386 Mas/en. — The Structure of Mesoxylon Sutcliffii [Scott). 
it is seen that the stem of the English form is larger than that of Poroxylon , 
which had comparatively slender stems less than 2 cm. in diameter. 
The outer surface of the stem has a very irregular contour, well 
seen in the transverse sections (PI. XXXIII, Fig. 1), which is due to the 
numerous crowded leaf-bases which completely cover the surface of the 
stem. Usually sections of some six or seven adherent leaf- bases are shown 
in the transverse sections (PI. XXXIII, Fig. i, /. b.), and evidently the 
leaves covered the surface of the stem in a close spiral. Crowding of the 
leaves occurs in several of our species of Mesoxylon [M. Sutcliffii , M . poroxy- 
loides , M. multirame ), while in others (M. Lomaxii , M. platypodium) the 
leaf-bases are more scattered, but in no other known form are the leaves so 
densely crowded as they are in M. Sutcliffii. 
In the crowding of the leaves Mesoxylon Sutcliffii is in strong contrast 
to Poroxylon , in which the leaves were separated by long internodes, some- 
times as much as 2 or 3 cm. in length. On the other hand, in some forms 
of Cordaites the leaves appear to have been crowded on the stem, although 
in the species hitherto described there was probably always a free inter- 
nodal surface between the leaf insertions, and the leaves were therefore less 
crowded than in Mesoxylon Sutcliffii . 
The stem probably bore its leaves in seven or eight vertical rows, or 
orthostichies, but some difficulty has been experienced in determining the 
exact phyllotaxis of M. Sutcliffii , owing partly to the flattening by pressure 
of most of the specimens, and partly also to the more or less patchy preser- 
vation of the internal tissues, including the leaf-traces, which is characteristic 
of the plants preserved, as these are, in the roof nodules. PI. XXXIII, 
Fig. i, shows a transverse section of a stem which is only slightly flattened, 
and around it there are seven leaf-bases, which, counting also the petiole 
which would have been opposite to the bud shown on the top of the photo- 
graph, would indicate eight orthostichies. The angular divergence between 
two successive leaf-traces (PI. XXXIII, Fig. 1, l.t, l.t.) appears to be about 
140°, and the phyllotaxis is probably -£ T , in which case the angular diver- 
gence would be 13 7 0 . The phyllotaxis is thus of a higher order than that of 
Poroxylon , which was determined by MM. Renault and Bertrand to be T \. 
Although the leaves of Mesoxylon Sutcliffii are at present known only 
by their adherent bases and by a small portion of the petiole (PI. XXXV, 
Fig. 18), these show that they were probably broad strap-shaped leaves 
essentially similar to those of Cordaites (indeed some of the familiar impres- 
sions of Cordaitean leaves in the English Coal Measures may really belong 
to Mesoxylon), and of Poroxylon y the leaves of which have recently been 
identified by M. Grand’Eury in the form of impressions (see p. 383). That 
the leaves of Mesoxylon Sutcliffii were deciduous appears to be shown by 
the presence of a well-marked meristematic layer at their base (PI. XXXIV, 
Fig. 13, a). 
