Another indication, equally
significant, is found in the tubular structure of the wood in Ferns. On
a vertical section of any well-preserved Fern-trunk from the old forests
the little tubes may be seen very distinctly running up its length; or,
if it be cut through transversely, they may be traced by the little
pores like dots on the surface. Trees of this description are found in
the Carboniferous marshes, standing erect and perfectly preserved, with
trunks a foot and a half in diameter, rising to a height of many feet.
Plants so strongly bituminous as the Ferns, when they equalled in size
many of our present forest-trees, naturally made coal deposits of the
most combustible quality. It is true that we find the anthracite coal of
the same period with comparatively little bituminous matter; but this is
where the bitumen has been destroyed by the action of the internal heat
of the earth.
Next to the Ferns, the Club-Mosses (_Lycopodiacae_) seem to have
contributed most largely to the marsh-forests. They were characterized,
then, as now, by the small size of the leaves growing close against the
stem, so that the stem itself, though covered with leaves, looks
almost naked, like the stem of the Cactus. Beside these, there are the
tree-like Equiseta, in which we find the articulations on the trunk
corresponding exactly to those now so characteristic of those
marsh-grasses which are the modern representatives of this family of
plants, with cone-like fructifications on the summit of the stem.
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