| Monocotyledons |Palms, lilies, |Endogens |
| | grasses | |
| Dicotyledons |Most European |Exogens |
| | trees and shrubs | |
-------------------------------------------------------------------

Adolphe Brongniart termed the coal era the "Age of Acrogens," because, as
we shall see, of the great predominance in those times of vascular
cryptogamic plants, known in Dr Lindley's nomenclature as "Acrogens."

[Illustration: FIG. 10.--_Spenophyllum cuneifolium._ Coal-shale.]

Two of these families have already been dealt with, viz., the ferns
(_felices_), and the equisetums, (_calamites_ and _equisetites_), and we
now have to pass on to another family. This is that which includes the
fossil representatives of the Lycopodiums, or Club-mosses, and which goes
to make up in some coals as much as two-thirds of the whole mass.
Everyone is more or less familiar with some of the living Lycopodiums,
those delicate little fern-like mosses which are to be found in many a
home. They are but lowly members of our British flora, and it may seem
somewhat astounding at first sight that their remote ancestors occupied
so important a position in the forests of the ancient period of which we
are speaking. Some two hundred living species are known, most of them
being confined to tropical climates. They are as a rule, low creeping
plants, although some few stand erect. There is room for astonishment
when we consider the fact that the fossil representatives of the family,
known as _Lepidodendra_, attained a height of no less than fifty feet,
and, there is good ground for believing, in many cases, a far greater
magnitude. They consist of long straight stems, or trunks which branch
considerably near the top. These stems are covered with scars or scales,