308 
PROEESSOK W. C. WILLIAMSON ON THE ORGANIZATION 
There yet remains to be considered the relations which subsist between the respective 
cortical layers of the extinct and living Lycopodiacese. 
In the former we have substantially three layers — an inner one of parenchyma com- 
posed of cells having a tendency to become arranged in vertical lines, an intermediate 
layer of prosenchyma, in which, in old stems, a peculiar, vertically elongated tissue tends 
to develop itself, and an outer parenchyma of the ordinary type, and which also con- 
stitutes the principal tissue of the leaves. If we combine what we find in the cortical 
investments of the recent Lycopods Selaginella Martensii and Lycopodium chamcecy- 
parissus, we shall be furnished with all that we require to illustrate the identity between 
these tissues in the living and the extinct forms. In Selaginella Martensii we have an 
inner layer of parenchyma enclosed in an outer one of prosenchyma, which latter becomes 
more compact, in consequence of the increasing thickness of the ligneous deposit within 
its cell- walls, as we proceed from within outwards. No true epiderm invests the stem. 
In Lycopodium chamcecyparissus, on the other hand, we have no inner parenchyma, but 
the prosenchymatous layer, very much thickened*, closely invests the central vascular 
axis. External to this prosenchyma we have a distinct parenchymatous layer, which 
Sachs describes as being an extension of that composing the leaves. Thus these two living 
plants combine to furnish us with the three layers of bark found in the fossil ones. It 
is interesting to remember that in one of the fossil Lepidodendroid stems from Lancashire 
and Yorkshire described in my last memoir, I found the very thick prosenchymatous 
layer apparently in close contact with the vascular tissues, as in Lycopodium chamcecy- 
parissus. It will be noted that no true epiderm invests the stems of either of these 
recent species, but it exists in the leaves in a well-defined form and with the usual 
stomata. In that position it rests immediately upon the foliar parenchyma, which, as we 
have seen, extends over the entire stem of L. chamcecyparissus , as it does over the fossil 
stems. Hence in the latter I have designated this superficial parenchyma the subepi- 
dermal layer, though I have seen no trace of true epidermis investing it ; but this term 
assists us in maintaining correct relationships between the nomenclature of the recent 
and fossil types. 
I have hitherto said nothing about the probable roots of the plant described in this 
memoir ; but since the Burntisland beds are permeated in every direction by Stigmarian 
rootlets, specimens of the thick roots also being far from rare, I have come to the con- 
clusion that they belonged to the same plant as the Lepidodendroid stems and branches. 
I am the more inclined to adopt this conclusion from the circumstance that I have not 
yet seen in this deposit a single fragment of a true Sigillaria to which these numerous 
roots could have belonged. Mr. Binney has more than once affirmed the probability 
that Lepidodendron had a Stigmarian root, which opinion I fully endorse. 
Having satisfied myself of the soundness of these conclusions, I venture to suggest that 
Plate XLY. fig. 37 may be regarded as a diagrammatic representation of a vertical section 
of a typical Lepidodendroid tree, drawn in accordance with the various details described 
* Sachs, ‘ Lehrbuch,’ fig. 89 B. 
