204 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
feel constrained to differ from the above conclusion, to which, I think, Mr. Binney has 
been accidentally led by uniting two plants, which, though very closely allied, are never- 
theless distinct, viz. the one which I have just described and that now under considera- 
tion. The dark central portion of the medulla is a compact mass of cells, as is well 
seen in Plate XXV. fig. 9, a. No vessels appear in their midst, and the dividing line, 
extending on each side to the woody zone, is a prolongation of the same cellular tract*. 
The cells average from -01 to -0025 in diameter, their length being variable. As in the 
preceding instance, they are generally arranged in vertical piles, but with great irregularity 
in the obliquity of their horizontal or transverse septa. This peculiar obliquity of many of 
these cell-partitions in the cells traversing the long axis of the stem appears to be a 
characteristic feature of most of the Lepidodendroid plants. All these cells in the plant 
before us have had barred walls. External to this cellular axis we have a dense ring of 
barred vessels (figs. 8 & 9, c). At the inner portion of the ring they are detached from 
one another, masses of the barred cells ramifying between them ; but towards its exterior 
portion the vessels become a compact mass. They have a varying diameter of from -01 
to *0025, but the most peripheral series in immediate contact Avith the ligneous zone 
are not more than -0012. The entire series is arranged, in transverse sections, in paren- 
chymatous fashion, being wholly devoid of any linear disposition, and small tubes being 
packed into the interstices amongst the larger ones. 
One of the striking and characteristic features of this plant is its well-developed ligneous 
zone (figs. 8 & 9, d). This consists of barred vessels, arranged in very regular radiating 
lines. In the specimen figured, there are not more than seventeen or eighteen of these 
vessels in each radial series; but in another section, in my cabinet, of a stem which, though 
deprived of its epidermal layers, has been fully inches in diameter, the woody zone 
has a breadth of *37, and each linear row contains about 80 vessels. As usual the inner- 
most of each series are the smallest, and they increase in size as they proceed outwards. 
The medullary rays are very abundant (Plate XXV. figs. 9 & 10,/). In the tangential 
sections (fig. 10) they are easily recognized ; but, owing to the delicacy of their texture, a 
superficial observation easily leads to their details being overlooked in the radial sections. 
They are nevertheless most distinct, sweeping across the vessels in straight and parallel 
lines from the medullary to the cortical surface of the ligneous zone ; precisely as they 
would be seen to do in a corresponding section of any exogenous wood. The exogenous 
growth of this portion of the stem is sufficiently obvious. We have the radiating 
arrangement, and the regular increase occurring in the number of vessels in each linear 
row, as the stem enlarges its diameter. The new vessels have not been intercalated, 
but added to the exterior of each series, — a fact often rendered evident by the circum- 
stance that, from their walls being less strengthened by ligneous deposits than in the 
case of the older vessels, they are much more liable to be disturbed and disarranged by 
lateral pressure. 
* Other specimens have come under my notice, in which the medullary vessels encroached almost entirely 
upon the inner parts of the pith ; nevertheless there remained the central spot, of which the transverse line 
dividing the medulla into two halves is the lateral extension. 
