62 DR. D, H. SCOTT ON THE HETERANGIUMS 
the circumference no secondary xylem can be distinguished. Thus secondary 
growth had just started, the newly formed elements being, as usual, small, 
corresponding rather to the centrifugal primary tracheides on which they 
abut, than to the wide elements of the metaxylem. 
The primary strueture of the peripheral strands was thus, as we should 
expect, mesarch, but with the centrifugal portion of the primary xylem very 
small in amount (fig. 5). 
Deferring the important subject of the leaf-traces, we come next to the 
pericycle, for the phloem is practically lost. 
The inner margin of the pericyclie zone is more or less crenulated, its 
concavities corresponding to the convexities of the xylem cylinder (fig. 7). 
Hence they must originally have fitted on to each other, with the inter- 
vention of the phloem only. Now they are almost everywhere separated by 
a wide gap, containing some Stigmarian rootlets. In places some thickness 
of thin-walled tissue is preserved ; its inner portion may include some remains 
of the phloem (fig. 6). In other places the pericycle is represented almost 
wholly by sclerotic tissue, most of the elements having apparently very thick 
walls, like those of the sclerotic cell-plates in the cortex. In the pericycle, 
however, the sclerotie tissue is less continuous, a certain number of thin- 
walled elements being interspersed. In these more resistant parts of the 
pericycle the crenulations are specially well marked, the sclereides extending 
into the projections between the hollows (Pl. 8. fig. 7). The pericyclic 
sclereides are unusually abundant in Zeterangium shorense, forming exten- 
sive tracts, but they occur also in most of our other Coal-Measure forms. 
Cortex. 
The cortex, as in other Heterangiums, may be divided into inner cortex 
and hypoderma—the former, of course, much wider than the latter. The 
inner zone consists of a matrix of thin-walled parenchyma, in which great 
sclerotic plates are imbedded (Pl. 3. figs. 1 & 7). These plates sometimes 
border on the pericycle, and elsewhere may almost reach the hypoderma. 
As a rule they consist, as seen in transverse section, of hexagonal cells with 
their lumina more or less completely blocked with a brown substance, which 
may be altered cell-wall. In other cases the sclereides are flattened and 
ranged in regular rows, suggesting the activity of a special meristem. These 
appearances are like what we find in other forms, in longitudinal sections 
through the sclerotic plates, and are no doubt seen owing to displacement of 
certain of these plates. 
The great development of the cortical sclerotie masses is characteristic of, 
though not peculiar to, the Shore Zeterangium. The thin-walled cortical 
tissue between the plates often consists of large cells tangentially dilated ; in 
other places the cells are hexagonal and small, resembling those of the 
sclerotie plates in form and size. 
TU C PT ie h 
