OP THE FOSSIL PLANTS OF THE COAL-MEASURES. 
285 
tendency in the cells of the inner bark to arrange themselves in a line parallel with the 
larger axis of the section, partly in the more uniform size of the cells, and partly in the 
less dense character of the tissue. On turning to the central vascular bundle, we find 
that it has undergone a considerable change. The vessels have become much more 
numerous, and the transverse sections of the larger ones have a larger axis of •0033. 
The vascular and somewhat compressed cylinder itself has a longer diameter of ‘022, and 
an uncompressed circular one belonging to a twig of about the same size as Plate XLI. 
fig. 1 has a diameter of '01 7. Fig. 4 represents a transverse section of one of these 
larger cylinders, drawn to the same scale as fig. 2. 
Vertical sections of specimens in this stage of growth reveal yet more distinctly the 
changes that have occurred. Plate XLI. fig. 5 represents such a section made across the 
shorter diameter of an example like fig. 1 ; figs. 6 & 7 are the radial sections of the bark 
of two other similar specimens. 
Plate XLI. fig. 5 reveals at a a slender column of very delicate cells elongated in the 
vertical direction ; these cells are obviously those of a rudimentary medulla. At d are the 
barred vessels of the vascular cylinder ; h, h is a cellular mass considerably disorganized, 
but of which we shall learn the true structure from other examples ; at i is a layer of 
elongated cells with oblique overlapping extremities, a true prosenchyma, the cells of 
which occasionally become so much elongated as to approach the general condition of 
bast-tissue*; whilst at i! the prosenchymatous cells gradually become broader and 
shorter, thus passing into a parenchyma (figs. 6 & 7, k), which usually forms the exterior 
of the plant, but which is not well represented in the specimen, fig. 5. I think there 
can be no doubt that the inner parenchymatous tissue (A) represents the middle 
cortical layer of my previous memoir, whilst i represents the outer bark, and k the epi- 
dermal layer of the same memoir, but which latter may be more accurately termed the 
sub epidermal layer. In the present instance h appears to be the innermost layer of the 
bark; but I have previously applied the term inner to a very delicate structure, found in 
some plants (e. g. Stigmaria), which I do not detect in the specimens under consideration. 
The distinctive features of the three layers of bark just described are sufficiently obvious. 
The cells of the layer h are arranged in rather regular vertical columns, each column 
having a diameter of about *00022, the entire layer being about ’0025 in thickness. 
These cells are almost destroyed in fig. 5, but in figs. 6 & 7 their true aspect is well 
shown. They have square and not overlapping extremities. The longer ones are about 
*0008 in length, being about three times longer than broad, but in many of them length 
and breadth are about equal. The prosenchymatous layer (i) is thicker than the more 
internal parenchymatous one, whilst the largest of its cells are as much as '0025 in 
length, mingled, however, with numerous others of much smaller dimensions. The 
cells of the subepidermal or outer parenchyma (k) are of the ordinary character, 
* The use of this term is not intended to imply that the part of the bark in which these elongated cells occur 
is homologous with the liber of Dicotyledonous stems, but that the individual cells are similar to those to which 
the liber owes some of its chief peculiarities. 
2 q 2 
