, they curve rapidly (owing to the growth in thickness of this zone) and 
assume a more nearly horizontal direction. After leaving the wood the 
inclination again increases, and in the phloem and pericycle the bundles are 
cut nearly transversely. One of the longitudinal sections, 1 a tangential one 
passing through the outer part of the secondary xylem, shows a pair of 
1 Slide No. 2 666 (S). 
