298 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
slight effort to strengthen its buttresses by exogenous additions to their exteriors. In 
tangential sections of the cortical layer the vascular bundles exhibit the regular arrange- 
ment characterizing the taxis of all Lepidodendroid steins. 
Before attempting to draw any general conclusions from the preceding facts, I would call 
attention to two interesting modifications of the same Lepidodendroid type that have 
recently come under my notice. One of these, represented in Plate XLY. figs. 33 & 34, 1 
found in one of the Oldham nodules ; the other is in the cabinet of Mr. Nield, of Oldham. 
Plate XLY. fig. 33 represents a transverse section of the first of these plants ; it is a 
young Lepidodendroid shoot a little more advanced in growth than Plate XLI. fig. 1 ; 
in other respects the general appearances of the two closely correspond. The chief 
difference lies in the centre of the medullary axis, which in Plate XLY. fig. 33 is 
very large and well defined. On turning to the longitudinal section of the medullary 
cylinder (fig. 34) we see that this medulla («) is a cellular structure ; but instead of 
the cells being nearly cubical, they are elongated vertically and almost fusiform ; still 
they retain much of the disposition to arrange themselves in vertical columns that is so 
common a feature of the Lepidodendroid plants. Mr. Nield’s plant, represented in 
fig. 35, is a very different one ; its central axis is of the usual type, consisting of a 
medullary vascular cylinder (e) enclosing a cellular medulla ; but whilst the latter is 
very small, approximating to the condition of Plate XLI. fig. 4, the former is com- 
paratively large, being composed of very numerous vessels of nearly uniform size. The 
most remarkable feature of the plant is seen in the large size of the bases of the 
leaves (l), which must have approximated in form to thick scales. They are composed 
of the usual slightly elongated parenchyma. Unfortunately the importance of this 
remarkable specimen was not appreciated when it was found, and I have seen no vertical 
section that was made from it ; hence I am ignorant of the shape which the leaves assumed 
in a vertical direction. The maximum diameter of the transverse section is nearly three 
quarters of an inch*. 
* It appears that Mr. Butterwortii prepared other sections of the above specimen, which he recently sold, 
through Mr. Carrttthers, to the Trustees of the British Museum. Mr. Garreteers has described these speci- 
mens in a paper which he read before the Royal Microscopic Society since the above descriptions were penned. 
In this paper he describes the vascular medullary cylinder, but does not refer to the vacant space in the centre 
of his own figure, which I believe was originally occupied by medullary cellular tissue. I think that the section 
which I have represented in Plate NLV. fig. 35 displays indications of this cellular medulla. Speaking of the 
vascular cylinder, Mr. Carrttthers says, “ Professor Williamson, in his recent investigations into the organi- 
zation of Lepidodendron, proposes to call this axis a medulla.” This is certainly not an exact representation of 
the idea put forth in my last paper ; I spoke of the vessels in the centre of Lepidodendron selaginoides, ivJiere 
they are intermingled ivith cellular tissue, as belonging to a medullary axis in contradistinction to the exogenous 
ring which enclosed them, and I then proceeded to show how, in other species, these vessels receded from the 
centre to the periphery of that medullary axis, where they formed in every Lepidodendroid plant, except 
L. selaginoides, a distinct cylinder, and which I described not as a medulla, but as being homologous with the 
medullary sheath of the higher Exogens, which is a very different thing. The true medulla is the cellular 
element. All my subsequent researches have tended to confirm these views. I never doubted for a moment 
that these axial vessels represented the vascular bundles of living Lycopods. 
