486 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
to node, at which points, in the type under consideration, their extremities appear to termi- 
nate as cul-de-sacs. Their inner or medullary wall is almost invariably composed of very 
narrow elongated cells (fig. 11, b) with square extremities, being in fact elongated modi- 
fications of the ordinary medullary cells. I have not succeeded in discovering any cells 
in their peripheral wall ; this appears to be composed of the innermost and first formed 
barred vessels of the woody wedge (11, g), to which each canal belongs. Their diameter 
varies from -03 to T of an inch, their width being uniform throughout their entire 
length, except at their extremities, where they rapidly contract until they disappear. 
The Bark or Epidermis . — In a lecture delivered before the Royal Institution of Great 
Britain (April 16, 1869), Mr. Carrutiiers incidentally mentions that the Calamitean 
stem possessed “ a thin cortical layer.” I am not aware, however, that the true bark 
has hitherto been either figured or described. The discovery by Mr. Butterworth,' 
of the small stem represented in figures 9 & 10 removes much of the existing obscurity 
on this subject. Of all the scores of microscopic sections of stems of which I have 
either made or examined, it is remarkable that two examples alone exhibited evi- 
dences of the existence of a bark, all the rest having been decorticated. The same 
observation is applicable to those figured by Mr. Binney, in none of which is this tissue 
seen. These facts show that the cohesion between the bark and the woody zone must 
have been exceedingly slight, at least in the small fragments inclosed in the ironstone 
nodules from which the majority of our specimens retaining their structure have been 
derived. 
The entire diameter of the small stem referred to is - 2 of an inch. The thickness of 
the woody zone at the internode is about ‘05. At its narrowest part the thickness of 
the bark is -025, whilst it becomes very much thicker at the nodes. It has in fact been 
a rather thick parenchymatous layer, in which the cells were very irregular both in size 
and distribution. Some few cells of large size appear dispersed amongst others of smaller 
dimensions. There is no trace of linear or other special arrangement of these cells, 
either in the transverse (fig. 9) or vertical (fig. 10) sections. Where the bark crosses the 
node, in the latter section, its surface has become somewhat disintegrated ; but though 
Mr. Butterworth and myself have prepared several sections of the specimen, I have 
not been able to discover the slightest trace of vascular bundles, or even of solitary 
vessels crossing the cellular parenchyma. The peripheral outline of the vascular woody 
zone at this point is sharp and well defined, and the course of the vessels undisturbed *. 
The only difference seen between the longitudinal, tangential, and transverse sections, 
is a slight vertical elongation of the cells in the two former. The cells of the outer- 
most surface differ little if at all from those of the interior, and I detect no trace of a 
special cuticular layer. It exhibits no indication of being an exogenous bark, no trace 
of the tripartite division seen in that of the gymnospermous Conifera existing in it. 
* Unfortunately the arching vessels of the node exhibit no trace of a branch crossing them at this point, as 
is frequently the case with parallel sections of other specimens. Consequently we miss a valuable opportunity 
of seeing the relations of these branches to the bark. 
