XXIIl] PSAKONIUS 415 



elongated cone, may have served an important mechanical 

 purpose analogous to the secondary thickening in a Dicotyledon 

 or a Conifer. A specimen of Psaronius Gottai in the Hof 

 Museum, Vienna, is cited in illustration of the enormous breadth 

 of the root-system : the radii of the stem proper and of the 

 encasing cylinder of roots bear the ratio 17 to 165. The 

 comparatively frequent occurrence of a lacunar cortex in the 

 roots points to the growth of the stems in swampy ground, a 

 conclusion in harmony with the evidence afforded by the ana- 

 tomical features of many other Palaeozoic genera. 



Psaronius may be briefly defined as follows : — 



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 difierent form of meri- 

 stele. 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 endarchi. 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, even- 

 tually 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 



1 Scott (08) p. 302. 



