The stem on which this genus is founded was discovered by Dr Gordon[470] in the Calciferous Sandstone series of Pettycur: a specimen collected by Dr Kidston in Berwickshire may be specifically identical with the Pettycur plant. We know nothing of the leaves or reproductive organs of _Rhetinangium_. The stem, approximately 2 cm. in diameter, was probably cylindrical; it possesses a single stele consisting mainly of a central primary region occupied by anastomosing groups of tracheids, 130–150μ in diameter, embedded in parenchyma containing numerous secretory sacs and ducts. In the peripheral region of the stele the groups of tracheids consist of narrower elements characterised by exarch protoxylem. Each peripheral group forms the base of a wedge of secondary xylem (fig. 450, _x²_), the primary medullary rays being in direct connexion with some of the parenchyma of the primary xylem. The secondary tracheids, 45–85μ in diameter, have multiseriate bordered pits on their radial walls and the rays are broad and deep as in _Heterangium_ and _Lyginopteris_. With the exception of the external position of the protoxylem, the stele of _Rhetinangium_ is practically identical with that of _Heterangium_, though in _Rhetinangium_ the primary tracheids form larger and fewer groups. The inner cortex is composed of thin-walled cells with many secretory sacs: there are no stereome elements. In the outer cortex (fig. 449, _cr_) radially disposed bands of stereome form a reticulum with narrow and very long meshes like that of Medullosan petioles. The recently recorded occurrence of polydesmic petioles[471] in _Heterangium_ is of special interest from the point of view of the comparison of that genus with _Rhetinangium_ and the Medulloseae. The very broad decurrent petiole-bases are a striking feature, the major diameter of the pulvinus-like base of the leaf-stalk exceeding that of the stem (fig. 449, _p_). Several xylem-strands from the peripheral region of the primary xylem go to form a single leaf-trace: these U-shaped strands of xylem destined for a leaf are connected laterally by parenchyma and form an irregularly corrugated band. Fig. 450 shows a petiole-trace still enclosed on each side by the secondary xylem cylinder _x²_. [Illustration: Fig. 449. _Rhetinangium Arberi_ Gord. Transverse section of stem with large leaf-base; _cr_, outer cortex of stem; _p_, petiole. (After Gordon; × 1¾.)] There are several protoxylem groups in a leaf-trace, one on the outer face of each xylem-strand. In the petiole the xylem groups are more intimately connected and the trace has the form of a flat band with abaxial protoxylem. There is no indication that a leaf-trace undergoes division into separate strands. The roots are described as tetrarch with well-developed secondary xylem. [Illustration: Fig. 450. Transverse section of leaf-trace of _Rhetinangium Arberi_ showing the exarch structure; _x²_, secondary xylem. (After Gordon.)]