OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
229 
we have already seen, this is the condition existing in the small Coalbrookdale speci- 
men; but it is not the usual state of the pith in larger Dadoxylons, in which the Stern- 
bergian laminae are only enclosed within a very thin cylinder of undivided medullary 
parenchyma. 
Fig. 42 is a transverse section of a large branch of the natural size. The medulla 
has been three quarters of an inch in diameter, and the entire stem, exclusive of its 
bark, was, when perfect, 2 J inches in diameter, the ligneous cylinder having a maximum 
thickness of nearly an inch. Though the woody wedges composing this cylinder have 
been disturbed by mineralization, it is obvious that the medullary extremities of the 
larger ones projected into the medulla, as is so commonly the case in the branches of 
living Conifers ( e . g. Araucaria imbricata), giving to the transverse section of the pith a 
stellate outline, large pointed rays of the latter being prolonged outwards between the 
convex extremities of the wedges. This specimen, for which I am indebted to John 
Aitken, Esq., of Bacup, came from the Halifax Bullion or Ganister bed at South Owram. 
The matrix is full of small Goniatites. The longitudinal section (fig. 43) of the above 
branch displays some peculiarities. Thus the medullary layer lining the vascular cylinder 
is very thin, soon breaking up into thin disks, but which are very regularly thickened at 
their circumferential portions ; hence, in the vertical sections, each disk appears as a pillar 
standing upon a rapidly swelling base (fig. 47, a), which latter rests upon the interior of 
the woody cylinder ( b ). Then the laminae of the several disks, instead of extending 
straight across the medullary cavity, subdivide into two, one half joining the corre- 
sponding half of the lamina above, and the other half being united in a similar manner 
to the lamina below. Thus there is formed a central pile of lenticular cavities, a , which 
alternate with a more marginal series of lenticular rings, a\ a'. 
Fig. 44 is a transverse section of a small branch from one of the bullions above the two- 
foot coal at Oldham ; and fig. 45 is a vertical section of the same specimen. Its diameter 
is half an inch, of which the medulla occupies nearly T9. This latter tissue not only 
divides into the usual disks, a , but each one of these again exhibits the strongest tendency 
to split up into numerous yet thinner layers, each one of which consists apparently of a 
single horizontal layer of cells*. It is obvious that in this instance the medullary cells 
have arranged themselves in successive vertical series of horizontal layers, and thus pre- 
disposed the tissue to break up into thin laminae along the lines intervening between 
* In pointing out the fact that the Balsam fir (Abies bdlsamea) has a discoid pith, Dr. Dawson says : — 
“ This modern Sternbergia is not produced by the mere breaking of the cellular tissue transversely by elonga- 
tion of the fibre, hut, as I pointed out many years ago in the case of the Coal-formation Sternbergia (‘ Cana- 
dian Naturalist and Geologist,’ 1857), is a true organic partitioning of the pith by diaphragms of denser cells 
opposite the nodes, as in Cecropia joeltata and some species of Ficus. The transverse diaphragms are com- 
posed of denser cells flattened horizontally, and they are, as in Sternbergia, accompanied by constrictions of the 
medullary cylinder.”. — Nature, May 15, 1873. 
In my Sternbergia described above I find no evidence of two kinds of cells ; but there is an unmistakable 
horizontal stratification of them which obviously approximates closely to Dr. Dawson’s description. 
MDCCCLXXVII. 2 K 
