One of the largest specimens of a petrified Stigmaria which I have seen is one lent to me by Mr Lomax from the Coal-Measures of Halifax in which the flattened transverse section measures 18 cm. × 3·5 cm., the cylinder of wood being 1·1 cm. × 7 mm. in diameter.
In French examples of Stigmaria or Stigmariopsis it has been demonstrated by Renault[596] that primary xylem strands occur very like those in the stem of some species of Sigillariae (see p. 219). If a well-preserved section of an English Stigmaria is examined it will be seen that the edge of the secondary wood consists of a few narrower elements which do not exhibit the radial seriation characteristic of secondary elements.
A type of Stigmaria characterised by centripetal primary wood has been described by Weiss[597] and referred by him to Bothrodendron mundum; the main results of his observations are stated in the account of Bothrodendron on a subsequent page. This discovery is of considerable interest not only as rendering our knowledge of Bothrodendron remarkably complete but as confirmatory of Renault’s account of French Stigmarian axes in which centripetal primary wood is well developed between the secondary xylem and the centre of the stele. The Stigmarian axis of Bothrodendron was originally figured by Williamson as Lepidodendron mundum[598]. The chief difference between Weiss’s specimen and those described by Renault[599] as the Stigmarian axes of Sigillaria Brardi, is that in the English plant the centripetal wood forms a cylinder of uniform breadth instead of a band with a crenulated inner margin as figured by Renault.
STIGMARIA
Fig. 210. Stigmaria.
- A. Transverse section of vascular bundle of rootlet and part of outer cortex. t, tracheae. (After F. E. Weiss.)
- B, C. Vascular bundle of rootlet; in C a series of small tracheae are shown extending from the protoxylem.
- D. Rootlets from the outer cortex of E.
- E. Part of a large Stigmaria: St, stele; s, intruded rootlet.
- F. Vascular bundle and tracheae passing obliquely towards the outer cortex, c3.
- G. Outer cortex of Stigmaria.
- H. Diagrammatic section of Stigmaria: p, phelloderm; r, rootlets.
An interesting agreement between the French and English specimens is the occurrence in the cortex of groups of reticulate elements: in Weiss’s section these are short and wide and occur in the middle cortex; in Renault’s plant they are more fusiform and occur in the secondary cortical tissue. These elements appear to have been arranged as an interlacing network in the middle cortex and were in close connexion with the rootlet-bundles, comparable, as Weiss points out, with the transfusion tracheids accompanying Lepidodendron leaf-traces.
It is probable that these short and wide tracheal elements served for water-storage and thus afford another indication of the xerophilous character of the Carboniferous Lycopods, a feature possibly connected with a salt-marsh habitat.
The presence of conspicuous medullary rays gives the secondary xylem of Stigmaria the appearance of being divided into several more or less distinct groups ([fig. 210], E, St). In tangential longitudinal section the xylem assumes the form of a broad reticulum with lenticular meshes filled with medullary-ray tissue through which strands of xylem are cut across in a transverse direction as they pass outwards from the inner edge of the wood to supply the rootlets. In addition to these broader or primary medullary rays, there were numerous secondary rays composed of narrow plates of parenchymatous cells one or several elements in depth. As Williamson pointed out, the medullary-ray tissue consists in part of radially elongated tracheal elements with spiral or scalariform thickening bands like those described in the same position in Lepidodendron stems.