The Story of a Piece of Coal | Page 8

a coat of sandstone, whilst the bark has
become transformed into an envelope of an inch, or half an inch of coal.
But many are found lying in the strata in a horizontal plane. These have
been cast down and covered up by an ever-increasing load of strata, so
that the weight has, in the course of time, compressed the tree into
simply the thickness of the double bark, that is, of the two opposite
sides of the envelope which covered it when living.
Sigillarae grew to a very great height without branching, some
specimens having measured from 60 to 70 feet long. In accordance
with their outside markings, certain types are known as
_syringodendron_, _favularia_, and clathraria. Diploxylon is a term
applied to an interior stem referable to this family.
[Illustration: FIG. 16.--Stigmaria ficoides. Coal-shale.]
But the most interesting point about the sigillariae is the root. This was
for a long time regarded as an entirely distinct individual, and the older
geologists explained it in their writings as a species of succulent
aquatic plant, giving it the name of stigmaria. They realized the fact
that it was almost universally found in those beds which occur
immediately beneath the coal seams, but for a long time it did not strike
them that it might possibly be the root of a tree. In an old edition of
Lyell's "Elements of Geology," utterly unlike existing editions in
quality, quantity, or comprehensiveness, after describing it as an extinct
species of water-plant, the author hazarded the conjecture that it might
ultimately be found to have a connection with some other well-known
plant or tree. It was noticed that above the coal, in the roof, stigmariae
were absent, and that the stems of trees which occurred there, had
become flattened by the weight of the overlying strata. The stigmariae
on the other hand, abounded in the _underclay_, as it is called, and

were not in any way compressed but retained what appeared to be their
natural shape and position. Hence to explain their appearance, it was
thought that they were water-plants, ramifying the mud in every
direction, and finally becoming overwhelmed and covered by the mud
itself. On botanical grounds, Brongniart and Lyell conjectured that they
formed the roots of other trees, and this became the more apparent as it
came to be acknowledged that the underclays were really ancient soils.
All doubt was, however, finally dispelled by the discovery by Mr
Binney, of a sigillaria and a stigmaria in actual connection with each
other, in the Lancashire coal-field.
Stigmariae have since been found in the Cape Breton coal-field,
attached to Lepidodendra, about which we have already spoken, and a
similar discovery has since been made in the British coal-fields. This,
therefore, would seem to shew the affinity of the sigillaria to the
lepidodendron, and through it to the living lycopods, or club-mosses.
Some few species of stigmarian roots had been discovered, and various
specific names had been given to them before their actual nature was
made out. What for some time were thought to be long cylindrical
leaves, have now been found to be simply rootlets, and in specimens
where these have been removed, the surface of the stigmaria has been
noticed to be covered with large numbers of protuberant tubercles,
which have formed the bases of the rootlets. There appears to have also
been some special kind of arrangement in their growth, since, unlike
the roots of most living plants, the tubercles to which these rootlets
were attached, were arranged spirally around the main root. Each of
these tubercles was pitted in the centre, and into these the almost
pointed ends of the rootlets fitted, as by a ball and socket joint.
[Illustration: FIG. 17--Section of stigmaria.]
"A single trunk of sigillaria in an erect forest presents an epitome of a
coal-seam. Its roots represent the stigmaria underclay; its bark the
compact coal; its woody axis, the mineral charcoal; its fallen leaves and
fruits, with remains of herbaceous plants growing in its shade, mixed
with a little earthy matter, the layers of coarse coal. The condition of
the durable outer bark of erect trees, concurs with the chemical theory

of coal, in showing the especial suitableness of this kind of tissue for
the production of the purer compact coals."--(Dawson, "Structures in
Coal.")
There is yet one other family of plants which must be mentioned, and
which forms a very important portion of the constituent flora of the
coal period. This is the great family of the _coniferae_, which although
differing in many respects from the highly organised dicotyledons of
the present day, yet resembled them in some respects, especially in the
formation of an annual ring of woody growth.
The conifers are those trees which, as the name would imply, bear their
fruit in the