DAWSON—SIGILLARIA, CALAMITES, AND CALAMODENDRON. 151
It has been held to be an objection to the identification of the
discigerous tissues above mentioned with those of Sigillaria, that
the Stigmarioe, when their structure happens to be preserved, show
merely scalariform tissue. To this it maybe answered:—(1.) That,
as Corda has shown*, some Stigmarioe have reticulated or multiporous tissues. (2.) The tissue of Stigmaria is not essentially different
from the pseudo-scalariform fibres of the stem, and is arranged
in a similar manner, showing that it is homologous rather with
woody than with vascular tissue. (3.) Many Stigmarioe probably
belong to Favularia and similar forms, or possibly even to Lepidodendroid plants†. In either case the structure would be unlike that
of the stems of Sigillaria proper. (4.) Inasmuch as the proportions
of pseudo-vascular and discigerous tissue may differ greatly
in the stems of Stigmarioe, it would not be unreasonable to suppose
that the tissue, which is more particularly important for the
strengthening of the stem, should be absent, or in a feeble state of
development, in the root. Something of this kind occurs in the
roots of Cycads, and perhaps, if detailed examinations were made,
might be found to be more general than is commonly supposed.
(5.) The outer part of the axis, being left exposed by the decay of
the loose cellular matter of the inner bark, may, in most cases, have
perished. In my specimen of the axis of Sigillaria, above described,
it is in parts much disorganized, and has disappeared, or been converted into coal, on one side.
The evidence included under the above heads is sufficient to show that the ordinary ribbed Sigillarioe referred to in my previous papers, possessed in their main trunks the following kinds of tissue, in proceeding from the circumference to the centre:— (a) A dense cellular outer bark, usually in the state of compact coal—but when its structure is preserved, showing a tissue of thickened parenchymatous cells.
(b) A very thick inner bark, which has usually in great part perished, or been converted into coal, but which, in old trunks, contained a large quantity of prosenchymatous tissue, very tough and of great durability. This "bast-tissue" is comparable with that of the inner bark of modern Conifers, and constitutes much of the mineral charcoal of the coal-seams.
(c) An outer ligneous cylinder, composed of wood-cells, either with a single row of large bordered pores‡, in the manner of Pines
- Beitrage zur Flora der Vorwelft.
† Brown, in 1847, described, in the 'Proceedings' of this Society, Stigmaria-roots of Lepidodendron. Baily seems to have shown that such roots belong to the singular Lepidodendroid Cyclostigma of the Devonian of Ireland; and Schimper asserts a connexion of Stigmaria roots with trees which he refers to Knorria.
‡ These are the same with the wood-cells elsewhere called discigerous tissue and to which I have applied the terms uniporous and multiporous. The markings on the walls are caused by an unlined portion of the cell-wall placed in a disk or depression, and this often surrounded by an hexagonal rim of thickened wall; but in all cases these structures are less pronounced than in Dadoxylon, and less regular in the walls of the same cell, as well as in different layers of the tissues of the axis.
