5. Leioderma, Goldenberg. Type, S. Sydnensis, Dawson.—Ribs obsolete. Cortical and ligneous surfaces striate. Vascular scars double, elongate longitudinally, and alike on cortical and inner surfaces. Areoles in rows and distinct; stigmaria-roots striate, with small and distinct areoles.
6. Clathraria, Brongniart. Type, S. Menardi, Brongniart.—Areoles hexagonal, not in distinct rows, but having a spiral appearance. Some of the plants usually referred to this group are probably branches of Favularia. Others are evidently fragments of plants of the genus Lepidophloios.
3. Internal Structures of Sigillaria-Stems.
I long ago pointed out, on the evidence of the external markings and mode of growth, that the stems of Sigillariæ must have been exogenous, and this conclusion has now been fully confirmed by the microscopic researches of Williamson, not only in the case of Sigillariæ, but of Lepidodendra and Calamodendra as well. Confining myself to my own observations, three types of Sigillariæ are known to me by their internal structures, though I cannot certainly correlate all of these with the external markings referred to above.
1. Diploxylon, in which the stem consists of a small internal axis surrounded by a very thick inner bark and a dense outer cortex. A fine example from the South Joggins is thus described:[CU]
[CU] “Journal of the Geological Society of London,” November, 1877.
"The axis of the stem is about six centimetres in its greatest diameter, and consists of a central pith-cylinder and two concentric coats of scalariform tissue. The pith-cylinder is replaced by sandstone, and is about one centimetre in diameter. The inner cylinder of scalariform tissue is perfectly continuous, not radiated, and about one millimetre in thickness. Its vessels are somewhat crushed, but have been of large diameter. Its outer surface, which readily separates from that of the outer cylinder, is striated longitudinally. The outer cylinder, which constitutes by much the largest part of the whole, is also composed of scalariform tissue; but this is radially arranged, with the individual cells quadrangular in cross-section. The cross-bars are similar on all the sides and usually simple and straight, but sometimes branching or slightly reticulated. The wall intervening between the bars has extremely delicate longitudinal waving lines of ligneous lining, in the manner first described by Williamson as occurring in the scalariform tissue of certain Lepidodendra. A few small radiating spaces, partially occupied with pyrites, obscurely represent the medullary rays, which must have been very feebly developed. The radiating bundles passing to the leaves run nearly horizontally; but their structure is very imperfectly preserved. The stem being old and probably long deprived of its leaves, they may have been partially disorganised before it was fossilised. The outer surface of the axis is striated longitudinally, and in some places marked with impressions of tortuous fibres, apparently those of the inner bark. In the cross-section, where weathered, it shows concentric rings; but under the microscope these appear rather as bands of compressed tissue than as proper lines of growth. They are about twenty in number. This tree has an erect, ribbed trunk, twelve feet in height and fifteen inches in diameter, swelling to about two feet at the base."
2. Favularia Type.—This has been well described by Brongniart and by Renault,[CV] and differs from the above chiefly in the fact that the outer exogenous woody zone is composed of reticulated instead of scalariform tissue, and the inner zone is of the peculiar form which I have characterised as pseudo-scalariform.
[CV] “Botanique Fossile,” Paris, 1881.
3. Sigillaria Proper.—This I have illustrated in my paper in the “Journal of the Geological Society” for May, 1871, and it appears to represent the highest and most perfect type of the larger ribbed Sigillaria. This structure I have described as follows, basing my description on a very fine axis found in an erect stem, and on the fragments of the woody axis found in the bases of other erect stems: