OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
467 
teristic of the Halonice are sufficiently conspicuous, and in the sections we can readily 
distinguish between the vascular bundles going off to these protuberances, and those 
supplying the leaves. Three of the former are seen at /,/,/. They have a mean 
diameter of about '004. In the case of the Halonia previously described, the corre¬ 
sponding bundle was formed by the detachment of a small but complete segment of 
the vascular cylinder leaving a break in the continuity of the latter. In the present 
case the bundle is formed in the way represented in the three transverse sections, 
figs. 22, 23, and 24. In all these sections, a represents a portion of the solid vascular 
axis of the branch. In fig. 22 two indentations at b, b are separating the clusters of 
vessels, / from the main bundle. In fig. 23 the bundle, / is almost entirely separated, 
whilst in fig. 24 it is completely detached from the axial bundle, and is moving out¬ 
wards through the middle bark as in fig. 21,// These bundles are distinguished 
from the foliar bundles by their size ; fig. 25, a, represents a portion of the cylinder 
giving off a foliar bundle, b. 
This section, drawn to the same scale as the other three, exhibits the relative sizes 
of these two classes of bundles. We further see that many of the larger ones pro¬ 
ceeding to the tubercles are surrounded by small foliar bundles of their own— 
destined, doubtless, to supply the leaves clothing these prominences characterising the 
Halonia. 
This hitherto undescribed form of Halonia raises anew the question whether or not 
Plate 10, figs. 1 and 2, of my Part X. is a younger state of the plant represented in 
figs. 3 and 4 of the same plate. The two are identical in every feature other than the 
structure of these axial bundles, which is solid in the one and a hollow cylinder 
enclosing medullary cells in the other. The present position of the question is as 
follows :— 
As stated in my previous memoir, the central axis of each of the very young 
Lepidodendroid twigs, so abundant in the Laggan Bay deposit, consists of a solid 
vascular bundle having a diameter of '012. The similar solid axis of the new Halonia, 
belonging to a branch of larger size, has a diameter of T4. I have recently obtained 
from the same locality an ordinary Lepidodendroid branch of about the same dimen¬ 
sions as the Halonia, of which the axial bundle is also a solid one, with a diameter of 
'14. This is the largest example I have met with having a solid bundle. On the 
other hand, the smallest of the many Lepidodendroid branches and stems from Arran 
which my cabinet contains, possessing a hollow cylindrical vascular axis, is that 
figured in my memoir, Part X., figs. 3 and 4. In that specimen the vascular zone 
surrounding the central medulla has a diameter of '2, from which figure all these 
cylinders increase steadily in diameter, in the number of their component vessels, and 
in the dimensions of the cellular medulla which they enclose, until we reach the largest 
axial bundles of the arborescent stems.* The fact that all the specimens hitherto 
found below a certain diameter have the solid vascular bundle whilst all above that 
* This increase seems to be partly explained by what I have said on p. 466 about fig. 20. 
