356 
PROFESSOR W. WILLIAMSON ON THE ORGANIZATION 
memoir on Sigillaria spinulosa .* In tlie case of the latter plant, the fragments of 
the bark, with its peculiar network of prosenchymatous structures, attracted the 
notice of the late M. Brongniart, who gave to it the provisional name of Dictyoxylon, 
a name which I had previously given to what is now the Lyginodendron Oldhamium. 
M. Renault, however, successfidly demonstrated ( loc . cit.., Plate IV., fig. 24) that 
Brongniart’s Dictyoxylon was merely part of the bark of his Sigillaria spinulosa. 
The elaborate and well -illustrated description of the structure of the bark of this 
plant by M. Renault shows that in all its essential features it agrees thoroughly 
with that which I now describe. All these facts tend to justify my conclusion that 
the latter is Sigillanan, and it would further appear probable that the peculiar 
network formed by the peripheral portion of the prismatic layer (which layer 
M. Renault designates the corky layer, or “ couche subereuse ’) are characteristic 
of most of the Sigillarice. As shown in some of my previous memoirs, the bark of 
Lepidodendron exhibits, even in its young state, the same subdivision (Phil. Trans., 
1877, Plate 41, figs. 6 and 7) into the three layers found in Sigillaria, viz., an 
inner parenchyma, a median prosenchymatous layer, and a second external paren- 
chyma. But I have not yet found in any Lepidodendron the peculiar reticulated 
arrangements of the prosenchyma which I have just described. But in this latter 
respect the Lepidodendra agree with the Sigillaria elegans ol Brongniart.! The 
physiological conclusions which these studies justify are, I think, obvious; viz., that 
there are two vertical zones in the bark of these plants in which active meristem 
growth has gone on. In other words, there were two planes of genetic activity. 
One of these was at the junction of the vascular zone and the innermost bark, 
representing the true cambium of ordinary exogens, adding periodically, though 
perhaps irregularly, to the thickness of the vascular zone, and possibly also, though 
on this point I am more doubtful, making additions to the inner surface of the inner 
cortical parenchyma. The other genetic plane was a peripheral one, multiplying the 
cells of the superficial parenchyma, and also making additions to the exterior of the 
prosenchymatous analogue of the corky layer of living exogens. 
The prevalence of this peculiar regular form of prismatic cellular tissue amongst 
these carboniferous cryptogams is a curious fact. We have seen that it prevails alike 
in the bark of the matured Calamite, and that it forms the medullary rays of the 
same plant. (See my memoirs, Part I., Tab. 23, fig. 8; Tab. 24, fig. 11.) We 
find it in Favularia (Memoirs, Part II., Plate 27, fig. 29), in Lepidodendron Harcourtii 
(idem, Plate 26, fig. 13, i), in Syringodendron (idem, Plate 29, fig. 42, i), in Stigmaria 
(idem, Plate 30, fig. 41, h ; Plate 31, figs. 55-56), in the Diploxyloid Lepidodendron of 
Burntisland (Memoirs, Part IIP, Plate 43, fig. 17), and in Asterophyllites (Memoirs, 
Part V., Plate 1, fig. 7). I have not met with its long continuous lines of cells in any 
of the ferns. The tact that it constitutes a very peculiar form of medullary ray in the 
* ‘ Memoires presen tes par divers savants a l’Academie des Sciences de l’lnstitnt National de France,’ 
tom. 22, No. 9. t ‘ Archives du Museum,’ 1839. 
