380 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
arrange themselves in perpendicular rows, as is so often the case amongst the Lepidoden- 
droid plants ; with this trifling exception the tissue is an ordinary form of parenchyma. 
I shall in the course of the memoir give my reasons for concluding that the history of 
this tissue in Dyploxylon Oldhamium is similar to that of the Lepidodendroid plant 
described in Part III., viz. that it had scarcely any existence in the very young twigs, 
the central axis of each of which has been chiefly occupied by a mass of vascular 
tissue ; but as the plant grew, the primitive cells which lurked amongst the vessels 
multiplied themselves, forming a true medulla, which pushed the vascular tissues out- 
wards, where they permanently occupy a position analogous to, if not even physio- 
logically identical with, that of the vascular medullary cylinder common to plants of the 
Lepidodendroid type. Thus in fig. 2 we have these medullary vessels at c, constituting an 
almost unbroken ring enclosing the true medulla. The large and but partially detached 
masses of vessels occupy the greater portion of the medullary area ; but as we proceed to 
examine a series of larger and older examples, we discover that these vascular masses 
become widely separated from each other (Plate XXII. fig. 1, c, and opposite to c c 
in fig. 4). They now form irregularly triangular bundles of vessels, projecting into the 
medulla from -0165 to - 02 in ordinary specimens, and adhering by their peripheral 
margins to the innermost surface of the ligneous zone, as represented in fig. 3, c, and in 
Plate XXIII. figs. 6, c, & 7, c. These masses do not appear to increase in size with the 
growth of the stem, since in a larger example of the latter, in which the ligneous 
zone alone has had a diameter of at least an inch, these vascular bundles do not project 
into the medulla more than from '0166 to '025. The component vessels of each 
bundle vary much in size. As in the Lepidodendroid medullary cylinders, they are 
arranged irregularly, and not in radiating series. They have all reticulated walls. In 
fig. 4 there are some conditions which I have not found in any other example, and 
which may have been the accidental results of partial desiccation or some similar 
disturbing cause : the continuity of the cellular medullary tissue is interrupted at the 
centre, and the cells exhibit a tendency to dispose themselves in irregular lines, 
radiating towards the periphery of the medulla. 
The Ligneous Zone . — This is a cylinder of vessels arranged in radiating planes, which 
latter are really vertical laminae separated by large and very remarkable medullary rays. 
In its normal state the cylinder has been an unbroken one, as represented in Plate XXII. 
fig. 4, d ; but, owing to various causes, this continuity is frequently disturbed in the fossils, 
as seen in fig. 1. Each lamina commences, at its inner margin, either at the medullary 
cells or at one of the triangular medullary vascular bundles. The innermost vessels are 
usually the smallest ones, a gradual increase taking place in their diameter as we follow 
them towards the periphery. This enlargement, however, soon reaches definite limits. 
Their diameter in the smaller stems rarely exceeds ’0033, and in the largest ones I have 
found none more than '005. In the young plants many of these laminae consist of a 
single linear series of vessels, as shown in the several figures 2,. 3, G, and 7 ; and in such 
examples we have rarely more than four or five linear series in any undivided wedge. 
