_Ptychoxylon_[482] _Levyi_ Renault. Like many Palaeozoic genera founded on anatomical features, _Ptychoxylon_[483] is represented only by stems, our knowledge of the leaves being confined to the leaf-traces in the stem which appears to have a phyllotaxis of ⅜. The stem of this Permian species has a diameter of 5–6 cm.: the comparatively broad cortex contains numerous secretory canals, but in place of hypodermal strands of stereome there is a superficial periderm. The vascular tissue, consisting of secondary xylem and phloem, assumes different patterns at different levels. There is an outer vascular cylinder of centrifugally developed xylem and phloem; the xylem is manoxylic and the tracheids have 3–5 rows of bordered pits on the radial walls. At intervals the continuity of the main stele is broken by the formation of leaf-gaps and before one gap is repaired a second may be produced, thus converting the cylinder into two crescentic and infolded bands (fig. 453, A). A striking character is the occurrence in the large parenchymatous central region of internal vascular bands or arcs varying in size and number at different levels and composed of centripetally developed secondary xylem and phloem. These internal bands differ from the outer and broader cylinder both in their inverse orientation and their limited vertical range. The connexion between the inner and outer vascular tissue and the alteration in plan of the conducting tissue at different levels are illustrated by fig. 453, 1–4, simplified from some of Renault’s figures of successive sections through a vertical distance of 4–5 cm. In section 1 the main cylinder is continuous except for a small gap where a leaf-trace is about to be given off: there are three internal vascular bands similar in structure to the outer stele but inversely orientated. At a higher level (section 2) the leaf-gap is larger and in it is a double leaf-trace of two collateral strands consisting of primary centripetal xylem and a fan-like group of secondary xylem and phloem. The free edges of the outer stele of section 1 have curved inwards and united with the two lateral medullary bands, while the lower internal band of section 1 has increased in extent and forms a discontinuous arc with the upper portions enclosed by the loops formed by the infolded ends of the outer vascular tissue. In section 3 a second leaf-gap has been formed in the outer stele and its invaginated ends have fused with the internal bands. In section 4 the first leaf-gap is closed and the invaginated bands of section 3 have broken up into an irregular circle of shorter bands. The section reproduced in fig. 453, A, shows the main cylinder in the form of two curved and flattened loops, each composed partly of the centrifugally developed xylem and phloem of the main stele and in part of the inversely orientated tissue of the inner bands. At a lower level the two bands _b_, _b_, will become detached as the