The single cylindrical stele has a relatively large solid pith, the perimedullary region being characterised by the occurrence of a row of primary crescentic strands of centripetal xylem of exarch type, though not improbably in some cases slightly mesarch, varying in size and shape and forming single or paired bundles. These strands represent the xylem of collateral leaf-traces similar to those of _Lyginopteris_ but differing in the absence of well-defined centrifugal elements: the curved form of some of the xylem strands gives them an appearance similar to that of the leaf-traces of _Lyginopteris_. The leaf-traces, except in the lower part of their course through the pith, are double and pass through several internodes before the centripetal tracheids die out. The secondary xylem (fig. 463) is manoxylic and very similar to that of _Lyginopteris_ though rather less parenchymatous. The secondary phloem and cambium are often very well preserved. No endodermis and no distinct pericycle has been recognised. The cortex is parenchymatous and, like the pith and to some extent the phloem, contains numerous secretory sacs; in the outer cortex the presence of hypodermal strands is a prominent feature. At an early stage in the growth of the stem a deep-seated phellogen forms secondary tissue both externally and internally and decortication ensues. [Illustration: Fig. 463. _Poroxylon Edwardsii_. A. Transverse section of stem. B. The central region of another stem of the same species. (A, × 9; University College Collection; B, from a photograph supplied by Prof. Bertrand of a specimen in the Renault Collection, Paris.)] The bundle of each leaf-trace is accompanied by an arc of secondary centrifugal xylem as it passes through the secondary wood and this is retained in the leaf except in the finer veins. After entering the petiole the leaf-trace branches and an arc of bundles is produced, the concave side facing the upper surface of the thick lamina (fig. 464, A). Further reference is made to the structure of the leaves in the description of _Poroxylon stephanense_. The specimens of roots so far described are characterised by a diarch plate of primary xylem and two masses of secondary vascular tissue separated by two medullary rays opposite the protoxylems. Bertrand mentions the occurrence of roots of _P. stephanense_ with more than two protoxylem strands. The phellogen was produced in the pericycle as in the roots of recent Gymnosperms. It is suggested by Lignier[540] that some silicified rootlets from Grand’ Croix (Loire) described by him as _Radiculites reticulatus_ and at first compared with roots of _Sequoia_ may belong to some Cordaitalean plant, possibly _Poroxylon_. _Poroxylon Edwardsii_ Renault.