n or runs along the ground, takes in the tree-fern
a different direction, and, rising erect, climbs slowly upwards in the
character of a trunk or stem, and sends out atop, year after year, a
higher and yet higher coronal of fronds. And in order to impart the
necessary strength to this trunk, and to enable it to war for ages with
the elements, its mass of soft cellular tissue is strengthened all round
by internal buttresses of dense vascular fibre, tough and elastic as the
strongest woods. Now, not a few of the more anomalous forms of the Coal
Measures seem to be simply fern allies of the types Lycopodiaceae,
Marsileaceae, and Equisetum, that, escaping from the mediocrity of mere
herbs, shot up into trees,--some of them very great trees,--and that had
of necessity to be furnished with a tissue widely different from that of
their minuter contemporaries and successors. It was of course an
absolute mechanical necessity, that if they were to present, by being
tall and large, a wide front to the tempest, they should also be
comparatively solid and strong to resist it; but with this simple
mechanical requirement there seems to have mingled a principle of a more
occult character. The Gymnogens or conifers were the highest vegetable
existences of the period,--its true trees; and all the tree-like fern
allies were strengthened to meet the necessities of their increased
size, on, if I may so speak, a _coniferous_ principle. Tissue resembling
that of their contemporary conifers imparted the necessary rigidity to
their framework; nay, so strangely were they pervaded throughout by the
coniferous characteristics, that it seems difficult to determine whether
they really most resembled the acrogenous or gymnogenous families. The
Lepidodendra,--great plants of the club moss type, that rose from fifty
to seventy feet in height,--had well nigh as many points of resemblance
to the coniferae as to the Lycopodites. The Calamites,--reed-like,
jointed plants, that more nearly resemble the Equisetaceae than aught
else which now exists, but which attained, in the larger specimens, to
the height of ordinary trees, also manifest very decidedly, in their
internal structure, some of the characteristics of the conifers. It has
been remarked by Lindley and Hutton of even Sphenophyllum,--a genus of
plants with verticillate leaves, of which at least six species occur in
our Coal Measures, and which Brogniart refers to one of the humblest
families of the fe