OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
225 
Ulodendron the innermost surface of the vascular ligneous cylinder is present, though 
very small compared with the large medullary one, approximating very closely, in this 
respect, to the Lepidodendron represented in Plate XXVI. fig. 13, Plate XXV. fig. 14. 
Not having a transverse section of the central axis of Favularia (Plate XXVIII. fig. 31), 
I cannot be certain about its details ; but we have in the longitudinal section evidence 
of a vascular mass, though whether it be medullary or ligneous I am not able to affirm ; 
but Brongniart’s Sigillaria elegans, which is a true Favularia , demonstrates the close 
resemblance which its central axis bears to that of a Diggloxylon. Wherever we are 
able to trace the origin of the vascular bundles going to the leaves, in Diploxylon, w 7 e 
I imagine, only be fonnd in a strobilus, which must be regarded as a branch that has undergone an arrested 
development at a very early stage of its growth. 
Guided by these new observations, I have reexamined the curious specimen found by the late Mr. James 
Wilde and referred to on p. 222. This is a semidecorticated branch of an ordinary Lepiclodendron, having a 
diameter, as it appears in its stony matrix, of about 2| inches. This stem divides into two smaller branches, 
one of which is also that of an ordinary Lepiclodendron ; the other displays the same Lepidodendroid features 
on its upper half, but what constituted its underside, when a growing plant, exhibits rows of the cha- 
racteristic tubercles of Hcdonia. We here learn two things : — First, that Halonia belongs to the upper branches 
of a Lepidodendroid tree, consequently it cannot be a root. This may be regarded as finally settled. The same 
truth is demonstrated by Mr. Whittaker’s specimens : in these the large vascular bundle going to each 
tubercle bends upwards and outwards in the same way as the foliar bundles with which it intermingles. This 
fact alone would be a conclusive one against the root hypothesis. Secondly, we learn that Halonia is a spe- 
cialized branch of cl Lepidodendroid tree that is not itself an Hcdonia ; and as I have already given reasons for 
believing that each tubercle sustained an abortive branch, it appears to me that we arc shut up to the conclusion 
that these arrested developments could only exist in the form of strobili. 
I think there can be little doubt that the innermost cortical layer, prolongations of which invest cell the 
vascular bundles proceeding from the medullary vascular sheath to the periphery, must be regarded as the 
homologue of what Sachs, following Nageli and Leitgeb, has termed the procambian layer in living Lycopods, 
and which, as we shall see, reappears in Stigmaria. 
The important truth demonstrated by the specimen in the Manchester Museum, and one with which all the 
other specimens that I have mentioned appear to harmonize, is, that the projecting tubercles of Halonia and 
Ulodendron were confined to the inner prosenchyma of the bark, of which they were conical extensions 
surrounding and accompanying a fibro-vascular bundle on its way outwards to the surface, but that they did not 
appear in any marked form, if at all, save as a scar, on the exterior of the plant. No such tubercular provision 
was made for the very numerous leaf-bundles, and we have abounding proofs that the tubercles had nothing 
to do with the ordinary branches of the plant. It appears to me that nothing remains with which we can 
associate them but strobili, and with these I believe them to have been connected. Every new fact that w r e 
discover appears to me to bring the two genera Hcdonia and Ulodendron into nearer relationship) than has hitherto 
been recognized. I have very little doubt that the Ilalonice were young branches sustaining rows of cones : 
after the cones fell off, they would leave permanent cicatriculac impressed upon the bark, and which would 
enlarge as the stems increased in magnitude, the latter process being probably accompanied by the development 
of an exogenous zone around the medullary cylinder. Specimens of these old and matured fruiting stems 
may exist among what we have hitherto termed Ulodendra. This explanation would give us the reason why we 
never find cones or other appendages of a magnitude corresponding with the cicatricula of Ulodendron. The 
chief argument against the idea that the eicatricuke of Ulodendron may be those of Halonice enlarged by age 
and growth, lies in the fact that the leaf-scars of Ulodendron do not appear to have undergone any corresponding 
enlargement. — April 15, 1872. 
