The Story of a Piece of Coal | Page 4

the more possible since the fossilised impressions had been retained in so distinct a manner. Here before us is a specimen in a shale of _pecopteris_, as it is called, (_pekos_, a comb). The leaf in some species is not altogether unlike the well-known living fern osmunda. The position of the pinnules on both sides of the central stalk are seen in the fossil to be shaped something like a comb, or a saw, whilst up the centre of each pinnule the vein is as prominent and noticeable as if the fern were but yesterday waving gracefully in the air, and but to-day imbedded in its shaly bed.
[Illustration: FIG. 5.--Pecopteris Serlii. Coal-shale.]
_Sphenopteris_, or "wedge-fern," is the name applied to another coal-fern; _glossopteris_, or "tongue-leaf"; _cyclopteris_, or "round-leaf"; _odonlopteris,_ or "tooth-leaf," and many others, show their chief characteristics in the names which they individually bear. Alethopteris appears to have been the common brake of the coal-period, and in some respects resembles pecopteris.
[Illustration: Fig. 6.--_Sphenopteris Affinis._ Coal-shale.]
In some species of ferns so exact are the representations which they have impressed on the shale which contains them, that not only are the veins and nerves distinctly visible, but even the fructification still remains in the shape of the marks left by the so-called seeds on the backs of the leaves. Something more than a passing look at the coal specimens in a good museum will well repay the time so spent.
What are known as septarian nodules, or snake-stones, are, at certain places, common in the carboniferous strata. They are composed of layers of ironstone and sandstone which have segregated around some central object, such as a fern-leaf or a shell. When the leaf of a fern has been found to be the central object, it has been noticed that the leaf can sometimes be separated from the stone in the form of a carbonaceous film.
Experiments were made many years ago by M. Goppert to illustrate the process of fossilisation of ferns. Having placed some living ferns in a mass of clay and dried them, he exposed them to a red heat, and obtained thereby striking resemblances to fossil plants. According to the degree of heat to which they were subjected, the plants were found to be either brown, a shining black, or entirely lost. In the last mentioned case, only the impression remained, but the carbonaceous matter had gone to stain the surrounding clay black, thus indicating that the dark colour of the coal-shales is due to the carbon derived from the plants which they included.
Another very prominent member of the vegetation of the coal period, was that order of plants known as the Calamites. The generic distinctions between fossil and living ferns were so slight in many cases as to be almost indistinguishable. This resemblance between the ancient and the modern is not found so apparent in other plants. The Calamites of the coal-measures bore indeed a very striking resemblance, and were closely related, to our modern horse-tails, as the equiseta are popularly called; but in some respects they differed considerably.
Most people are acquainted with the horse-tail (_equisetum fluviatile)_ of our marshes and ditches. It is a somewhat graceful plant, and stands erect with a jointed stem. The foliage is arranged in whorls around the joints, and, unlike its fossil representatives, its joints are protected by striated sheaths. The stem of the largest living species rarely exceeds half-an-inch in diameter, whilst that of the calamite attained a thickness of five inches. But the great point which is noticeable in the fossil calamites and equisetites is that they grew to a far greater height than any similar plant now living, sometimes being as much as eight feet high. In the nature of their stems, too, they exhibited a more highly organised arrangement than their living representatives, having, according to Dr Williamson, a "fistular pith, an exogenous woody stem, and a thick smooth bark." The bark having almost al ways disappeared has left the fluted stem known to us as the calamite. The foliage consisted of whorls of long narrow leaves, which differed only from the fern asterophyllites in the fact that they were single-nerved. Sir William Dawson assigns the calamites to four sub-types: calamite proper, _calamopitus, calamodendron_, and eucalamodendron.
[Image: FIG. 7.--Root of Catamites Suckowii. Coal-shale.]
[Image: FIG 8.--Calamocladus grandis. Carboniferous sandstone.]
Having used the word "exogenous," it might be as well to pay a little attention, in passing, to the nomenclature and broad classification of the various kinds of plants. We shall then doubtless find it far easier thoroughly to understand the position in the scale of organisation to which the coal plants are referable.
[Illustration: FIG. 9.--Asterophyllites foliosa. Coal-measures.]
The plants which are lowest in organisation are known as Cellular. They are almost entirely composed of numerous cells built up one above the other, and possess