OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
73 
I have already referred to the fact that the ligneous reticulations are deposited on the 
inner surfaces of the walls of the vessels of Aster ophyllites, and that these deposits are 
more distinct on those sides that are in contact with the medullary rays than on those 
which are tangential to the circumference of the stem. In the axis of the root this 
difference becomes yet greater, since on the tangential surfaces the reticulations dis- 
appear altogether. When we remember that precisely similar conditions exist in the 
distribution of the circular disks on the corresponding portions of the fibres of Coni- 
ferous stems, we may suspect that some similar influence regulates that distribution in 
both cases. Not that I mean to infer the existence of any systematic relationship 
between Asterophyllites and the Conifera, but of some influence common to both 
which has produced differences between the deposits and secondary growths of the 
radial and tangential surfaces of their fibro-vascular elementary tissues. That some 
relations subsist between the intervascular disks of Coniferous fibres and the ligneous 
deposits within the same fibres is well known*. 
The most determined opponent of exogenous development must admit that we have 
here circumferential growth, as unmistakable as that of an oak-tree both in the stems 
and in the roots. Not only have we the lines of growth, so distinctly seen in Plate IX. 
fig. 56, but the newer laminae not unfrequently split off from the older ones, as in Plate 
IX. fig. 59. In both stems and roots these new vascular layers can only have 
originated in the inner portions of the inner bark, g. This unity of function leads us 
to the conclusion that the parenchymatous cells of the inner bark of the stem 
(Plate II. fig. 10, g ) are homologous with the non-radiating parenchyma of the roots 
(Plate VIII. figs. 47 & 49, g), and that in both cases this layer has been the chief seat of 
genetic activity. I have already shown that in the case of 16 the roots the outer layer of 
the bark has clearly been derived from the outer surface of the inner parenchyma by 
the development of new and parallel tangential divisions within the interior of its con- 
stituent cells ; so that, in this organ at least, the parenchyma has been subject to a 
double waste of its cell-supply — an inner one to furnish new vessels, and an outer one 
thickening the outer bark-layer. This exhaustion, whether it was continuous or 
periodic, must necessarily have been antagonized by a corresponding activity of cell- 
reproduction. It thus appears that this inner parenchyma was a meristem layer, in a 
more or less constant state of intense genetic activity. I have not been able to satisfy 
myself in the same conclusive manner respecting the origin of the prosenchymatous outer 
bark of the aerial stems. How far the latter is the altered homologue of the exterior 
of the outer bark of the roots is also an open question. It is not improbable that both 
these tissues may be genetically identical, but modified in their form to suit the aerial 
and subterranean conditions under which they had to fulfil their respective functions. 
The most difficult question of all yet remains to be considered — What is Asteropkyl- 
litesl In the first place, it has the closest possible affinity with Sphenophyllum. I 
* See on this point my memoir “ On the Structure and Affinities of some Exogenous Stems from the Coal- 
measures,” Monthly Microscopical Journal, August 1, 1869. 
MDCCCLXXIV. L 
