224 
PROEESSOE W. C. WILLIAMSON ON THE OEGANIZATION 
gated as to constitute a distinct bast-layer, which has exhibited a constant tendency to 
separate itself from the other subjacent cortical tissues. Outside this bast-layer we have 
the superficial epidermis, consisting of thick-walled parenchyma, which also constitutes 
the tissue composing the bases of the leaves. 
These arrangements are repeated with variations of detail throughout the entire 
Lepidodendroid series. In Mr. Binney’s Sigillaria vascularis (Plate XXV. fig. 8) we 
find the vascular part of the medullary axis retreating towards its periphery, but with an 
undefined inner margin. In Diploxylon there is reason to believe that it had become 
altogether peripheral, and had a sharply defined inner boundary line, though this latter 
fact cannot be absolutely affirmed until a specimen is found with the whole of the 
medullary tissues preserved. In the same two plants we find a corresponding advance 
in the thickness of the radiating woody cylinder and in the development of the medullary 
rays. The other genera allied to Lejgidodendron exhibit structures of the same type. In 
structure of the entire hark, as "well as some other important points in their history. Immediately surrounding 
the medullary vascular cylinder is a layer of delicate parenchyma, the cells of which average about -166 in 
diameter ; these cells are arranged in columns which proceed obliquely upwards and outwards, diverging from 
the perpendicular at an angle of about 35°. The entire thickness of this innermost parenchyma is about the 
eighth of an inch (-125). Externally to it is the ordinary coarser parenchymatous layer, invested in its turn by 
the prosenchymatous one, which again is enclosed in what I have recently designated the subepidermal paren- 
chyma. Thus we here see distinctly exhibited the four layers of bark of which I have spoken in other parts of 
this memoir. The ordinary foliar vascular bundles, given off in great numbers from the outer surface of the 
medullary vascular cylinder, ascend upwards and outwards at the same angle as the cells just referred to (35°), 
until they reach the exterior boundary of the innermost parenchyma, when they suddenly bend outwards in a 
horizontal direction, describing a slight curve as they do so, the concavity of which is directed upwards. 
Each vascular bundle is invested with a delicate cellular sheath, which is a prolongation of the innermost 
parenchyma of the bark. 
But in addition to these bundles, I have now obtained the larger ones, which proceed to the tubercles charac- 
teristic of Hcilonia, and which are very different from the ordinary foliar ones. In the first place, the former are 
very much larger, consisting of many more vessels than is the case with the latter ; they are accompanied in 
their outward course by a yet thicker investment of the cells of the inner bark-layer. But the most remarkable 
difference is seen at their point of departure from the vascular medullary cylinder ; they are not merely derived, 
like the foliar bundles, from the exterior of that cylinder, but the entire mass of the vessels of the latter, immedi- 
ately beloiu the bundle, are absorbed into it. The consequence is that directly above the bundle there is a slit 
in the medullary cylinder unprovided with vessels, and where the parenchyma of the pith and that of the inner- 
most bark blend their cells into a continuous tissue. This slit ascends for some little distance up the stem, but 
the vessels on each side of it gradually converge and ultimately close it up. These peculiarities in the origin 
of the vascular bundle in question appear to me to be of great physiological importance ; they can only be 
understood when compared with conditions connected with the branching of Lepidodendroid plants that I have 
described in the third memoir of this series read to the Royal Society on the 7th of March last. I there showed 
that, prior to dividing into two branches, the vascular cylinder split into two halves, bringing the cells of the 
pith and of the bark into direct contact. It is evident to me that the arrangements in the Halonia just described 
are of the same nature, only instead of half the entire cylinder being split off, but a small portion of it is so sepa- 
rated. I infer, therefore, that the vascular bundle, thus originated, proceeded to some modification of a branch 
— birt which modification was of smaller dimensions than branches usually attained to, and which, consequently, 
required a less abundant supply of vascular tissue than ordinary branches needed. Such a modification would. 
