OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
69 
whilst the vessels themselves exhibit, in this aspect, very distinctly marked reticulations 
on the inner surfaces of their walls. In Plate VIII. fig. 49 we find the bark of this section 
exhibiting very similar appearances to those of fig. 47. We have the same inner layer 
of parenchyma (g), some of the cells of which (g 1 ) are again subdivided by secondary 
septa, whilst those of the outer bark (h) are arranged in centrifugal columns, similar to 
those of the transverse section. We thus learn that the outer bark consists of numerous 
horizontally disposed lines of compressed cells, whose parallel sides are lines more or 
less parallel with the periphery of the bark. The origin of this peculiar structure is 
sufficiently obvious ; but were it otherwise, Plate IX. fig. 54 would make it plain, as I 
shall shortly demonstrate. 
Plate VIII. fig. 50 is a tangential section of part of the vascular axis, magnified seventy- 
five diameters. The vessels do not exhibit their characteristic reticulations on the 
surfaces which are parallel to this section. In every one of the sections which I have 
made in this plane I have obtained the same result. The reticulated structure is con- 
spicuous in the vascular walls parallel with the medullary rays ; but instead of being 
continued round the entire tube, the reticulations appear to terminate abruptly, and with 
defined though irregular margins, as represented at d on reaching the tangential surfaces, 
which latter merely exhibit a thin, uniform, and apparently structureless layer of car- 
bonaceous matter. These facts must be correlated with what I have previously mentioned, 
viz. that in the stems of Asterophyllites (see anted , p. 44) the reticulations of the vessels 
were more definite on such of their walls as are parallel with the medullary rays than 
are those tangential to them. The rays themselves ( f ) are numerous, and, like those 
of the stem (Plate II. fig. 13), consist of from one to four cells (rarely five) arranged 
in a single vertical line. Plate VIII. fig. 51 represents the lateral aspect of one of these 
medullary rays, where the cells ( f ) are seen to have a regular mural arrangement. 
They have a vertical diameter of about ‘0015. 
lieturning to Plate VII. fig. 46, we find that at two points, one to the left and the 
other at the upper margin of the bark, the outer layer of that tissue divides and allows 
of the centrifugal extension of the inner parenchyma. The former rapidly thins out 
as it accompanies the latter for a very short distance, when it soon disappears. The 
inner parenchyma soon spreads out into a cluster of diverging rootlets ( o ), which 
stream in every direction. Some of these are seen in longitudinal ( o "), others in oblique 
(o'), and others again in transverse ( o ) sections. In the longitudinal section (Plate VIII. 
fig. 48) we again see one of these root-clusters, commencing as a large globular mass (o'), 
dividing into separate rootlets. Extending directly inwards from this root-cluster we 
see a column of the cells of the inner bark (g') prolonged through the outer one. In 
the particular portion of the section here represented much of this inner layer has been 
accidentally destroyed, but in the remainder of the specimen the tissue appears in its 
normal state. At n we see a bundle of vessels passing horizontally outwards through the 
vascular zone to reach the root-cluster. On turning to Plate VIII. fig. 52, we have one 
of these root-bundles divided tranversely (n), and pushing aside the vascular laminae as it 
