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 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.
The vascular region is surrounded by a parenchymatous cortex, which in younger plants, or in the apical region of an older plant, forms the surface of the stem to which the leaf-stalks are attached. From the peripheral steles, or from the more external bands of the vascular network, roots are given off which pass in a sinuous vertical course through the cortex, appearing on the surface between the leaf-bases. In older stems, after leaf-fall, the tissue immediately external to the vascular region produces secondary parenchyma with which the roots become intimately associated by their outermost cells. As a result of the secondary cortical development and the gradual increase in the number of roots invading the cortical tissue from above, the stem is enclosed by a cylinder of roots and associated parenchymatous tissue of secondary origin. In still older portions of a stem the more external roots are free from the stem-cortex and form a thick felted mantle, which increases in thickness towards the base of the tree.
The roots ([fig. 296], E) are polyarch, 5–10 groups of xylem alternating with strands of phloem, and similar in structure to those of recent species of Marattia and Angiopteris; the stele is enclosed by an inner cortex of compact or lacunar tissue containing secretory sacs, and this is surrounded by a cylinder of mechanical tissue. In one or two instances secondary xylem has been observed wholly or partially enclosing the root-stele[1065].
Fig. 296.
- Psaronius infarctus (P, peripheral steles; L, leaf-traces).
- P. infarctus, longitudinal tangential section through the peripheral region of the stem.
- P. coalescens.
- P. musaeformis.
- P. asterolithus (root).
(A—C, E, after Zeiller; D, after Stenzel.)
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.