OF THE FOSSIL PLxlNTS OF THE COAL-MEASURES. 
199 
Plate XXIV. fig. 1 represents a transverse section of Lepidodendron selaginoides, from 
the cabinet of Mr. Butterwortii, magnified six diameters. The medullary axis (a) con- 
sists of a very peculiar admixture of barred cells and vessels also barred. I abstain, as I 
have already done in my previous memoir on Calamites, from designating these vessels 
scalariform , because I have not yet found them to be thickened at their angles with 
continuous deposits of lignine, as is the case with the true scalariform vessels of ferns. 
Fig. 2 represents a longitudinal section of the same specimen, magnified four diameters. 
Fig. 3 is a small portion from the centre of the medullary axis of fig. 1, more highly 
magnified, and fig. 4 is a corresponding enlargement of the same structure, though less 
highly magnified, of fig. 2. The cells of this structure in the specimen figured exhibit 
a tendency to diverge into two forms. We have one thick- walled series, arranged in 
vertical rows (fig. 4, b), the transverse septa of which are sometimes rectangular in rela- 
tion to their sides, but much more frequently oblique, the obliquity tending sometimes 
in one direction, and sometimes in another even in the same pile. The sides and ends 
of these cells are alike richly barred. Sometimes the bars are regularly parallel with each 
other, and arranged transversely as in the vessels ; but very frequently they describe a 
series of curves as if one, two, or even three of the angles of the cells had been centres 
from which corresponding series of concentric segments of circles had been drawn. In 
the transverse section these cells also appear barred on their transverse partitions (fig. 3, b), 
the bars being usually arranged in two opposed systems of curves. These barred cells 
vary in diameter from •005 to ‘0015'*. The cells of the other class are much smaller, 
have very thin walls, and appear to be small masses of ordinary parenchyma intermingled 
with the other medullary tissues ; it is possible, but not probable, that this difference is 
due to mineralization, a point to which I shall return. The vessels (figs. 3 & 4, c) are 
often almost undistinguishable from sections of the barred cells ; indeed we appear to 
have here strong evidence of their primarily cellular character. In the specimen figured, 
those of the centre of the medulla are somewhat widely separated by the two kinds of 
cellular tissue as shown in fig. 4 ; but this separation only extends over a small area. 
In the peripheral portions of the medullary axis they are closely conjoined, the cellular 
element becoming less abundant, especially the delicate parenchymatous tissue which is 
so much more copious in the centre of the structure. These vessels range from ’0014 
to ’002 in diameter. 
Immediately investing the medullary axis is a thin cylinder (figs. 1 & 2, e) of small 
barred vessels arranged in parallel series radiating from the medulla outwards. These 
represent the ligneous zone. The innermost ones are exceedingly minute, and though 
they increase in size as we proceed outwards, they rarely exceed -016 in diameter, the 
great majority of them being very much smaller. It is from the innermost surface of 
this cylinder that the vascular bundles are given off to the leaves, a point of importance 
in determining the homologous relationships of the various portions of the Lepidodendroid 
plants. The radiating arrangement of these vessels suggests, as the quotation already 
* All these dimensions refer to decimal divisions of an inch. 
2 D 2 
