The Geological History of Plants

by Sir John William Dawson

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,[CW] in the manner of pines and cycads, or with two, three, or four rows of such pores sometimes inscribed in hexagonal areoles in the manner of Dadoxylon. This woody cylinder is traversed by medullary rays, which are short, and composed of few rows of cells superimposed. It is also traversed by oblique radiating bundles of pseudo-scalariform tissue proceeding to the leaves. In some Sigillariæ this outer cylinder was itself in part composed of pseudo-scalariform tissue, as in Brongniart’s specimen of S. elegans; and in others its place may have been taken by multiporous tissue, as in a case above referred to; but I have no reason to believe that either of these variations occurred in the typical ribbed species now in question. The woody fibres of the outer cylinder may be distinguished most readily from those of conifers, as already mentioned, by the thinness of their walls, and the more irregular distribution of the pores. Additional characters are furnished by the medullary rays and the radiating bundles of scalariform tissue when these can be observed.

[CW] 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.

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.)