The Geological History of PlantsDawson, John William, Sir
Science
The Geological History of Plants
Dawson, John William, Sir
Paleobotany
_d._ An inner cylinder of pseudo-scalariform tissue. I have adopted the
term pseudo-scalariform for this tissue, from the conviction that it is
not homologous with the scalariform ducts of ferns and other acrogens,
but that it is merely a modification of the discigerous wood-cells, with
pores elongated transversely, and sometimes separated by thickened bars,
corresponding to the hexagonal areolation of the ordinary wood-cells.
A similar tissue exists in cycads, and is a substitute for the spiral
vessels existing in ordinary exogens.
_e._ A large medulla, or pith, consisting of a hollow cylinder of
cellular tissue, from which proceed numerous thin diaphragms towards the
centre of the stem.
These structures of the highest type of _Sigillaria_ are on the one
hand scarcely advanced beyond those of Calamopitus, as described by
Williamson, and on the other approach to those of _Cordaites_, as seen in
specimens presented to me by Renault.
Finally, as to the fruit of _Sigillariæ_, I have no new facts to offer.
The strobiles or spikes associated with these trees have been variously
described as gymnospermous (Renault) or cryptogamous (Groldenberg and
Williamson). 1 have never seen them in place. Two considerations,
however, have always weighed with me in reference to this subject. One
is the constant abundance of Trigonocarpa and Cardiocarpa in the soil
of the Sigiliaria forests, as I have studied this at the South Joggins.
The other is that the rings of fruit-scars on the branches of Sigiliaria
are homologous with leaf-scars, not with branches, and therefore should
have borne single carpels and not cones or spikes of inflorescence. These
are merely suggestions, but I have no doubt they will be vindicated
by future discoveries, which will, I have no doubt, show that in the
family _Sigillariaceæ_ we have really two families, one possibly of
gymnospermous rank, or at least approaching to this, the other allied to
the Lepidodendra.
Cryptogamia.
(_Acrogenes._)
_Family_ Lepidodendreæ; _Genus_ Lepidodendron, Sternberg.
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