198 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
vascula,r zone appear at h, whilst the inner cortex, c, of fig. 12 is now seen to consist of 
vertically elongated cells, amongst which those with rectangular and oblique septa 
are intermingled, whilst the thin-walled innermost ones, c, present an obvious 
procam bial aspect. 
In fig. 14 we find the medullary vascular cylinder dividing in the regularly 
dichotomous manner seen among the larger forms of Lepidodendra. In fig. 15, on 
Plate 5, we have a section of my only example of this species in which the medullary 
vascular cylinder, a, b, is invested by a secondary, exogenously-developed zone, d. 
The plant just described again illustrates the gradual development of a medulla 
within the interior of a vascular bundle where, in the youngest state of the bundle, 
no traces of cellular structure could be discovered, but the germs of which must 
necessarily have been furnished by the procambium from which, in the youngest 
twigs, the entire bundle originated. The probable philosophy of these facts may be 
considered after describing some additional undescribed types. 
Figs. 16, 17, and 18 represent sections of a Lepidodtndron from Halifax, for which 
I propose the name of L. intermedium., and which is remarkably distinct from any 
other form with which I am acquainted. Its vasculo-medullary axis, a, h, differs 
from all other known Lepidodendra, with one exception. The true medulla, a, 
consists of well-defined parenchymatous cells. The medullary vascular cylinder, h, is 
composed of numerous large barred tracbeids. The external boundary of this cylinder 
is sharply and regularly defined, but its inner border is very irregular, some of its 
largest vessels being detached from it and isolated amongst even the most central 
cells of the medulla.* In addition to this characteristic feature, we have an exogenous 
zone, d, which is equally characteristic. The entire thickness of this zone, from its 
medullary to its cortical boundary, is only about the one-hundredth of an inch ; yet 
many of its radial lines of tracheids consist of twenty-four distinct vessels. Measured 
separately, we find these tracheids ranging between the thirteen-hundredth and 
the twenty-six-hundredth of an inch in diameter. On examining these vessels 
under high powers, as they appear in tangential sections (fig. 17), we find them, 
d', ascending and descending in extremely tortuous courses. They are here largely 
intermingled with cells, d", d", the diameters of which are equal to those of the 
vessels. These cells unmistakably correspond to those which constitute the medullary 
rays of the larger forms of Lepidodendra. (See ‘Phil. Trans.,’ 1872; Memoir, 
Part III., Tab. 42, fig. 13,/!) Fig. 18 is a vertical section made through the centre of 
the stem. At a we have the large vessels of the medullary vascular cylinder, inter¬ 
mingled irregularly at certain points with the medullary cells, as shown in fig. 16. 
At d we have the exogenous layer on one side of the central medulla and its vascular 
cylinder. In this zone we find the vessels of minute sizes, and again copiously inter¬ 
mingled with cells, many of which are arranged in radial mural lines like medullary 
* In this respect the plant resembles Lepidodendron selaginoides ; but it differs in the entire absence 
from its medulla of the barred medullary cells so characteristic of the latter type. 
