OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
497 
to, three-fourths the natural size. At d we have the medulla; the inner, non-exogenous 
ring is seen at a; and at i we have the, now very much thickened, vascular exogenous 
zone. It is unnecessary to enter into a detailed description of these structures, since 
they correspond in the closest manner with similar ones described in my previous 
memoirs—especially with those of the Burntisland plants. I would only remark that 
we notice in their inner or medullary vascular cylinder the same remarkable proof of 
the increase which has taken place in the number of the medullary vessels, a, as the 
stem increased in size, as I have already referred to in previous memoirs. I can only 
conclude that these additions to this vascular zone must have been made by a con¬ 
version into vessels of the cells of the central or medullary parenchyma by a centri¬ 
petal process of development. If this has been the case, this centripetal growth of the 
medullary vascular ring affords another feature of resemblance to the vascular bundles of 
the living Lycopods, of which I believe it to be the true homologue. Fig. 6a represents 
a portion of a photograph of a section similar, but further enlarged, to fig. 6, for which 
I am indebted to Mr. Wunsch. It shows the exact relations of size and number of 
vessels which the medullary and exogenous zones bear to each other. All the vessels 
in these stems exhibit with great distinctness the innumerable vertical parallel lines of 
lignine (fig. 4*, b), connecting the transverse bars (fig. 4"% a) which I described and 
figured in my memoir, “ On the Structure and Affinities of some Exogenous Stems 
from the Coal Measures,” (‘Monthly Micros. Journal,’ Aug. 1, 1869, p. 71, plate 20, 
fig. 10). Before examining the condition in which the bark of the large Arran stems 
is found, we may advantageously recall some observations made by Dr. Dawson, of 
Montreal. In his ‘Report on the Fossil Plants of the Lower Carboniferous and Mill¬ 
stone Grit Formations of Canada,’ that observer remarks : “In older stems (of Lepido- 
dendron) three modes of growth are observed. In some species the expansion of the 
bark obliterates the leaf-bases and causes the leaf-scars to appear separated by wide 
spaces of more or less wrinkled bark, which at length becomes longitudinally furrowed 
and simulates the ribbed character of Sigillaria This description accurately repre¬ 
sents the condition of the exteriors of the larger Arran stems. All that remains of 
their bark is a cylinder, usually about two inches thick, which consists almost entirely 
of the prosenchymatous or bast-layer—the cells of which are always seen, in transverse 
sections, arranged in radiating lines, and which is interposed in younger stems between 
the coarse outer-bark parenchyma, fig. 5, g ,—and the somewhat similar outermost or 
sub-epidermal parenchyma. As I have already pointed out, the surface parenchyma 
has entirely disappeared from these Arran stems. Since the true leaf-bases and their 
subjacent leaf-scars belong solely to the outer surface of this parenchyma, it follows 
that when it was thrown off through the pressure of internal growth, combined with 
atmospheric agencies, all definite traces of the outlines of these leaf-scars would dis¬ 
appear. Such a change brings the bast-layer to the surface where it forms a protective 
or “ healing ” tissue corresponding teleologically to the corky layer of ordinary exogens. 
* Loc. cit., p. 41, 
