OF THE EOSSIL PLANTS OF THE COAL-MEASURES. 
15 
series obviously varies with the age of the axis. Even in the transverse sections the 
existence of medullary rays is sufficiently obvious. In fig. 31, which represents a small 
portion of the periphery of one of the primary wedges of fig. 34, these medullary rays 
are clearly shown at f, where they appear as parenchymatous cells elongated radially. 
Fig. 30 is a tangential section of a stem intersecting three vascular wedges, the central 
one {cl) having been cut through at its innermost portion close to the central vascular 
axis. In this section the medullary rays appear at ff, as clusters of cells arranged 
vertically as single linear series. In fig. 27 we have a tangential section of a similar 
wedge at its more peripheral extremity. These two sections combined illustrate the 
increase in diameter of the vessels composing each primary wedge as we proceed from 
within outwards ; and fig. 27 further demonstrates a corresponding increase in the 
number and dimensions of the medullary rays (f). In the latter figure we find that many 
of these rays are lengthened vertically by a material increase in the number of their 
component cells ; and, further, that from time to time we meet with portions of these 
rays, as at fig. 27, f, in which two and even three vertical lines of cells coexist in the 
same ray. Fig. 29 represents a portion of a slightly oblique, radial, longitudinal section 
crossing two vascular laminae {e, e'), with parts of the medullary ray (f) which separate 
them. The latter exhibits the usual mural arrangement of its cells seen in radial 
sections of so many others of the Carboniferous plants. In the tangential section 
(fig. 27) the vertical length of these cells ranges from -001 to -0025. 
The Cortex . — This part of the organism exhibits some very characteristic features 
which distinguish the plant under consideration from any of those which I have hitherto 
described. The six large primary wedges of the vascular zone are separated by a mass 
of delicate thin-walled parenchyma. The arrangement of the cells of this structure 
is best shown in the two figures 26, h & 32, h. The cells are parenchymatous, but are 
elongated radially, showing a tendency to arrange themselves in lines more or less 
parallel with the sides of the two contiguous primary vascular wedges. On approaching 
the broad outer extremities of these wedges, the elongated cells exhibit a strong tendency 
to diverge in two groups, each one bending round the peripheral extremity of the 
woody wedge nearest to it, whilst in the angular space between and external to the 
diverging portions we have ordinary regular parenchyma. I do not mean to convey the 
impression that all these portions of the cellular tissue are separated by clearly defined 
boundaries ; such is not the case ; but whilst these respective areas graduate into each 
other, there is little difficulty in tracing broadly their respective characteristic features, 
which are well shown in fig. 34, i". The same figure further shows, at g, g, another 
remarkable feature ; the cellular masses {li) do not bend round the outer extremities of 
the primary wedges ( d , d) as just described, in immediate contact with those wedges ; but 
they enclose a small semilunar area {g) coextensive with the 'diameter of the wedge, and 
which is occupied by a distinct form of cellular tissue. I shall shortly give my reasons 
for believing that this latter tissue is a quasi-cambial meristem layer, which is concerned 
in the formation of the newest exogenous vascular growths. The general features of 
