OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
51 
Tangential sections of the exogenous layers (Plate IV. fig. 22) show the intersected 
extremities of the medullary rays (/). These are arranged in precisely the same way 
as in the Oldham plant (Plate II. fig. 13), the longer axes of the individual cells being 
frequently inclined (/') to the vertical line separating the two vessels enclosing the ray. 
The vertical length of the cells in this section varies from *0017 to ‘00087. Each ray 
frequently consists of a single cell, rarely of more than two or three : such cells are 
more densely crowded together at fig. 22,/, than is usual. Their more general aspect is 
that presented by those on the left of the same series, and by the second lower series in 
the figure. On endeavouring to trace these medullary rays in sections made parallel 
to their course (Plate IV. fig. 23), if we employ low magnifying-powers, we have no 
difficulty in seeing that such sections are crowded with them ; but, owing partly to 
the small number of the cells disposed vertically in each ray, and partly to the exceeding 
delicacy of their cell-walls compared with the coarser tissues of the vessels, it is a more 
difficult thing to decipher their details where high powers are used. Plate IV. fig. 23,/, 
exhibits their appearance under such powers. In many instances we see that the cells 
in these latter sections appear to be almost cubical ; but, owing to their limited number 
in each ray, and the consequent absence of mutual pressure, their sides are more turgid 
and their general aspect less mural than in other examples of Coal-measure plants. This 
difficulty in tracing the details of their form is further increased by the fact that their 
vertical divisions frequently coincide with those of the vessels, owing to the dimensions 
of the two being correspondent in the radial direction. Nevertheless there is no real 
difficulty in demonstrating that these organs exhibit substantially the ordinary structure 
and arrangement of exogenous medullary rays. 
I have already stated that the vascular elements of these stems differ from those of 
the Oldham plant, in consisting wholly of barred vessels instead of reticulated ones. 
Their general aspect is seen in the highly magnified portion of one of them represented 
in Plate IV. fig. 24. But small portions of vessels frequently exhibit the aspect 
shown in Plate IV. fig. 25 ; and it is easy to see that the reticulations of Plate I. 
.fig. 6 are but modifications similar to those of Plate IV. fig. 25, only carried to a 
much greater extent and occurring in every vessel. They are both modifications 
of fibro-vascular tissue, and have no relationship to the discigerous fibres of the 
Conifera. 
Though my sections of the Burntisland plant are numerous, all the fragments have 
been short ones, and I have not been fortunate enough to meet with one exhibiting 
clearly a nodal foliiferous disk with associated leaves. One small and crushed twig, 
however, appears in one of my slides, which I do not doubt is of this character ; but 
it is too distorted by pressure for me to make much use of it. It appears to me that 
this species has been characterized by very long internodes. 
Attention has been directed to the fact that, amongst the Oldham specimens, I have 
in no one instance discovered any trace of a lateral deflection of the vessels of the vas- 
cular axis either into leaves or into branches. Amongst my Burntisland examples I 
h 2 
