OF THE FOSSIL PLANTS OF THE COAL-MEASUBES. 
403 
tinuously, in a quoit-like form, across the twig. The prosenchynra, 1c, is in all respects, 
except its thinner dimensions, like that already described. 
Whatever these twigs may be, homologically, I have no doubt that they are the free 
prolongations of the lateral diverticula from the parent axis, described in pages 400 & 401. 
There can, I think, be little question that they are petioles rather than branches; 
but as yet I have not succeeded in connecting them with any thing resembling leaves. 
They bear so close a general resemblance to corresponding sections of the petioles of 
the Lepidodendra as to leave little room for doubting their petiolar character. If this 
be a correct interpretation, it follows that Dictyoxylon Grievii has been a stem giving off 
large foliar appendages, at somewhat distant intervals and from its entire circumference. 
That these appendages have been of considerable dimensions relatively to the diameter 
of the main stem, is indicated by the large size of each of the petiolar bundles of vessels. 
Whether it was an erect stem or a creeping rhizome is doubtful ; but the circumstance 
that the exogenous zone is always so much thicker on one side than on the other, sug- 
gests the possibility of an excess of vital action on that side, due to the stimulus of 
light upon the upper surface of a creeping stem. Supposing this inference to be cor- 
rect, the question recurs, What is the large divergent vascular bundle of fig. 36 ? Most 
probably the vascular bundle of a large adventitious root rather than a true branch. It 
bears a most striking resemblance to the bundles seen in corresponding sections of recent 
Lycopodiaceee, which supply the roots or root-bearers; only in the latter case the 
vessels originate in the vascular bundles of the main axis. It bears a still closer 
resemblance to that of I). Oldhomium , seen in Plate XXIII. fig. 7, n. In both plants the 
reticulated vasculo-cellular mass appears to be projected from cellular tissues; only in 
I). Oldhamium the intervention of a thick ligneous zone shuts out the root-like organ from 
direct association with the pith, and compels it to spring from it indirectly through the 
cells of the medullary rays, whilst in D. Grievii it starts directly from the axial 
medullary cells. 
The supposed petioles just described, whether we regard their structure or their probable 
origin as illustrated by Plate XXX. figs. 37-44, help to throw some light upon the 
very similar portion of Dictyoxylon Oldhamium represented in Plate XXVI. fig. 25. The 
latter bears strong indications of having been one of the double vascular bundles seen 
in the inner and middle layers of the bark, but which is here moving outwards to form, 
with its surrounding parenchyma, the base of a petiole. But if this has been the case, 
why are such examples so seldom met with 1 and where are the petioles which, considering 
how abundant Dictyoxylon Oldhamium is, cannot have been rare objects'? 
. In the c Proceedings of the Eoyal Society ’ (xx. p. 438) I have pointed out the existence, 
in the Lancashire nodules, of abundance of small stems or petioles, to which I gave the 
provisional name of Pdraxylon. I have since succeeded in connecting these petioles 
with the leaflets of a Pecopteris. I think it far from impossible that these may prove 
to belong to Dictyoxylon Oldhamium ; but since I have not yet succeeded in correlating 
them with any certainty, I shall add no more respecting them at present ; the details 
3 H 2 
