Now this little Selaginella is of all living plants
the one most like some of the gigantic trees of the
coal-forests. If you look at this picture of
a coal-forest (Fig. 51), you will find it difficult
perhaps to believe that those great trees, with diamond
markings all up the trunk, hanging over from the right
to the left of the picture, and covering all the top
with their boughs, could be in any way relations of
the little Selaginella; yet we find branches of them
in the beds above the coal, bearing cones larger but
just like Selaginella cones; and what is most curious,
the spores in these cones are of exactly the same kind
and not any larger than those of the club-mosses.
These trees are called by botanists Lepidodendrons,
or scaly trees; there are numbers of them in all coal-mines,
and one trunk has been found 49 feet long. Their
branches were divided in a curious forked manner and
bore cones at the ends. The spores which fell
from these cones are found flattened in the coal, and
they may be seen scattered about in the coal-ball.
Week 23
Another famous tree which grew in the coal-forests
was the one whose roots we found in the floor or underclay
of the coal. It has been called Sigillaria,
because it has marks like seals (sigillum, a seal)
all up the trunk, due to the scars left by the leaves
when they fell from the tree. You will see the
Sigillarias on the left-hand side of the coal-forest
picture, having those curious tufts of leaves springing
out of them at the top. Their stems make up
a great deal of the coal, and the bark of their trunks
is often found in the clays above, squeezed flat in
lengths of 30, 60, or 70 feet. Sometimes, instead
of being flat the bark is still in the shape of a
trunk, and the interior is filled with sane; and then
the trunk is very heavy, and if the miners do not
prop the roof up well it falls down and kills those
beneath it. Stigmaria is the root of the Sigillaria,
and is found in the clays below the coal. Botanists
are not yet quite certain about the seed-cases of
this tree, but Mr. Carruthers believes that they grew
inside the base of the leaves, as they do in the quillwort,
a small plant which grows at the bottom of our mountain
lakes.
But what is that curious reed-like stem we found in
the piece of shale (see Fig. 47)? That stem
is very important, for it belonged to a plant called
a Calamite, which, as we shall see presently, helped
to sift the earth away from the coal and keep it pure.
This plant was a near relation of the “horsetail,”
or Equisetum, which grows in our marshes; only, just
as in the case of the other trees, it was enormously
larger, being often 20 feet high, whereas the little
Equisetum, Fig. 52, is seldom more than a foot, and
never more than 4 feet high in England, though in
tropical South America they are much higher.
Still, if you have ever gathered “horsetails,”
you will see at once that those trees in the foreground
of the picture (Fig. 51), with leaves arranged in
stars round the branches, are only larger copies of
the little marsh-plants; and the seed-vessels of the
two plants are almost exactly the same.
