OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
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the fistular cavity, a, is very large. In fact, the thin layer of medullary cells does 
little more than link together the large primary medullary rays, c, which separate the 
large vascular wedges, f These wedges, each with its internodal canal, e, at its inner 
angle, are here thirteen in number. The cortical parenchyma is seen at h. 
My cabinet contains other similar specimens besides those figured ; the series clearly 
illustrates one type of structure seen in various degrees of development, and figs. 12 
and 13 unmistakeably demonstrate the type to be identical with the Cctlamodenaron 
of Brongniart and those who accept his views. We here see the gradual formation 
of the central fistular cavity, and of the vascular wedges, from a condition in fig 8, m 
which they exactly resemble what we find in living Equisetums, viz., canals with 
extremely few vessels at their outer border, to that of figs. 12 and 13, where exogenous 
additions have developed a well-formed wedge composed of numerous radiating laminae 
of vessels separated by medullary rays. The structure of these wedges was fully 
described in my first memoir, and my recent researches require that nothing should 
either be added to or subtracted from those descriptions. It is obvious enough that 
if such examples as figs. 1 1 and 1 3 had been deposited horizontally, parallel to the 
stratification of a bed of shale, and subjected to vertical pressure, they would have 
become as flat, and almost as thin, as a piece of brown paper. In ether words, they 
would have resembled the Calamites of Brongniart, and yet they are all true examples 
of the genus Calamodendron of that author. 
We may now turn to the different conditions seen in figs. 14 and 15, which repre- 
sent, two-thirds their natural size, transverse and vertical sections of a fine Calamite 
found near Oldham by Mr. Nield. In the transverse section, fig. 14, the fistular 
cavity, a, has a mean diameter of about '62. It is surrounded fly an extremely 
scanty pith, b, which in turn is invested by a cylinder of vascular tissue fully 
2 inches in thickness. At its inner surface this zone is divided into the usual 
0 
radiating wedges,^ which have been either fifty-eight or fifty-nine in number. Each 
of these wedges starts internally from the characteristic canal, and they are separated 
from each other by the usual radiating extensions of the pith which I have always 
designated the primary medullary rays. These can easily be traced, extending 
outwards through the vascular cylinder as much as '25, but beyond that distance the 
tissues become so blended together that the rays cease to be visible. In tangential 
sections made near to the pith these primary rays are very regular in form and arrange- 
ment, but long before reaching the middle distance between the pith and the hark they 
are only represented by a very small number of vertically elongated lenticular masses 
of cells, such as Mr. Binney has accurately represented in Plate iii., fig. 6, of his 
‘Monograph on Calamites and Calamodendron.’ These cellular areas are arranged at 
very irregular intervals in all matured stems. 
The aspect of the secondary rays in radial vertical sections is shown in fig. 16c/. 
The vessels, g, are usually arranged vertically with great regularity, especially when 
seen in radial sections ; but in one of my tangential sections there is an irregular 
