OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
385 
the radiating laminae have a diameter varying from -0023 to ‘0006, their more usual 
dimensions being from -0012 to -0008. It is very difficult to measure their length, 
because, owing to its amount, which is considerable, and to their interlacing freely within 
their respective bundles, it is almost impossible to see the two extremities of each fibre ; 
it is, however, sufficiently great to give them an almost vascular character. Practically 
they are bast-fibres. When we do see their extremities, as in Plate XXIV. fig. 15, 7c', we 
observe distinctly the fusiform terminations of the cells ; but where they come in 
contact with the parenchyma enclosed within the areolse, we sometimes notice a very 
rapid transition from the one type to the other, through some elongated square-ended 
modifications of cellular tissue. 
I have referred to the cellular tissue filling up the lenticular or rhomboidal areolse as 
parenchymatous, but it varies immensely in the same plant. Thus in the transverse 
section (Plate XXII. fig. 1) we find these spaces occupied throughout the greater part 
of the section by ordinary parenchyma, as seen in Plate XXIV. fig. 10, 7c" ; but on the 
extreme right of fig. 1 they are filled with transversely elongated prismatic cells, which 
often almost extend from one fibrous lamina to another, their long axes being parallel 
with the circumference of the bark. We see similar differences in the tangential sections : 
thus, in Plate XXIV. fig. 15, 7c", these cells are parenchymatous, whereas in the greater 
part of fig. 13, 7c", & Plate XXV. fig. 14, 7c", they are prismatic, or even mural, their long 
axes being transverse to that of the stem. In the radial vertical section (Plate XXIV. 
fig. 11 ,7(7) we see the prosenchymatous layer forming an irregular wavy line. The cells 
occupying the areolse vary much in their dimensions ; but when they assume the paren- 
chymatous form they have a somewhat larger diameter than characterizes the contiguous 
fibres, or from ‘0033 to '0015. The internal cavities of the fibres are usually more or 
less filled with carbonaceous matter (Plate XXIV. fig. 10, 7c'), their ligneous walls having 
evidently possessed some peculiar osmotic properties not possessed by the ordinary cellu- 
lose of the parenchyma. 
In the large majority of my specimens I detect no tissue external to the remarkable 
one just described ; but in some young stems, more perfect than the rest, the parenchyma 
of th,e rhomboidal areolse spreads out into a thin parenchymatous subepidermal layer 
(Plate XXII. fig. 1, l, & Plate XXIV. fig. 10, /), at the outer surface of which we often 
see small irregular projections. Nothing approaching a true epiderm has yet been 
found*. The thin layer just named seems to have disappeared during growth, as if it 
were an epiderm and was not renewed. 
In no instance has a trace of a foliar vascular bundle, like those of the Lepidodendra, 
been found in the cellular areola? of the bark. This fact alone demonstrates one differ- 
ence between these rhomboidal areolse and the leaf-scars of Lepidodendron and Sigillaria ; 
* At the Meeting of the British Association in 1871, Mr. Bixney stated that he had found a yet more peri- 
pheral layer. I have examined some hundreds of these stems, but have seen no trace of any thing like what he 
there described. 
3 F 
MDCCCLXXIII. 
