OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
355 
shorter axes are radial. In a word, these cells stand upon their thin bevelled edges, 
with their two flat parallel surfaces severally directed towards the medulla and the 
periphery of the bark. It is also clear that they are grouped into lenticular, vertically 
extended, wedge-shaped masses, whose thin edges project inwards between the rows 
of prismatic cells. 
But fig. 101, which is a portion of fig. 95, enlarged 16 diameters, reveals some 
further peculiarities. The prismatic cells, a , a, now exhibit their somewhat prosen- 
chymatous aspect, whilst, in the parenchyma, the darker outlines, c, obviously indicate 
the boundaries of the tabular cells seen in figs. 93 and 94, c ; but we now discover that 
these cells are in the condition of extremely active meristem, most of them containing 
secondary subdivisions, their thin secondary cell walls, c', intersecting the cavities 
of the primary cells, c, in every direction. It is thus clear that the network ol 
prismatic or prosen chymatous cells, a, a , a", represents permanent, or formed tissue, 
whilst the cellular masses, c, which that network encloses, have been in an active 
state of cell multiplication ; the same activity also appearing, though less con- 
spicuously, in the peripheral parenchyma, b, of the bark. 
I can arrive at no other conclusion than that we have here a sub-epidermal plane of 
parenchymatous tissue in a state of genetic activity, and that it is from this growing 
tissue that the additions have been made to the exterior of the prismatic tissue, a, 
which is thus developed into a thick modification of corky structure, only unlike the 
phellem-layers of living exogens, growing by additions to its outer side instead of 
being produced by a more internal “phellogen,” or cork-cambium. Whilst func- 
tionally this prismatic tissue appears to have been like the phellem layer of living 
exogens, it is impossible to overlook the morphological resemblance between the 
tangential sections (fig. 95) of this tissue, and corresponding sections of the phellem 
layer of the bark of many exogens, in which the bast-layers form a similar network to 
that formed by the prismatic cells, a, of the above figure. 
In my memoir, Part II., I have already described (p. 220) and figured (Plate 31, 
figs. 54-57) a small fragment of bark similar to that now described, but the specimen 
I then possessed left me in doubt as to which was the medullary and which the 
peripheral side of the example figured. This point now is no longer doubtful. 
The next question is — to what plant does this bark belong ? I think the answer 
must be, to Sigillnria. The prismatic or prosenchymatous layer of the bark is as 
I have shown in previous memoirs, common to Sigillaria and Lepidodenclron, in 
a smaller degree to Stigmaria, and, as I have shown in the earlier part of this 
memoir, to Calamites. It is obviously a protective analogue of the corky layer of 
living exogens. But the characteristic feature of the example now described is found 
in the peculiar arrangement represented in fig. 95. In my previous memoir ( loc . cit.) 
I have pointed out that a similar condition exists in association with a Diploxyloid 
stem now in my cabinet. I find it again in a modified shape in Syringodendroid 
Sigillarice, and it has been described by M. Renault, in a most marked form, in his 
