An illustration published by Martin in 1809[562] shows a characteristic feature of Stigmarian casts, namely the presence of a smaller axis, usually occupying an eccentric position inside the larger. This represents the cast of the fairly broad parenchymatous pith which, on decay, left a space subsequently filled by sand or mud: at a later stage the surrounding wood and cortex were removed and the cavity so formed was similarly filled. A thin layer of coal formed by the carbonisation of some of the tissues frequently surrounds the medullary cast, and Steinhauer, whose account of the genus is much fuller and more scientific than those of other earlier and many later writers, recognised the true nature of this internal cast. Artis[563] regarded it as the remains of a young plant, which he described as “perforating its parent,” at length bursting it and assuming its place, a gratuitously drastic interpretation. In 1838[564] Lindley and Hutton figured a partially petrified specimen of _Stigmaria_ obtained by Prestwich from Carboniferous rock of Shropshire. This example showed a fairly broad cylinder of secondary wood penetrated by medullary rays. The medullated stele consisted of a pith surrounded by a small amount of primary xylem and by a cylinder of secondary scalariform tracheae. The preservation of the tissues abutting on the edge of the wood is usually very imperfect, and the middle cortex of lacunar parenchyma has practically in every case eluded the action of mineralising agents; the outer cortex, on the other hand, consists of more resistant elements and is frequently well preserved. As in _Lepidodendron_ and _Sigillaria_ stems, meristematic activity produced a broad band of secondary cortex; and beyond this were attached to cushion-like pads the numerous appendages, each supplied with a single vascular bundle which arose from the primary xylem and passed outwards through a medullary ray. There is abundant evidence that the appendages were hollow, a fact in striking accord with the aquatic and semi-aquatic habitat (cf. _Isoetes_ root, fig. 133, G). [Illustration: FIG. 206. _Cyperus papyrus._ Piece of rhizome showing rootlet-scars. Nat. size. M.S.] The piece of dried rhizome of _Cyperus papyrus_ shown in fig. 206 is an almost exact counterpart of _Stigmaria ficoides_; the wrinkled and shrivelled surface and the circular root-scars containing the remains of a vascular bundle are striking features in common and, it may be added, the two plants, though very different in structure and in systematic position, illustrate anatomical adaptations to a similar environment. _Stigmaria ficoides_ Brongniart[565]. Figs. 204, 205, 207, 208. 1809. _Phytolithus verrucosus_, Martin, Petrifact. Derb. Pls. XI–XIV. 1818. _Phytolithus verrucosus_, Steinhauer, Trans. Phil. Soc. America, [N.S.] Vol. I. p. 268, Pl. IV. 1820. _Variolaria ficoides_, Sternberg, Flora der Vorwelt, p. 22, Pl. XII.