r, been met with which
was actually attached to the plants, and hence, when it was discovered
that some of them had long attenuated leaves not at all like those
possessed by ferns, geologists were compelled to abandon this
classification of them, and even now no satisfactory reference to
existing orders of them has been made, owing to their anomalous
structure. The stems are fluted from base to stem, although this is not
so apparent near the base, whilst the raised prominences which now form
the cicatrices, are arranged at regular distances within the vertical
grooves.

When they have remained standing for some length of time, and the strata
have been allowed quietly to accumulate around the trunks, they have
escaped compression. They were evidently, to a great extent, hollow like
a reed, so that in those trees which still remain vertical, the interior
has become filled up by 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