The
best bituminous coals seem to be most largely composed of these
spore-cases; whilst inferior kinds possess a progressively increasing
amount of the dull carbonaceous substance which is known as "mineral
charcoal," and which is undoubtedly composed of "the stems and
leaves of plants reduced to little more than their carbon." On
the other hand, Principal Dawson finds that the American coals
only occasionally exhibit spore-cases to any extent, but consist
principally of the cells, vessels, and fibres of the bark,
integumentary coverings, and woody portions of the Carboniferous
plants.
The number of plants already known to have existed during the
Carboniferous period is so great, that nothing more can be done
here than to notice briefly the typical and characteristic _groups_
of these--such as the Ferns, the Calamites, the Lepidodendroids,
the Sigillarioids, and the Conifers.
[Illustration: Fig. 108.--_Odontopteris Schlotheimii_. Carboniferous,
Europe and North America.]
[Illustration: Fig. 109.--_Calamites cannoeformis_. Carboniferous
Rocks, Europe and North America.]
In accordance with M. Brongniart's generalisation, that the Palaeozoic
period is, botanically speaking, the "Age of Acrogens," we find
the Carboniferous plants to be still mainly referable to the
Flowerless or "Cryptogamous" division of the vegetable kingdom.
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