OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
495 
the bark have been destroyed in every one of these twigs that I have examined, but 
the leaves (fig. I, c) are all in their normal positions, varying in shape according to the 
portion of the leaf intersected by the section; they present the usual structure of 
Lepidodendroid leaves, consisting of cellular parenchyma and with the single vascular 
bundle (fig. 1, c) running through it. The distorted outlines of the sections of these 
leaves have most probably been due to desiccation ; they are chiefly found imbedded 
in the volcanic ash, which probably fell in a heated condition, producing the shrivelled 
condition in which most of these leaves are found. 
On comparing this section with the similar one of the Burntisland Lepidodendron, 
represented in Plate 41, figs. 2 and 3, of my third memoir, the difference between the 
two specimens will be seen at a glance. In the Burntisland plant the bundle has a 
diameter of about '015. In the Arran specimen it is about '033. In the former, there 
is already a small irregular central medullary space from which vessels are absent— 
nothing of the kind exists in the latter plant. In it the bundle is more compact, the 
vessels more equal in size, and the structure altogether more symmetrical and robust 
than in the Fifeshire plant. Fig. 3 represents a section of a branch of larger dimensions; 
such branches vary from one-and-a-half to three or more inches in diameter. Next to 
the twigs these are the most common fragments found in the deposit. The bases of 
the leaves still adhere to the bark, but when these are removed, as they are in a part 
where the specimen from which the section, fig. 3, had been a little water-worn, they 
present the aspect of such Lepidodenclroicls as L. nothum of Unger, Hcdonia gracilis, 
&c. The central vascular bundle has now expanded into a vascular cylinder, fig. 3, a, 
enclosing a cellular medulla, d. The structure of this cylinder is further illustrated in 
fig. 4, which is enlarged 15 diameters. The cells of the medulla are much disorgan¬ 
ized,'"' but at the junction of this tissue with the inner surface of the vascular cylinder 
I observe that a few of them are strongly barred—as I have shown to be also the case 
with many of the medullary cells of Lepidodendron Selaginoides. t The diameter of 
the pith is about T, the mean diameter of the vascular cylinder enclosing it being 
about '2. Each transverse section of this vascular cylinder has a crenulated outline 
exactly corresponding with that seen in so many of the Diploxyloid stems described 
as Sigillarice. The mean size of the vessels composing the cylinder is about '01. The 
projecting points of the periphery, fig. 4, a, consist of very small vessels, elongated 
radially in the sections; these are the orienting numerous foliar vascular bundles, c, 
which evidently detach themselves from the cylinder at first obliquely and then ascend 
more vertically when passing through the inner bark. Their transverse sections in 
the latter position, as seen at c, are circular. All these vessels are barred. 
Returning to the section, fig. 3, we now find, beautifully preserved, much of the 
middle bark, e. It is a delicate parenchyma, the cells of which are very uniform in 
* Vertical sections stow that these medullary cells are arranged in perpendicular columns as is the 
case with so many of these Carboniferous Lepidodendroids. See memoir, Part II., Plate 25, fig’. 14. 
t Memoir, Part II., Tab. 24, fig. 3. 
3 S 2 
