OF THE FOSSIL PLANTS OF THE COAL-MEASTTEES. 
395 
to it the name of Dictyoxylon Grievii, after its discoverer. Since that period I have 
made numerous additional sections of the plant, some of which I will now proceed to 
describe. At the first glance no two plants could well appear to be more distinct than 
the two just named ; it is only when we compare the details of their organization that 
we learn how numerous and strong are their points of affinity, and how great the amount 
of light which each throws upon the other. The example now under consideration always 
appears in the form of straight, slender, unbranched stems ; but we also find associated 
with it much smaller twigs, having the characteristic structure of the larger ones, but 
with distinctive features in their central vascular axes, similar to those which distinguish 
a twig of a Lepidodendroid plant from its matured Diploxyloid stem. 
Plate XXVIII. fig. 30 represents one half of a transverse section of one of the more 
matured of the stems that have hitherto been met with. We here readily discern a large 
vasculo-cellular medullary axis (a), a very thin investing ligneous cylinder (d), a thin and 
delicate inner layer of bark (</), a thick parenchymatous bark-layer (A), and a well-defined 
outer or prosenchymatous layer of the same (A). 
The central axis consists of a mass of medullary parenchymatous cells, dispersed 
throughout which are numerous well-defined bundles of reticulated vessels, unprovided 
with any special sheaths. The cells have thin walls, and are devoid of all fibrous deposits 
in their interiors. Plate XXVIII. fig. 31 represents an enlarged portion of this medullary 
axis : the cells (A) are at once distinguished from the vessels (c) by their smaller size, 
their thinner walls, and their brown colour. The last distinction is an evidence, were 
such needed, of their cellular character : however thin the section, owing to their small 
size, we always look through one or more of their carbonized cell-walls ; whereas the long 
tubular vessels, being filled by infiltration with pure carbonate of lime, and having their 
oblique transverse divisions only at long distances from each other, their transverse sections 
transmit pure white light. The relations of the cells and vessels are further shown in the 
longitudinal sections (Plate XXIX. figs. 32 & 33, A & c ). Fig. 33, b, especially exhibits 
the aspect of these cells in vertical sections. The largest of them rarely exceeds -0025 to 
•0028 in diameter, and many of them are much smaller. The vessels are arranged in 
clusters, transverse sections of no two of which exhibit the same shape ; and they are 
equally variable in the number of their component vessels. They are of large size, many 
of them being *0052 in diameter; but they are frequently much smaller than this, and at 
the peripheral portion of the medullary axis they are invariably so. In the latter case, 
in the transverse sections, they are only distinguishable from the surrounding cellular 
tissue by their colour and their thicker walls. The clusters of the periphery also contain 
a larger number of vessels, as well as display a greater tendency to an irregular coalescence 
of the groups, than is the case nearer the centre of the axis. Plate XXVIII. fig. 34 
exhibits the lateral aspect of one of these vessels, enlarged 200 diameters. The entire 
diameter of the medullary axis in the larger specimens is about T3. This axis is 
invested by an exogenous layer of vessels (d), disposed in radiating lines widely separated 
by large medullary rays. The entire thickness of this zone is small, rarely exceeding 
3 G 2 
