Tree-fern stems, occasionally reaching a height of 50 feet or more, closely resembling in habit recent tree ferns, but exhibiting in the structure and arrangement of the vascular system a close agreement with recent Marattiaceae. Leaves, in such cases where a connexion between fronds and stems is known, large and highly compound and of the _Pecopteris_ type, borne in more or less crowded spirals (_Psaronius polystichi_), in four rows (_P. tetrastichi_), or in two opposite rows (_P. distichi_). Leaves deciduous, leaving a clearly defined oval scar containing the impression of the leaf-trace in the form of an open =U=, or a closed oval with a small inverted =V=-shaped band a short distance below the upper end of the long axis of the oval (figs. 297, 298); in _Megaphyton_ the alternate scars of the two opposite series are larger and characterised by a different form of meristele. The surface of the cortex below the leaf-scars occasionally shows impressions of pits similar to the lenticel-like organs on recent Tree-fern stems. The central region of the stem is occupied by a complex system of concentrically disposed steles (dictyosteles), which in transverse section present the appearance of flat or curved bands varying in extent and in degree of curvature. The vascular bands consist of xylem surrounded by a narrow zone of phloem; the xylem is composed either exclusively of tracheae or of tracheae and parenchyma; the protoxylem in the one instance in which it has been clearly recognised is endarch[1064]. The steles are embedded in parenchymatous tissue and in some species are associated with mechanical tissue (e.g. _P. infarctus_, fig. 296, A, B). The central or vascular region of the stem may be surrounded externally by a cylinder of mechanical tissue interrupted by outgoing leaf-traces and adventitious roots. The leaf-traces arise as single bundles from an internal stelar band and pursue an obliquely radial course towards the outside, eventually anastomosing with peripheral cauline steles, which in some species form with the leaf-traces the outermost zone of the vascular region. The leaf-traces have the form of loops which pass into the petioles as =V=-shaped meristeles or closed oval cylinders. As a leaf-trace passes out compensating strands occupy the foliar gap.