OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
203 
even with barred vessels, in no way militates against the conclusion that the vertical 
piles of mural cells which separate the laminae of the vessels constituting the woody 
zone, and which are constantly extending in a peripheral direction, are true medullary 
rays. Their earliest genesis, combined with their final functions, rather than the degree 
of differentiation which the several tissues have finally undergone, determine their 
nature. 
I shall shortly demonstrate that, simple as these rays are in their early form, they 
become very definite when we ascend to some of the higher developments of the ligneous 
zone which we shall find amongst these Lepidodendroid plants. 
If I am correct in these determinations, no question can arise as to the cortical nature 
of the thick, investing, and more external layers, with their prevalence of prosenchyma, 
so characteristic of Lycopodiaceous cortical structures. We also see that the retention 
on the epiderm of portions of the bases of the leaves hides what should otherwise 
represent the regularly arranged leaf-scars seen in the common Lepidodendroid stems. 
Whether, in the species under consideration, these leaf-petioles were persistent, or 
whether, as the stem advanced in age, they fell off, leaving a natural cicatrix of the forms 
represented in fig. 6, is doubtful ; but my present belief is that the latter was the case, 
and that the fact explains why we only find such examples as I have figured of compa- 
ratively small size. 
I have sections of one example of the above type from South Owram, near Halifax, 
and for which I am indebted to Mr. Neild of Oldham, hi which the cellular element 
of the medullary axis is reduced to its minimum. The axis consists mainly of barred 
vessels ; nevertheless cells exist in sufficient numbers to demonstrate the identity of the 
two forms. The next modification of the Lepidodendroid type to which I would call 
attention is one that Mr. Binney has included, together with that which I have just de- 
scribed, in his memoir on Sigillaria already referred to, but which I agree with Mr. 
Carruthers in regarding as a distinct plant. None of the specimens of it which I have 
had the opportunity of studying exhibit the outermost layer of the bark ; consequently 
I know nothing of its external contour, but the portions which I possess are interesting 
and instructive. Plate XXV. fig. 8 represents a transverse section of the central axis 
(a, c), with the ligneous zone (cl) and the inner portions of the bark ( g , i ), of a specimen 
in Mr. Butterworth’s cabinet, besides which I have made numerous dissections of 
some other specimens of the same type, with which Mr. Butterworth has kindly 
supplied me. The prominent features in the medullary axis of the specimen figured 
are the entire absence of vessels from its central portion, and its transverse division 
into two longitudinal halves, by a line extending on each side from its central cellular 
portion to its periphery. Mr. Bixney has correctly represented this structure*; but 
he says respecting it, “ The dark line across the axis, as well as the dark space in the 
centre, both seem to be the result of a disarrangement of the tubes during the process 
of mineralization, as similar appearances have not been observed in many other speci- 
mens examined by me, which in those parts are in a more perfect preservation"'!’. I 
* Loc. cit. pi. 32. fig. 1 & 2. 
Loc. cit. p. 587. 
T 
