_Heterangium Grievii_ has a radially symmetrical stem bearing compound leaves with decurrent petioles which give to the otherwise cylindrical axis an angular outline as seen in transverse section (fig. 411, A). The phyllotaxis appears to be ⅜. The stem rarely exceeds 1·5 cm. in diameter: in the centre is a comparatively large stele consisting in young stems of primary xylem and phloem, but in older stems these are separated by a cylinder of secondary vascular tissue which in this species is always narrower than in _Lyginopteris oldhamia_ and, as Williamson pointed out, often of unequal thickness on different radii. The medullated stele of _Lyginopteris_ is replaced by a solid xylem-cylinder consisting mainly of groups of large tracheids, reaching ·3 mm. in diameter, with multiseriate bordered pits (fig. 411, D) embedded in an anastomosing parenchymatous tissue-system. In the stele reproduced in fig. 411, B, which with the exception of a very narrow zone of secondary xylem, _x_², consists entirely of primary xylem, _x_¹, the parenchyma is represented by a darker reticulum (_cf._ fig. 415, B) dividing the metaxylem into islands as in _Gleichenia_. In the peripheral portion of the xylem the tracheids are rather narrower and arranged in more definite groups in many of which is a single strand of narrow spiral elements (fig. 411, A′, _px_) close to the outer margin. These peripheral primary bundles in which protoxylem is recognisable may be described as leaf-traces of mesarch structure consisting of centripetal xylem and, to a much less extent, of smaller centrifugal elements for the most part with dense spiral bands in place of the multiseriate pits of the rest of the metaxylem. The structure of these leaf-traces is practically identical with that of the primary bundles of _Lyginopteris_. There is, however, a difference to which attention is drawn by Williamson and Scott. While in _Lyginopteris_ in any transverse section the primary bundles in the stele are equal in number to the leaf-traces in the pericycle and cortex, in _Heterangium_ the peripheral groups in the stele may be as many as twenty, a number considerably in excess of the leaf-traces beyond the limits of the primary xylem of the stele. It may be that the leaf-trace of each leaf, which joins the stele at a distance of 6–10 internodes below its entrance into the cortex from the leaf-stalk, may branch in its descent in the axial region, or some of the primary groups of xylem may be confined to the axial region and independent of the leaf-traces. Portions of the peripheral region of the stele may be occupied by metaxylem groups without protoxylem and identical with those which make up the bulk of the metaxylem.