4 
180 Transactions of the 
sizes, more or less circular or polyhedral in section, and arranged in 
irregular order ; and 2nd, an external portion, in wMcli the vessels 
have an irregularly radiating direction, their longest diameter heing 
in the line of the radius (this is not vi^ell rendered in the plate). The 
circumference of the axis is regular, from the interspaces between 
the large vessels being filled in by some of smaller diameter. The 
vessels are of considerable length (Fig. 2 a), so that I have not been 
able to obtain a longitudinal section which would show with certainty 
both terminations of one. Some of those in the centre of the axis 
are divided into chambers by horizontal septa, or rather they appear 
to be made up of a series of short obtuse cells, whose transverse as 
well as longitudinal sides are marked with scalariform bars. Such 
interrupted vessels are scattered irregularly through the others. I 
can detect no trace of any other structure in the axis than scalariform 
vessels. 
Surrounding the axis is a narrow cylinder of radiating scalari- 
form tissue (Fig. 1?>), differing from that of the axis only in the 
method of its arrangement, and in the smaller diameter of the 
vessels. They are seldom more than a quarter of the size of those 
of the axis, and are smallest at their origin in the interior of the 
cylinder. Their form in transverse section is sub-quadrangular. 
They are separated into groups of one, two, three, or four rays by 
a horizontal radiating structure composed of somewhat elongated 
cells with truncate ends, and delicate walls without any markings 
on them (Fig. 3). The cells are from two to four times longer 
than the diameter of the scalariform vessels of the cylinder, while 
their transverse diameter is only half that of their vessels. In a 
longitudinal section at right angles to the radius these cells are seen 
to be arranged either singly, in longer or shorter linear series, or 
occasionally in larger wedges composed of two or three cells in thick- 
ness (Fig. 4). In addition to this cellular structure there also are 
seen passing outwards, between meshes in the cylinder of scalari- 
form vessels, bundles of similar but more delicate vessels connected 
with the leaves. Both these structures are described by M. Bron- 
gniart in his Sigillaria elegans. The first are his medullary rays. 
It is obvious that this term is scarcely admissible here, as the axis 
of the stem is not occupied with a cellular or medullary tissue, 
but with scalariform vessels. In a recent paper on Sigillaria * I 
ventured to doubt that these were medullary rays, but I was not 
then able to show that whatever the structure is it cannot be inter- 
preted as similar to that of the medullary system of dicotyledons. 
The vascular cylinder is surrounded by a considerable thickness 
of delicate parenchyma (Figs. Ic and 2 c), which in most specimens 
has completely disappeared, but in that figured some remains of it 
exist in the transverse section near the vascular cylinder, and a 
larger quantity is seen in the longitudinal section (Fig. 2 c). The 
* ' Quart. Journ. Geol. Soc.,' vol. xxv,, p. 248. 
