OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
205 
The next distinctive feature in all the examples of this type which I have examined 
is seen in the inner and middle bark. Instead of a thick parenchyma, we have here 
very little of that tissue. I had for some time a difficulty in satisfying myself that any 
existed ; but I think that the crushed and disturbed structures represented in figs. 8 & 9, g, 
have been parenchymatous. Almost immediately after leaving the woody zone, the tissues 
of the bark become conspicuously prosenchymatous, the cells, as seen in the transverse 
section (fig. 8, i), being arranged in radiating lines, and bearing the closest possible 
resemblance to a corresponding section of the ivood of an ordinary coniferous plant. But 
on turning to the radial and tangential sections (fig. 9, i), we see that this tissue consists 
of simple prosenchyma, the walls exhibiting no traces of the pits or extensions of the 
protoplasm through the ligneous cell-layers to the primary cell-wall, seen in the true 
pleurenchyma of Conifers and of the hard endocarps of fruits. These cells have a length 
of ’022 and a diameter of ‘0025. Their general aspect is represented in Plate XXY. 
fig. 11. The outer bark is not represented in the figure; but in the transverse section 
it merely presents a continuation, outwards, of the same linear series of cells as is shown 
in fig. 8, i, while in the radial sections we find that the prosenchymatous cells are now 
drawn out into very long tubes, such as are found immediately beneath the epidermal 
layer of Lepidodendron selaginoides. The entire thickness of the bark in my specimens, 
deprived of its epidermal layer, is about ’62. We have distinct evidence that bundles of 
vessels are given off from the woody zone of this plant to the leaves. Three such are 
represented in fig. 8, m ; but these are very much less obvious than in the case of Lepi- 
dodendron selaginoides. We do not find here regularly disposed channels ploughed 
conspicuously through the bark ; without careful observation the bundles would easily 
be overlooked. 
The next type to be described, and which I believe to be identical with the Lepidoden- 
dron Harcourtii , leads us in the opposite direction from that just discussed. In its 
general aspect it approaches nearer to L. selaginoides ; but it has its own distinctive 
features, which lead us further away than any of the other instances from the exogenous 
type of structure. Plate XXV. fig. 12 represents a transverse section of the natural 
size, in the cabinet of W. Boyd Dawkins, Esq. It was prepared from a fine specimen 
collected by J. Aitken, Esq., of Bacup. In Plate XXVI. fig. 13 a small portion of this 
section is more highly magnified. Plate XXY. fig. 14 is the central axis, with a small 
portion of the inner bark, yet further enlarged ; and Plate XXYI. fig. 15 represents the 
outer surface of the outer bark on the removal of the epidermal layer, but taken from 
another specimen of the same type as that from which the sections were prepared, and 
for which I am indebted to Mr. Butterwortii. 
The medullary axis (a-c) is about - 25 in diameter, of which the central cellular por- 
tion (a) occupies about T8. This consists of cells arranged in irregular vertical rows, 
and frequently with the oblique transverse septa so common amongst the Lepidodendra. 
I can detect no trace of barred structure in these cells; but, as their walls are thickened 
with a deposit of brown carbonaceous matter, I think it very possible that this deposit 
mdccclxxii. 2 E 
