The Story of a Piece of Coal | Page 4

this
was peculiar in the possession of masses of aerial roots grouped round
the stem. Some of the smaller species exhibit forms of leaves which are
utterly unknown in the nomenclature of living ferns. Most have had
names assigned to them in accordance with certain characteristics
which they possess. This was 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