200 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
The next species, which I propose to designate Lppidodendron ‘parvuliim, is the 
smallest of the Lepidodendroid family that I have hitherto met with. I have a 
considerable number of examples of it in my cabinet, but there is a remarkable 
similarity of shape, size, and structure throughout the entire series, the mean diameter 
of the transverse sections, including the leaves, being ouly about one-tenth of an 
inch. 
Though so small, every example in my cabinet obviously possessed a cellular 
medulla, Plate 8, fig. 28, a. This was surrounded by a medullary vascular cylinder, h, 
from two to three tracheids in thickness, the tracheids being fairly uniform in 
size, though, as uoual, the outermost are rather less than those occupying the 
inner margin of the ring, which they combine to form. The inner cortex has 
disappeared. The middle cortex (fig.. 23, c) is always an unbroken circle of 
parenchymatous cells. Still more uniform in size and shape are the parenchymatous 
cells of the outermost cortex, d, as well as of the leaves. But between these two zones 
of the cortex, c and d, we have in most of my specimens the verticils of empty areas 
(figs. 23, e, and 24, e). These spaces are separated by the radial bands of cortical 
parenchyma (fig. 24, e). The empty areolae, e, were long unintelligible to me, but at 
length I obtained specimens in which, as in fig. 25, e, I found them to be normally 
occupied by a thin-walled parenchyma very distinct from that of which the remainder 
of the cortex, including the leaves, was composed. The latter is represented in 
fig. 26 ; the former in fig. 27. What the functions of these vertical bands, e, of 
specialised tissue may have been I can form no opinion. I have seen nothing like 
them in any other Lepidodendroid stem. 4die vascular bundle, h, of fig. 25, is sub¬ 
dividing in the usual Lepidodendroid manner when the branch is about to dichotomise. 
I have an ascending series of five sections, from the lowest, in which the vascular ring 
is unbroken, to the highest, in which each of its two halves has not only almost 
reconstructed its perfect cylinder, but is enclosed in a distinct cortex of its own, just 
about to separate into two distinct branches ; each of these branches retains the 
features characteristic of the species. I have obtained the plant both from Oldham 
and from Moorside, in Lancashire. 
A few remarks on the general relations of the objects described in the previous 
pages may not be out of place. 
The form of ramification illustrated in figs. 1-6 is not wholly ne^v. In my Memoir, 
Part II. (‘Phil. Trans.,’ 1872, p. 224), I demonstrated that the tubercles characteristic 
of the genus Halonia were merely branches the development of which had been 
arrested, and that, as in the case of the plant (figs. 1-6) of the present Memoir, the 
vascular bundle supplying such branches was given otf from the medullary vascular 
cylinder, in a manner intermediate between a perfect dichotomy of that cylinder, 
representing ordinary I’amifications, and the detachment of a few small vessels from 
its periphery, constituting the bundles supplying the leaves. In Memoir, Part 
XII. (‘Phil. Trans.,’ 1883, p. 459), I further showed that the Halonial tubercular 
