Maslen.— The Structure of Mesoxylon Sutclijfii (Scott). 405 
both Poroxylon and Lyginodendron . In both these genera the phellogen 
arises in the pericycle, and in Lyginodendron periderm bounds the pericycle 
externally, forming an almost continuous layer, which arches out opposite 
to the leaf-traces just as is usually also the case in Mesoxyloji Sutclijfii 
(PI. XXXIV, Fig. 10). In the new form, however, the periderms are formed 
in a much more irregular manner, at varying depths, cutting across the 
cortex and leaf-bases in arcs. 
The leaf-trace bundles continue to divide in traversing the cortex. 
PI. XXXIV, Fig. 9, shows a leaf-trace in the inner part of the cortex ; each 
of the original bundles is now represented by three, so that the trace as 
a whole consists of six bundles. On PL XXXIV, Fig. io, another trace is 
shown somewhat further out in the cortex, but still consisting of six bundles. 
P'ach of the bundles possesses the same characters as were described above 
for the bundles in the pericycle, excepting that the bundle sheath is 
now a complete one, that part of it on the outer side of the bundle con- 
sisting of tissue which has been carried out from the pericycle (cp. p. 403). 
In some slides a trace consisting of as many as eight bundles may be seen in 
the cortex, or near to the junction between the cortex and the leaf-base. 
The leaf-bases and petioles . As already described, in Mesoxylon Sut- 
clijfii seven or eight of the large leaf-bases serve to cover the stem, as seen in 
transverse sections (PL XXXIII, Fig. 1). The general structure of the leaf- 
bases has been already given, and it remains only to describe now the leaf- 
trace bundles in the adherent leaf-bases and in the free petioles. Division 
continues as the trace is followed outwards from the cortex, and ten or more 
bundles may be seen in the leaf-base while it is still adherent to the stem. 
The lower portion of the free petiole is sometimes preserved in Meso- 
xylon Sutclijfii , and one of these is shown on PL XXXV, Fig. 18, cut in 
transverse section. The petiole shown in the figure is of flattened form and 
has a length of nearly 9 mm. and a width of nearly 2 mm. Its outer 
surface presents a more or less crenulated margin, while the inner side is torn 
and disorganized. The petiole itself appears to consist entirely of paren- 
chymatous cells, many of which possess very dark contents. The 
petiole shown in Fig. 18 shows a row of about ten bundles, some of which 
present the appearance of being about to divide. As many as sixteen 
bundles have been seen in a petiole within a very short distance of its 
insertion on the stem, showing that the division of the bundles continues 
to take place after their entry into the leaf. 
One of the bundles in the petiole shown on PL XXXV, Fig. 18, is also 
shown more enlarged on PL XXXV, Fig. 19. Only the xylem is preserved. 
The bundles still preserve their original collateral structure, and in this 
respect Mesoxylon Sutclijfii agrees with both Poroxylon and Cordaites. The 
bundles in the petiole still possess a considerable amount of radially arranged 
xylem, x,, but it is distinctly less in quantity than in the more deeply seated 
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