OP THE POSSIL PLANTS OP THE COAL-MEASURES. 
223 
Having thus reviewed all the principal facts that have come under my personal 
observation, and I have almost entirely confined myself to such, it now remains to he 
seen what general conclusions can be drawn from them. We began with a Lepidoclen- 
droid plant, L. selaginoides, in which we found the medullary axis largely occupied by 
a great number of scalariform vessels ; but we saw that these were not arranged in 
radiating order, neither did they give off any vascular bundles to the leaves. These 
bundles were confined to the inner surface of a very narrow, but nevertheless distinct, 
enclosing circle of somewhat smaller vessels, between which, and passing radially out- 
wards, were vertically disposed rows of cells, which I believe to be true representatives 
of medullary rays, whilst the thin cylinder through which they pass is the woody zone 
separating medullary from cortical structures. The bark we found to be thick, consisting 
of varying elements of parenchyma and prosenchyma, but chiefly the latter ; and near 
the outer surface we discovered a layer of prosenchyma, where the cells are so elon- 
through the bark, being - , in fact, an investure of the vascular tissue accompanying the latter to whatever, 
organism the tubercle helped to sustain. 
It thus appears that these outer layers of bark, having an aggregate thickness of from three eighths to half an 
inch, filled up the deep valleys separating the conical hillocks of the Halonia, and almost reduced the entire sur- 
face of the plant, when living, to a uniform level. These determinations bring the minute and geometrically 
arranged punctations covering the surface of the Ualonici into homological relations with similar markings seen 
on other semidecorticated Lepidodendroid plants. 
The other specimen to which I have referred is a very large example of one of the round or oval scars so 
characteristic of Ulodendron, but which, instead of being more or less depressed, as is commonly the case, stands 
out as a projecting cone at least 3 inches above the semidecorticated surface from which it rises. If this cone 
represents, in Ulodendron, the mammillary protuberance of Halonia (and that it does so I entertain no doubt), 
its height gives us a measure of the extreme thickness of the prosenchymatous and subepidermal layers of the 
plant to which it belonged. 
The above specimens having again drawn my attention to Halonia, I gladly availed myself of some specimens 
collected and placed in my hands by my friend W. Boyd Dawkins, Esq. On making sections of these I disco- 
vered that the vascular axis consisted of a very distinct vascular medullary cylinder enclosing a well-marked 
cellular medulla ; there was no exogenous zone around the cylinder, but in its place a circle of remarkably 
numerous and closely disposed vascular bundles, each one of which originated from a groove in the exterior of 
the medullary cylinder, and which in the transverse section formed a little bay, with the corresponding section 
of the bundle in its concavity. The cortical tissue consisted of the parenchymatous layer (7t), with here and 
there slight traces of the more external prosenchymatous one ( i ), the remaining tissues having disappeared. 
It is thus clear that, as I have already suggested, the specimens of Halonia with which collectors are familiar 
are branches which have lost the two outer layers of their bark. It is also obvious that the structure of Halonia 
and that of the branch represented in Plate XXYI. fig. 24 are identical ; only in the latter specimen the exte- 
rior of the vascular medullary cylinder is not quite perfect, since throughout the greater part of it the external 
indentations with their enclosed vascular bundles have almost all disappeared. A few, however, remain showing 
that they were originally present, as in my sections of Halonia. On the other hand, fig. 24 & Plate XXYII. fig. 25 
exhibit the prosenchymatous and subepidermal layers of the bark, which are deficient in Mr. Dawkins’s specimen. 
Still more recently specimens of the greatest importance, and of most exquisite beauty, have been supplied to 
me by Mr. Wiuttakek. Prom these I can easily make out almost the entire structure of the stem of Halonia. 
The cellular pith and investing medullary cylinder are arranged in our new examples as already described. 
The vascular foliar bundles appear as in Mr. Dawkins’s specimen ; but we further learn from them the exact 
2 a 2 
