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]. CHAPTER XXXI. PTERIDOSPERMS REPRESENTED BY SEED-BEARING LEAVES ONLY OR LEAVES IN CONSTANT ASSOCIATION WITH SEEDS. Before describing other genera represented by petrified vegetative organs exhibiting in their anatomical features points of contact with the Medulloseae, a short account is intercalated of some imperfectly known seed-bearing fronds and seeds belonging to the Pteridosperms but which do not afford sufficient data to admit of their reference to a more precise position in a natural classification. _Pecopteris Pluckeneti_ (Schlotheim).