It is worthy of notice that the radial section of Spencerites ([fig. 192]) presents a fairly close resemblance to a corresponding section through a cone-scale of Agathis (Kauri Pine)[447]. In each case the megasporangium is attached by a narrow pedicel to the sporophyll and the latter has a similar form in the two plants, though the extent of the resemblance is somewhat lessened by Lang’s more complete account of the Palaeozoic type. If the Spencerites sporangia possessed an integument the similarity with the Agathis ovule would of course be much closer: recent palaeobotanical investigations have shown that ovules and sporangia are not separated by impassable barriers.
[Since this Chapter was set up in type a paper has appeared by Dr Bruno Kubart on a new species of Spencerites spore, S. membranaceus, from the Ostrau-Karwiner Coal-basin (Austria). The spores are larger than those of S. insignis and in some the cells of a prothallus are preserved. Kubart figures a section of a spore containing a group of seven cells, a central cell, which he regards as an antheridial mother-cell, surrounded by six wall-cells. Kubart (90).]
CHAPTER XVI.
Sigillaria.
i. General.
In view of the close resemblance between Lepidodendron and Sigillaria, another lycopodiaceous plant characteristic of Carboniferous and Permian floras, a comparatively brief description of the latter genus must suffice, more particularly as Lepidodendron has received rather an undue share of attention. Sigillaria, though abundantly represented among the relics of Palaeozoic floras, especially those preserved in the Coal-Measures, is rare in a petrified state, and our knowledge of its anatomy is far from complete. In external form as in internal structure the difference between the two genera are not such as enable us to draw in all cases a clearly defined line of separation.
In the Antediluvian Phytology, Artis[448] figured a fossil from the Carboniferous sandstones of Yorkshire which he called Euphorbites vulgaris on account of a superficial resemblance to the stems of existing succulent Euphorbias. Rhode[449] also compared Sigillarian stems with those of recent Cacti. The specimen described by Artis is characterised by regular vertical and slightly convex ribs bearing rows of leaf-scars in spiral series, like those on the cushions of Lepidodendron. A few years earlier Brongniart[450] had instituted the genus Sigillaria[451] for plants with ribbed but not jointed stems bearing “disc-like impressions” (leaf-scars) disposed in quincunx; the type-species named by the author of the genus Sigillaria scutellata is identical, as Kidston[452] points out, with Euphorbites vulgaris of Artis and with the plant afterwards figured by Brongniart as S. pachyderma[453]. Brongniart in 1822 figured another type of stem characterised by the absence of ribs and by prominent spirally arranged cushions bearing relatively large leaf-scars like the upper part of the specimen shown in [fig. 203]; this he named Clathraria Brardii, a well-known and widely distributed Carboniferous and Permian species now spoken of as Sigillaria Brardi (figs. [196], A–C; [203]). A third type of stem figured by Brongniart as Syringodendron striatum[454] agrees with Sigillaria scutellata in having ribs, but differs in the substitution of narrow oval ridges or depressions for leaf-scars; this is now recognised as a partially decorticated Sigillaria, in which the vascular bundle of each leaf is represented by a narrow ridge or depression. The name Syringodendron, originally used by Sternberg, is conveniently applied to certain forms of Sigillarian stems which have lost their superficial tissues. A fourth generic name, Favularia, was instituted by Sternberg[455] for Sigillarian stems with ribs covered with contiguous leaf-scars of hexagonal form and prominent lateral angles ([fig. 193], A; [fig. 200], G).
Fig. 193.