OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
211 
chyma (l) of the leaf-bases into the tubular prosenchyma (k) adjoining the inner layer of 
the epiderm. 
The medullary axis has been ruptured, leaving a cavity (fig. 29, a!) filled with carbo- 
naceous matter; it has consisted of cells (fig. 31, a) arranged in somewhat regular ver- 
tical piles, many of these cells having a quadrate shape with a diameter of -005, whilst 
others of the same form have only about half that diameter. Many others, again, are 
elongated vertically to a length of -012. Of the diameter of this axis I have no means 
of judging, owing to the derangement of these parts of the plant. This axis has been 
surrounded by a cylinder of barred vessels (figs. 29 & 31, d), which may have been disposed 
in radiating series, though I cannot be quite certain respecting this point ; since it is 
possible that this vascular zone may comprehend both medullary and ligneous vessels, 
the difference between them being masked by imperfect mineralization. But the opi- 
nion that some of them were arranged in a radiating series is further sustained by the 
circumstance that, in parts of the woody zone, there are straight lines of cells, having 
a muriform arrangement, but the cells are elongated vertically as in the medullary rays 
of Calamites. I only meet with these in certain portions of my longitudinal sections ; but 
they look exceedingly like medullary rays, and are of course suggestive of a radial arrange- 
ment of the vessels between which they pass outwards. They may, however, belong to 
the bark. The vessels have a diameter of -0012 ; in many of them the transverse bars 
have disappeared through imperfect mineralization, but in others they are sufficiently 
distinct to demonstrate their nature. Immediately external to the vascular zone, I dis- 
cover patches of oblong, fusiform prosenchyma (fig. 31, g ) ; but we now come to a hiatus 
(fig. 29, h) from which the tissues have been displaced, but which has been occupied by 
the middle bark. Small patches of the outer bark appear (fig. 29, i!) attached to the 
inner surface of the epidermal layer. All these patches consist of the same oblong fusi- 
form prosenchyma as that adhering to the exterior of the ligneous zone. Coupling these 
facts with the additional one that all the numerous detached fragments of bark seen in 
the specimen consist of beautiful examples of the same tissue of uniform size, unmixed 
with any other ; and arranged in parallel lines with the greatest regularity, I arrive at 
the conclusion that the entire bark has closely resembled that of the plant indicated by 
Plate XXV. fig. 8. At the junction of the outer bark (k) with the epidermal layer (l) 
we find the usual transition of the fusiform into the tubular form of prosenchyma 
(Plate XXVIII. fig. 32), which, as is seen in fig. 30, k, is still arranged in radiating lines, 
until it suddenly passes into the parenchyma of the external epiderm and of the bases 
of the leaves (figs. 30 & 32, l). This parenchymatous structure is one of the most regular 
and beautiful that I have met with. On making a tangential section of the bases of 
the leaves, we find that they consist entirely of parenchyma, but with a point in the centre 
of each scar, marking the spot where the vascular bundles penetrated the leaf, and where 
the parenchyma is much more dense, consisting of much smaller cells than elsewhere. 
The same conditions exist at the outer surface of each scar or petiole. I have not 
discovered any traces of the vascular bundles passing from the woody zone to the 
