Our knowledge of the anatomy of _Psaronius_ is based largely on the investigations of Stenzel considerably extended by Zeiller’s more intensive studies and, more recently, by the later work of Stenzel[1066] and that of Rudolph. A striking fact, which has led to various suggestions, is that in a transverse section of a _Psaronius_ stem with its encasing cylinder of roots no signs of leaf-traces are met with in the root-region. If the roots simply penetrated the cortex, as in some recent species of _Lycopodium_ (fig. 125, A) or as in _Angiopteris_, we should expect to find leaf-traces in the outer region (root-cylinder) of Psaronius stems. An explanation of the absence of leaf-traces which was suggested by Stenzel, is that the cortical zone formed a comparatively narrow band in the young leaf-covered stem; after leaf-fall it became the seat of active growth in its inner layers and so produced a constantly widening zone of secondary parenchyma, which pushed the superficial cortical tissue with the leaf-bases or leaf-scars farther out until it was exfoliated. Farmer and Hill[1067] find it difficult to accept this explanation; but, as Rudolph shows, the radial arrangement of the cortical cells between the adventitious roots and their elongation in a radial direction are arguments in support of the secondary nature of the cortical zone. In sections of the adventitious roots of _Psaronius Renaulti_ figured by Williamson[1068], the spaces between the cylindrical roots are partially occupied by cell-filaments which, at first sight, suggest root-hairs; it may well be, as Rudolph suggests, that these felted hairs represent the outermost and looser part of the growing secondary cortex which gradually passes into the covering mass of free extra-cortical roots. As Stenzel[1069] has shown, slender stems of _Zygopteris_ (= _Ankyropteris_) are occasionally met with growing through the web of Psaronius roots. _Psaronius infarctus_ Unger. Fig. 296, A, B. This species, which Zeiller[1070] has investigated from sections of Unger’s material, illustrates a type in which the vascular tissue is very richly developed and forms crowded concentric series of curved plates associated, in the more peripheral series, with bands of mechanical tissue. The outermost part of the vascular region consists of (i) a series of loops or variously curved bands of conducting tissue representing leaf-traces at different stages in their outward course, (ii) a series of similar vascular strands (peripheral steles of Zeiller) confined to the stem (cauline) and from which roots are given off, and (iii) bands of mechanical tissue associated with the leaf-traces and peripheral steles. The peripheral steles (fig. 296, A, B, p) form anastomoses with the leaf-traces and contribute to their formation.