388 Masien.— The Structure of Mesoxylon Sutciijjii [Scott). 
PI. XXXIII, Figs. 3 and 4. The centrifugal wood, s.b., which constitutes 
by far the larger part of the bundles, consists of rows of radially arranged 
spiral or scalariform tracheides (PL XXXV, Fig. 14) which pass outwards, 
at the external limit of the bundle, by intermediate forms, into the ordinary 
pitted tracheides which form the continuous zone of secondary wood. The 
centripetal primary xylem (PI. XXXIII, Figs. 3, 4, p.b.) of a trace 
bundle is much smaller in amount than the centrifugal wood, and consists 
of non-radially arranged elements, with spiral and scalariform thickenings, 
forming an arc. As far as we have been able to determine, the develop- 
ment of the inner arc of wood in Mesoxylon Sutciijjii was entirely in the 
centripetal direction, for we have been unable to demonstrate the presence 
of any centrifugal primary xylem elements, although it is possible that 
such may exist (see p. 393). 
In the possession of paired leaf-trace bundles at the margin of the pith, 
Mesoxylo 7 i Sutciijjii agrees with the other species of Mesoxylon briefly de- 
scribed in our preliminary note, although there is much difference in the five 
species in the rapidity of convergence of the twin-bundles after their entry 
into the inner part of the stele. Twin leaf-trace bundles are also a feature 
of Poroxylon, and here, as in Mesoxylon , the two strands of a trace remain 
distinct for a considerable distance down the stem. Another Carboniferous 
plant in which the leaf-traces are given off in pairs from the margin of the 
pith is Dadoxylon Spencerij a form which may prove to be closely related 
to Mesoxylon, and the same thing is shown in the living Ginkgo , 2 relatives 
of which probably existed as far back as the Carboniferous Period. 
Surrounding the pith and the leaf-trace bundles in all the specimens 
occurs a zone of secondary wood which has a thickness in Mesoxylo 7 i Sut- 
cliffii of about 0-3 cm. (PL XXXIII, Fig. 1, s.x.). As is commonly the case 
in fossil plants which are preserved in the roof nodules, the preservation is 
somewhat patchy, and in all the specimens of M. Sutciijjii this is especially 
the case in the secondary xylem zone, the middle portion of which is often 
entirely destroyed (see PL XXXIII. Fig. 1, s.). 
The secondary xylem consists entirely of radially-arranged tracheides 
of small size (usually about 0-025 mm * * n diameter) and narrow uniseriate 
medullary rays, usually from 1-6 cells in height. With the exception of 
those which form part of the leaf-trace bundles, nearly all the secondary 
tracheides possess rows of bordered pits on their radial walls. The appear- 
ance of the tracheides in radial section is shown on Pl. XXXV, Fig. 15, and 
in tangential section on PL XXXV, Fig. 16. The wood is thus of the 
dense type with (usually) uniseriate rays and relatively narrow pitted 
tracheides which is common to all the species of Mesoxylon , and which is 
1 Scott, Primary Structure of certain Palaeozoic Stems, &c. Trans. Roy. Soc. Edinburgh, 
vol. xl, pt. ii, 1902. 
2 Seward and Gowan, Ann. Bot., vol. xiv, 1900. 
