The stele consists of a bluntly triangular core of primary xylem, 3–4 mm. in diameter, composed almost entirely of reticulately pitted tracheids reaching a diameter of 160μ: a few parenchymatous cells occur in the peripheral region and a band of parenchyma extends from the middle of each of the three sides of the xylem to the centre of the stele, thus dividing the primary conducting tissue into three groups which are the expression of a phyllotaxis of ⅓. The tracheids near the outer face of each xylem-group are narrower than the others and have scalariform pitting. The secondary xylem first appears along the slightly concave sides of the primary stele, eventually enclosing the whole: it consists of tracheids with multiseriate pits on the radial walls and numerous deep medullary rays 1–6 cells broad. No phloem is preserved though it is probable that a narrow band was originally present. A characteristic feature is afforded by a zone of thick-walled cells, regarded as periderm, encircling the stele and formed by a deep-seated phellogen. On the outer face of this band there are projecting bosses, and similar sclerous nests are scattered in the cortex. The outer cortex has a _Sparganum_[475] type of hypoderm, that is long vertical strands of fibres alternating with parenchyma. The leaf-traces are formed from the blunt angles of the primary xylem; an angle becomes nipped off as a more or less cylindrical strand enclosed by a zone of secondary tracheids which is very narrow on the adaxial side (fig. 452). Protoxylem was recognised only in the leaf-traces and not in the rest of the stele. A pair of protoxylem strands occurs on the outer edge of a prominent angle of xylem before it becomes detached from the stele, and these form a single strand at a lower level. As a leaf-trace passes outwards, the exarch xylem strand becomes mesarch and there is a single protoxylem group except at a point near the bifurcation of a trace. In its passage through the cortex a leaf-trace divides repeatedly, the secondary xylem on the outer face of each strand being retained for a considerable time. Our meagre knowledge of the nature of the leaves is based on incomplete fragments found in association with the stem. The leaf is believed to have been simple and characterised by a thick lamina with a hypodermal zone of sclerous strands and several vascular bundles. [Illustration: Fig. 452. _Stenomyelon tuedianum_. Transverse section of stele. (After Kidston; × 7.)] As Kidston and Gwynne-Vaughan[476] remark, _Stenomyelon_ is a very distinct type; while resembling _Sutcliffia_ in some respects it differs from that genus not only in the structure of the primary stele but in the absence of the system of meristeles which form so characteristic a feature of the latter genus. =CYCADOXYLEAE.= =Cycadoxylon.= Renault.