PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
288 
Plate XLII. fig. 11 is a transverse section of a woody cylinder of a large stem. Cal- 
culating roughly the proportions which the vascular axis of fig. 9 has borne to the 
entire stem, I conclude that fig. 11 represents the vascular axis of a stem of about 
14 inches in diameter; its central medulla has a mean diameter of about half an inch, 
whilst that of the entire vascular area is nearly an inch and a half. The medulla (a) 
is present, though considerably disturbed ; but sufficient remains in a normal position to 
show that its cells were arranged in vertical columns, a disposition which is well illus- 
trated by another specimen in my cabinet to which I shall call attention. I pointed out 
in my last memoir that this disposition to a columnar arrangement of the medullary 
cells is a common feature amongst the Lepidodendroid plants. The medullary cylinder 
(<?) is very narrow in proportion to the diameter of the stem, not averaging more than 
•055. The thickness of the ligneous zone (d), on the other hand, is fully half an inch. 
On one side the medullary cylinder has been detached from the ligneous zone and 
forced inwards into the pith by some force that must have acted through one of the 
two extremities of the specimen, since the ligneous zone is but slightly disturbed at its 
inner surface, and not in the least so externally. In this specimen the large vessels of 
the medullary cylinder have a mean diameter of '0075, a large increase upon the ‘0025, 
which was the maximum diameter in the young twig, Plate XLI. fig. 2. The great 
thickness of the ligneous zone is due to an enormous increase in the number of vessels 
in each radiating line, they having increased from the 13 to 17 of Plate XLII. fig. 9 to 
from 84 to 100. There is not a corresponding increase in the diameter of these vessels >' 
the more peripheral ones are actually smaller than those in the central parts of the 
woody zone. This may readily be accounted for. The latter have now attained to 
their maximum development, whilst the former, being younger, have not done so. 
Plate XLII. fig. 12 represents a tangential section of a portion of this ligneous 
zone, magnified 10 diameters. We here see the vascular bundles ( m ) passing outwards 
to the leaves, arranged in regular quincuncial order. Fig. 13 exhibits a portion of 
fig. 12, enlarged 40 diameters: we here find that numerous medullary rays (f) pass 
outwards between the barred vessels ( e ); these rays sometimes have but from one to 
four or five cells in each vertical pile, but in other instances their vertical extension is 
considerable. The cells of the rays have disappeared, but the spaces they occupied 
are well marked by the deep indentations which their pressure has made upon the 
walls of the contiguous barred vessels. In radial sections of the stem these rays are 
seen proceeding towards the periphery horizontally (fig. 10, f), and as straight as if 
they had been drawn with the aid of a parallel ruler. Enough of their form can be 
ascertained to demonstrate that they consisted of the mural form of parenchyma. In 
the centre of fig. 13 we have one of the foliar vascular bundles (m) passing outwards 
through a lenticular space corresponding in all respects, save size, with a medullary 
ray. Like these latter appendages, the space not occupied by the vascular bundle 
was occupied by cells identical with those of the medullary rays ; and in many instances 
these lenticular spaces pass into and are continuous with true medullary rays. We 
