Fig. 204. Stigmaria ficoides Brongn. M.S. (See Vol. i. p. 73.)
Fig. 205. Stigmaria ficoides. From a specimen in the York Museum, from Bishop Auckland. a, base of rootlet showing vascular bundle scar. M.S.
The circular scars mark the bases of long single and occasionally forked appendages (rootlets) which spread on all sides into the surrounding medium (figs. [205], [208]). The occurrence of rootlets radiating through the shale or sandstone affords proof that the Stigmarias are often preserved in their position of growth. This was recognised by Steinhauer[557] and Logan[558], and has been more recently emphasised by Potonié[559] as an argument in favour of the view that the beds containing such specimens are old surface-soils.
Stigmaria usually shows regular dichotomous branching, the arms spreading horizontally or slightly downwards and always arising from four main branches in the form of a cross ([fig. 207]). The most remarkable specimens found in England are described by Williamson[560] in his monograph of Stigmaria. One of two large casts found near Bradford in Yorkshire, and now in the Manchester Museum, shows four large primary arms radiating from the base of an erect stump 4 feet in diameter. Each arm divides a short distance from its base into two, and the smaller branches extend almost horizontally for several feet[561].
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).
Fig. 206. Cyperus papyrus. Piece of rhizome showing rootlet-scars. Nat. size. M.S.