OF THE FOSSIL PLANTS OF THE COAL-MEASUKES. 
197 
a large cellular medulla is a conspicuous feature, such a medulla would, tliere can be 
little doubt, be developed in its interior, tbougli no traces of cells can yet be detected 
in its central portion, where such an axial parenchyma should ultimately make its 
appearance. 
Plate 6 and Plate 5, fig. ] 5, represent various stages of growth of a small species 
of Leindodendron, from Halifax, of which I have now several examples, and to which 
I propose to give the name of Lepidodendron mmuhim. Its cliaracteristic feature 
resides in its medullary vascular cylinder, h, which mainly consists, in young stems, 
of one, rather irregular, line of comparatively large and conspicuous scalariform vessels 
or tracheids, but which sometimes develops a second such series on the larger stems or 
branches. In both these cases this circle of conspicuous vessels is surrounded closely 
by a fringe composed of numerous, very much smaller, ones, from which the foliar 
bundles are solely derived. 
Fig. 7 represents a transverse section of the youngest example of L. mundum I have 
yet obtained. In it the outer cortex, c, consists of a parenchyma, the diameters of the 
cells of which become smaller from within outwards, assuming, as they do so, the form 
of a coarse prosenchyma with somewhat thickened walls. The middle bark has disap¬ 
peared from this, as also from all my other examples of this planr. At the centre, 6, 
we have the vascular bundle, which is represented, still further enlarged, in fig. 8. It 
appears to consist wholly of a central mass of large tracheids, h, surrounded by a 
fringe of smaller ones, h'. I can detect no traces of cells amongst these tracheids. 
In fig. 9 this solid axial cylinder has developed into a medullary vascular ring, 6, V, 
enclosing a few parenchymatous cells, a, forming a small but distinct medulla. In 
fig. 10, whilst the general conditions are smiilar to those of fig. 9, not only the 
tracheids of the vascular cylinder, h, h', have become more numerous, but the medul¬ 
lary cells, a, have done the same. In this instance the cells, a, are crowded, thin- 
walled, and of irregular form, as if they had recently passed through a meristemic 
stage of mulriplication. In fig. 11 this general enlargement and multiplication has 
not only gone still further, but the individual medullary cells have now assumed the 
hexagonal form of a v-ery regirlar parenchyma. Fig. 12 represents a specimen in 
which these progressive developments of the medulla, a, and of the medullary vascular 
cylmder, h, b', have attained to the maximum extension seen in any of the specimens 
in my cabinet. We also see in this specimen the well-preserved innermost cortex, c, 
which consists of a parenchyma the cells of which are uniformly of small size and 
shape, their mean diameters being about one-thousandth of an inch. 
Fig. 13 represents a vertical section through the vasculo-medullary axis of a speci¬ 
men of the same plant, the entire diameter of which, including its cortex, is about 
four-tenths of an inch. Here we have the medulla at a composed of cells with square 
or very slightly inclined transverse septa. The barred vessels"'' of the medullary 
* The transverse bars of these vessels are connected vertically, as shown on Plate 5, figs. 14a and 
11b. by delicate threads like those seen in the Arran plant (see Memoir X., fig. 4*). 
