290 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
zone. At c", c" we find two foliar-vascular bundles, belonging: to an outer series corre- 
sponding to c" of fig. 10, whilst in the curved line between these two bundles we have 
a portion of the exogenous zone, composed of numerous radiating wedges, li, of barred 
vessels. At h', h', where these wedges are deflected somewhat from their direct course, 
their true vascular character is demonstrated. I find this exogenous zone in so many 
specimens that have attained the right stage of growth as to leave no doubt whatever 
as to its being a normal feature of the plant. Whether or not we shall hereafter 
discover it developed into the more perfect Diploxyloid condition seen in other 
Lepidodendroids remains to be seen. We were long before we found L. selaginoides 
so developed ; meanwhile its presence in L. Uarcourtii , even in this rudimentary form, 
directly links that plant with the Diploxyloid or Sigillarian group of stems. In many 
of my specimens some large isolated cells, fig. 11,6, appear in the zone b". In the speci- 
*men from which fig. 11 was taken these are larger and more conspicuous than usual. 
In fig. 9, a", a considerable segment of the vascular medullary cylinder is detached 
from the rest, leaving a hiatus in the cylinder, and carrying along with it a semi¬ 
circular loop of the innermost bark, b', with its associated foliar vascular bundles. 
In my memoir, Part II., p. 224, I called attention to the fact that the large vascular 
bundles separated from the main cylinder to supply the tubercles of the Halonice were 
given off in the same way, and the bundle a" has doubtless had some similar destina¬ 
tion. Such a division of the primary vascular cylinder differs only in amount from 
the dichotomous subdivision of a branch illustrated in fig. 9. In another branch 
of the same stem, contained in the nodule in which the example fig. 9 occurred, 
I found a similar bundle, of which a representation is given in fig. 12, enlarged 
21 diameters. In the branch bundle of fig. 9, the central vasculo-cellular mass, a", 
retains the concavo-convex form belonging to it as a small segment of a circle, and the 
remark applies also to the dark-coloured layer of inner bark, b', which surrounds its 
convex peripheral border ; but in fig. 12, the bundle a has assumed a more cylindrical 
contour; the dark inner bark, b', now completely invests the bundle in the form of a 
six-sided cylinder, whilst the contiguous cells, cl, of the middle bark have re-arranged 
themselves in almost concentric lines around the developing axis. On further exami¬ 
ning the central bundle we find that what in fig. 9, a', was merely a small concavo- 
convex segment of a large circle, having some cells in the concavity of its inner border, 
has also changed its form. The extremities of the vascular segment have been bent 
inwards, convening the segment into a circular cylinder, ft', which has enclosed the 
cells, a , that primarily only lay on one side of it.* I am able to demonstrate that 
this branch, fig. 12, is merely a further development of a structure like fig. 9, a", b'. 
Mr. A it ken kindly allowed me to make a vertical section through the specimen from 
which the transverse section, fig. 12, was taken. This not only enabled me to trace the 
bundle downwards into the condition represented in fig. 9, but to a yet lower position 
* The gap left in the vascular cylinder of the parent root stem closes in the same way by the conver¬ 
gence of the separated extremities of the broken circle. 
