382 
PBOFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
cells of which exhibit the aspect of ordinary parenchyma. On turning to radial sections 
(Plate XXIII. fig. 9), we see that these rays (f) present as perfect an example of mural 
arrangement as could be found in any Dicotyledonous plant. In whatsoever direction 
we make sections of the woody zone of this plant, we equally obtain clear proof that 
the cellular tissues of the medullary rays constitute fully half of its substance. Occa- 
sionally, as at Plate XXIII. fig. 8,/", we find individual rays consisting of but three or 
four cells, but such restricted examples are rare. 
The Bark . — This consists of three very conspicuously distinct layers — an inner 
parenchyma, a prosenchymatous layer, and an outer parenchyma ; but practically four 
such may be distinguished, since the surface of the innermost one of the three, which is 
in immediate contact with the wood, becomes differentiated into a cambium-layer. 
Inner Parenchyma . — This innermost division is not always to be found in a distinctly 
marked form. When present, it consists of a uniform series of exceedingly delicate cells 
(Plate XXII. fig. 1, g, & Plate XXIII. fig. 6, g), which have a somewhat constant diameter 
of from *001 to ‘002. In the transverse section they present the ordinary parenchymatous 
form; in the longitudinal one they exhibit a tendency to arrange themselves in vertical lines. 
At its outer boundary this stratum merges in an almost equally delicate parenchyma, 
hut composed of somewhat larger cells (Plate XXII. fig. 1, h, Plate XXIII. fig. 7 , h, Sc 
Plate XXIV. figs. 10 ,/j, & 11, h). Both the inner and outer portions of this double paren- 
chymatous layer are characterized by the presence of large, irregularly star-shaped patches 
(Plate XXIV. fig. 11, h!) of cells of a dark colour. These might at first be supposed to 
be accidental discolorations due to local mineralization; but I am satisfied that such is not 
the case. These cells are rather larger and have denser walls than those amongst 
which they are imbedded ; and they form so constant an element in this portion of the 
plant, that I can have little doubt as to their having constituted a feature of the living 
structure, resembling possibly the dark-coloured cells so common in the rootlets of Ferns 
and of some Lycopodiacece. Those in the inner parenchyma are much larger than 
those of the outer parenchyma, the latter often consisting of but one or two well- 
marked dark-coloured cells. In studying the Dictyoxylons, these pigment-patches 
supply an excellent guide, enabling us to distinguish between the parenchyma of the 
inner bark and that of the medullary rays*. In the fossil specimens this cortical 
parenchyma frequently appears as if trailed about in a remarkably irregular manner; 
but this is probably a result of partial desiccation prior to fossilization. 
*■ Whilst describing this part of the bark, a specimen now under my microscope reminds me once more of 
the necessity of guarding against the misleading influence of Stigmarian rootlets, the great enemies of the 
young palaeophytologist who studies the organization of these coal-plants. In the specimen referred to, a long 
narrow strip of clearly defined cellular tissue of peculiar aspect runs longitudinally through the portion of the 
bark just described, imbedded amongst the true cortical tissues in a way that would inevitably mislead an 
observer whose eye was not thoroughly familiar with the aspect of these rootlets. In a previous memoir I have 
called attention to the universal diffusion and marvellous penetrating power of these intruders ; but I think the 
latter attribute has reached its acme in one specimen which I have recently met with, in which a large rootlet 
has been penetrated longitudinally by a smaller one ; and the latter, in its turn, contains a third within its 
