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.