1022 
Scott . — The Structure of 
expect the outer periderm to have been formed before the inner ; the latter, 
however, is sometimes the more developed of the two, and as the whole 
cortex appears to have been living up to the time when the plant itself 
met its fate, we cannot be certain as to the order in which the peridermal 
layers appeared. 
The inner cortex, like the pericycle, contains large sacs with dark con- 
tents, though they are not so numerous here. Except where a leaf-trace is 
passing out, the inner cortex is little developed ; we almost immediately 
reach the Dictyoxylon zone, which attains a considerable thickness — quite 
2 mm. in places. The radial bands of fibres do not, however, extend quite 
to the outside of the stem ; the fibres are little thickened, and the differen- 
tiation of the mechanical tissues, as seen in transverse section, not very 
marked. 
The outer surface of the stem is incompletely preserved, and hence 
the arrangement of the leaf-bases is obscured. At first sight they appear 
to be scattered, but further examination shows that they must have been 
fairly close together. Thus, in one section (slide 2353) two leaf-traces are 
seen in the outer cortex ; both consist of a number of bundles — probably 
eight in each case— though all are not preserved. The adjacent bundles of 
the two traces are only about 4 mm. apart. 1 The corresponding leaf-bases 
are not complete, but there can have been no great extent of stem-surface 
between them. The leaf-bases, however, were probably somewhat less 
closely set than in M. Sutcliffii. 
As in other species, where a leaf-trace passes out through the cortex 
into the leaf-base it is surrounded by a large mass of parenchyma, which 
encroaches upon the Dictyoxylon zone. This is consequently considerably 
thinner opposite the outgoing leaf-traces than elsewhere. 
It may be mentioned that the ‘ Cordaitean ’ leaves associated in slide 
239 7 with a stem, probably referable to M. poroxyloides, are of the type 
recently described by Prof. Margaret Benson under the name Cordaites 
Felicis (Benson, T 2 ). This is shown by the well-developed sheath, the 
presence of complete longitudinal fibrous partitions between the bundles 
and of thickened masses of hypoderm between the bundles and the parti- 
tions, the partial ‘ inner sheath ’, and the arrangement of the centripetal and 
centrifugal xylem, which is often identical with that shown in Prof. Benson’s 
text-figure ( 1 . c., p. 204). Some of the leaves associated with the stem in 
question are, however, thicker (i.e. cut, no doubt, nearer the base) than any 
of Prof. Benson’s specimens; one fragment reaches a thickness of r8mm. 
In these thickest parts some of the partitions are missing ; their absence 
may be correlated with a recent division of the bundles. 
1 The distance between the centre lines of the two traces would be about 8 mm. 
