III. A. STELOXYLEAE.
Steloxylon. Solms-Laubach.
Steloxylon Ludwigii (Goeppert and Stenzel). The genus is founded on a piece of stem from Siberia, possibly of Permian age though not improbably older, which was originally described as Medullosa Ludwigii[434]. It is characterised by numerous cylindrical and band-like vascular strands forming an irregular anastomosing system (fig. 441, B) and by crowded spiral leaf-scars on the exposed face. The appearance presented by the transverse section figured by these authors, while suggesting comparison with Medullosa, reveals a distinctive character, namely the absence of a definite peripheral system of vascular rings such as forms a striking feature of the continental Medulloseae. A more complete description was afterwards published by Schenk[435] who recognised more fully the peculiar features and hinted at the possibility that the species might more appropriately be regarded as a member of a distinct group. Solms-Laubach[436] went a step further and instituted the generic name Steloxylon, and in a later publication gave a fuller account of the anatomical characters. The complete stem must have reached a diameter of approximately 13 cm. The homogeneous ground-tissue forms a matrix enclosing an anastomosing vascular system of cylindrical or oval steles (fig. 441, C). Each strand consists of a band of secondary xylem tracheids with one or several rows of circular or oval bordered pits on the radial walls and narrow medullary rays usually 1–2 cells broad and 1–4 cells deep, though occasionally deeper. No phloem is preserved. The tissue in the centre of each stele is very imperfectly preserved, but it is clear that the secondary xylem enclosed a central region (‘partial pith’) like that in the steles of a Medullosa, doubtless consisting of primary xylem and conjunctive parenchyma.
The stem is covered with leaf-bases of oval or circular section and between them are small organs, probably multicellular hairs (fig. 441, A). A leaf-base consists of an outer zone of strengthening tissue and a parenchymatous ground-tissue traversed by two or more small vascular strands which assume various forms. These petiolar strands are simply portions of the main vascular system which bend outwards at the periphery of the anastomosing network. The more noteworthy features in which Steloxylon differs from Medullosa, particularly such species as M. stellata and M. Leuckarti, are (i) the crowded and comparatively small leaf-bases in place of the massive decurrent petioles of Medullosa; (ii) the supply of the leaves by compact branches of the stelar network instead of the bundles detached as leaf-traces from a stem-stele of Medullosa (the origin of a leaf-trace in Steloxylon is shown in fig. 441, C); (iii) the absence of a peripheral system of vascular plate-rings and the irregular distribution of cylindrical and plate-steles in the ground-tissue. Nothing is known of the reproductive organs or leaves beyond the structure of the attached leaf-bases. The opinion expressed by P. Bertrand[437] that the fossil described by Stenzel as Asterochlaena (Clepsydropsis) kirgisica is the petiole of Steloxylon was abandoned after the additional facts published by Solms-Laubach.
As regards the affinities of Steloxylon: the structure of the steles agrees closely with that of the star- and plate-rings of a Medullosa, while the pitting of the tracheids is more like that in Medullosa than Cladoxylon. In the tendency to a more radial than tangential disposition of the band-like steles Steloxylon recalls Cladoxylon rather than Medullosa, but in Cladoxylon the vascular system does not form an irregular network as in Steloxylon. The information as to the structure of the primary xylem is very meagre, but it points to a closer connexion with Medullosa than with Cladoxylon. On the whole Steloxylon may perhaps be defined as a genus allied to the Medulloseae in the anatomical features of the stem more closely than to other genera, but sufficiently distinct to be excluded from the Medulloseae as at present understood[438].