CHAPTER XXIII.

Psaronieae.

This family name, first suggested by Unger, may be conveniently adopted for the numerous species of petrified tree-fern stems characteristic of the Lower Permian and Upper Carboniferous strata. In his monograph Über die Staarsteine published in 1854, Stenzel[1053] referred to the Psaronieae as a special subdivision of the Filices most nearly allied to the Polypodiaceae. There is now a consensus of opinion in favour of including Psaronius in the Marattiales, or at least of regarding the genus as more closely allied to the Marattiaceae than to any other family. While admitting that the balance of evidence is in favour of this view, it is probably wiser to retain the distinctive term Psaronieae on the ground that species of Psaronius differ in several respects from any recent ferns, and because of our comparative ignorance in regard to the nature of the fructification.

Psaronius.

This generic name was proposed by Cotta in his classic work Die Dendrolithen[1054]. The stems so named, formerly included by Sprengel[1055] in the genus Endogenites, had long been familiar as petrified fossils. Most of the specimens described by the earlier writers were obtained from Lower Permian rocks in the neighbourhood of Chemnitz, Saxony. The mottled appearance presented by their polished surfaces is said to have given rise to the appellation Staarsteine (starling stones), a term expressing a resemblance, more or less remote, to a starling’s breast. It has been suggested that this word is a corruption of Stern Steine or star stones[1056], a descriptive term suggested by the stellate arrangement of the vascular strands in transverse sections of the roots. Parkinson[1057], in his Organic Remains of a former World, speaks of these stems as starry stones. The history of our knowledge prior to 1854 is summarised by Stenzel. At first compared with corals or the stems of sea-lilies, Psaronii were recognised by Sprengel, who first used a lens in the examination of the fossils, as fern stems most nearly allied to those of recent Cyatheaceae. By other authors, e.g. Schlotheim and Sternberg, they were referred to Palms, and by Brongniart considered to be the lower portions of Lycopodiaceous (Lepidodendron) stems. Corda and many subsequent authors selected the Marattiaceae as the most closely allied family among existing plants.

Psaronius is represented by specimens obtained from the Lower Permian of Saxony and Upper Carboniferous rocks in Central France, also from Bohemia, Brazil and North America. As yet a few fragments only have been found in the English Coal-Measures. The genus was recognised by Williamson[1058] who described the roots and a small piece of the vascular tissue of a stem which he called P. Renaulti, and this type has since been more fully described by Scott[1059]. The roots of another species have been described by Butterworth[1060] as P. Cromptonensis.

It was pointed out in the account of Lepidodendron that several generic names have been used for the same type of stem in different states of preservation; in Psaronius accidents of fossilisation have been responsible for a similar confusion in nomenclature. The name Psaronius is applied to petrified specimens which, as a rule, lack external features. Casts or impressions of Palaeozoic tree-fern stems provided with leaf-scars are described as species of Caulopteris, Megaphyton, and less commonly as Ptychopteris ([figs. 297–299]). The first name is applied to stems exhibiting spirally disposed leaf-scars like those of recent tree-ferns; in Megaphyton the scars are distichously arranged, in two rows, while Ptychopteris is applied to decorticated stems. These terms are used for stems belonging to one generic type and possessing the structure of Psaronius stems.

Fig. 295. Psaronius stem with roots. (Much reduced. After Grand’Eury.)

The researches of Grand’Eury[1061] led to the discovery that certain Psaronius stems bore fronds of the Pecopteris type some of which bore sori of the Asterotheca or Scolecopteris type. The same author[1062] has also contributed many interesting facts, obtained by an examination of the relation of Psaronius stems to the sediments of French Coal-fields in which they occur, in regard to habitat and manner of growth. The specimen represented in [fig. 295] shows a portion of a Psaronius stem, the upper part of which illustrates the Caulopteris state of preservation, while the lower part is covered by a mass of roots. It is probable, as Rudolph[1063] suggests, that this rich development of roots, which gives to an old Psaronius stem the appearance of an elongated cone, may have served an important mechanical purpose analogous to the secondary thickening in a Dicotyledon or a Conifer. A specimen of Psaronius Cottai in the Hofmuseum, 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 anatomical features of many other Palaeozoic genera.

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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.

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.

The form of some of the vascular bands in the section of Psaronius infarctus shown in [fig. 296], A, illustrates the occasional anastomosing of one dictyostele with another: the different degrees of looping of other bands represent stages in the giving off of leaf-traces which eventually pass out as V-shaped meristeles. Beyond the leaf-traces and sclerenchymatous bands the section consists of transverse sections of adventitious roots.

The surface-features of Psaronius infarctus are probably represented, as Zeiller points out, by the cast described by Lesquereux as Caulopteris peltigera ([fig. 298], A).

Fig. 297. Pecopteris Sterzeli: a, pinnule. (After Renault and Zeiller. ¹⁄₁₁ nat. size.)

The Psaronius shown in [fig. 297] is one of the few examples illustrating the connexion between fronds and stem. The leaf (Pecopteris Sterzeli Zeill. and Ren.[1071] is quadripinnate and is described as reaching a length of at least 3 metres; the ultimate segments are entire or lobed. The stem is characterised by elliptical scars, 6–8 cm. x 3·5–4 cm., with leaf-traces like those in Caulopteris peltigera. The fronds of Pecopteris Pluckeneti, a Pteridosperm, bear a very close resemblance to those of P. Sterzeli, which are as yet known only in a sterile state.

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Psaronius brasiliensis Unger, a species founded by Unger on a piece of silicified stem acquired by Martius in Brazil and now in the Rio Museum, is a good example of a tetrastichous species. Solms-Laubach[1072] has recently told the history of this type, which is represented by sections, cut from the Rio stem, in several European collections. A well-preserved section in the British Museum is figured by Arber[1073] in his catalogue of the Glossopteris flora and by other authors. Scott gives a concise description of the species in his Studies in Fossil Botany[1074]. The roots of P. brasiliensis are stated by Pelourde[1075] to have a lacunar cortex.

Psaronius musaeformis Corda[1076]. Fig. 296, D.

This species from the Lower Permian of Chemnitz and the Coal-Measures of Bohemia affords an example of the distichous type in which the leaves are borne in two rows. The vascular bands, as seen in a section of the dictyosteles, occur in regular parallel series. The stelar region is separated from the cylinder of encasing roots by a sclerenchymatous sheath, broken at intervals where roots pass out from the vascular region.

Psaronius coalescens[1077] ([fig. 296], C) illustrates a somewhat different arrangement of vascular tissue which approaches more closely to the polycyclic structure characteristic of such recent ferns as Matonia and Saccoloma. A still closer resemblance to the solenostelic type is seen in Psaronius Renaulti from the Lower Coal-Measures of England which Scott[1078] describes as characterised by a single annular stele, interrupted only by the exit of leaf-traces. As he points out, it is noteworthy that this species is distinguished by the simplest form of stele met with in the genus; it is the oldest species and may be regarded as the most primitive representative of the genus Psaronius so far discovered.

Fig. 298.

Caulopteris peltigera.
Megaphyton insigne.

(After Grand’Eury.) Much reduced.

Psaronius stems preserved as casts showing surface-features, or in a decorticated state.

i. Caulopteris.

This generic name was instituted by Lindley and Hutton[1079] for tree-fern stems from the English Coal-Measures showing circular or oval scars arranged quincuncially. The vascular tissue of the petiole is represented by a U-shaped impression on the scar, the ends of the U being incurved, or by a closed oval ring with a wide-open and inverted V near its upper end. The surface between the leaf-scars bears the impression of adventitious roots. Caulopteris is represented, in the Upper Coal-Measures of England, by C. anglica[1080] Kidst. The species C. peltigera ([fig. 298], A), originally described by Brongniart as Sigillaria, illustrates the closed form of leaf-trace and, as Zeiller suggests, it is the cast of a Psaronius stem which possessed a vascular system on the same plan as that of P. infarctus. C. Saportae[1081] illustrates the open U-shaped type of petiole stele.

Caulopteris peltigera has scars measuring 6–9 by 4–6 cm.; it occurs in the Commentry Coal-field of France in association with the fronds known as Pecopteris cyathea, a species which Kidston regards as identical with P. arborescens[1082].

ii. Megaphyton.

The first use of this name was by Artis[1083], who gave it to a long flattened cast, Megaphyton frondosum, found in Carboniferous strata in Yorkshire, characterised by two vertical rows of large scars and by impressions of sinuous roots. Kidston records the genus from the Middle and Upper Coal-Measures of Britain. A good example of this type of cast is afforded by M. McLayi Lesq.[1084] from the Coal-Measures of North America, which has been recognised in European Carboniferous rocks. The leaf-scars are rounded or oval, broader than high; the vascular impression has the form of a closed ring (5–8 × 3–6 cm.), more or less circular and with a tendency to a rectangular outline, characterised by a deep inverted U-shaped sinus in the middle of the lower surface and by a W-shaped impression of an internal strand ([fig. 298], B)[1085].

iii. Ptychopteris.

This generic name, instituted by Corda[1086], is applied to decorticated stems of Psaronius, the surface of which is that of the vascular region on which the form of the leaf-scars is more or less clearly defined. The scar-areas are limited by an impression of the sclerenchymatous sheath enclosing the leaf-meristele, and internal to this is the impression of the leaf-trace. In some specimens a layer of coaly material which represents the carbonised cortex and adventitious roots covers the Ptychopteris cast. The Ptychopteris cast represented in [fig. 299] shows the decorticated surface of part of a long stem on which the leaf-scars are arranged as in Megaphyton. An example of Ptychopteris is figured by Fontaine and White[1087] from Virginia as Caulopteris gigantea.

Fig. 299. Ptychopteris. ⅙ nat. size. From the Middle Coal-Measures of Lancashire. (The Manchester Museum.)

Fig. 300. Dicksonia antarctica (half of stem in transverse section): st, stele; s, sclerenchyma.

Position of Psaronius.

A comparison of Psaronius with the Marattiaceae and other recent ferns leads to the conclusion that, on the whole, the evidence is in favour of the view usually held, namely that this genus is more closely related to the Marattiaceae than to any other recent ferns. It is, however, important not to overlook the differences between Psaronius and recent genera of Marattiaceae, or the resemblances between the extinct genus and the Cyatheaceae. In habit Psaronius agrees closely with recent tree-ferns; in the vascular system and in the sequence of events connected with the production of leaf-traces, there are striking resemblances between Psaronius and the Cyatheaceous fern Saccoloma adiantoides (= Dicksonia Plumieri Hook.) as described by Mettenius[1088]. The piece of stem of Dicksonia antarctica represented in [fig. 300] exhibits a fairly close agreement with species of Psaronius, e.g. P. infarctus ([fig. 296], A, B). Moreover, the peripheral steles, which Zeiller has shown are confined to the stem and play an important part in the production of the roots and in the anastomoses with leaf-traces, are not represented in any Marattiaceous fern; on the other hand, they are comparable with the accessory strands met with in stems of recent Cyatheaceous tree-ferns[1089] (cf. [fig. 240]). The complex system of concentric dictyosteles is a feature more closely matched in Angiopteris (Marattiaceae) than in any Cyatheaceous genus, the chief difference being in the more band-like form of the steles in Psaronius, though in a stem of Angiopteris figured by Mettenius we see a close approach to the extinct type. The position of the protoxylem has unfortunately not been clearly defined in Psaronius stems, but in P. Renaulti it is stated by Scott[1090] to be endarch, a position which some of the protoxylem strands occupy in Angiopteris[1091]. The occurrence of large sieve-tubes described by Scott in P. Renaultii is another feature shared by recent Marattiaceae. In many of the continental species of Psaronius the phloem has not been preserved, and our knowledge of this tissue is comparatively meagre. In the Marattiaceae the roots arise mainly from the inner portions of the stele, while in Psaronius they are usually formed from the external vascular bands. The formation of secondary cortical tissue is a peculiarity of Psaronius; on the other hand, if Butterworth[1092] is correct in referring to that genus the roots with secondary xylem, which he describes as P. Cromptonensis, a comparison may be made with the occurrence of secondary tracheae in the stem steles of Angiopteris[1093].

The absence of mechanical tissue in the stem of Angiopteris is in contrast with its occurrence in the fossil stems and in recent tree-ferns; but this is a character of secondary importance and one which can be readily explained by the difference in habit between Angiopteris and Psaronius.

The roots of Psaronius, more especially as regards the stelar structure, are in close agreement with those of Marattiaceae.

The reference to Marattiaceae of the great majority of fertile fern-like fronds from Permian and Carboniferous rocks constituted a strong a priori argument in favour of including Psaronius stems in the same family, especially when it was known that leaves with Marattiaceous synangia were borne by species of this genus. It is, however, well to remember the change in our views as to the dominance of Marattiaceae in Palaeozoic floras consequent on the discovery of the Pteridosperms. The association of fronds bearing Asterotheca and Scolecopteris types of fructification with Psaronius stems recorded by Grand’Eury[1094] is a point in favour of the Marattiaceous affinity of this extinct genus, but it is not impossible that Psaronius stems bore fronds which produced Pteridosperm organs of reproduction. In this connexion the specimen represented in [fig. 297] is of interest, as the fronds (Pecopteris Sterzeli) borne on the Psaronius stems are hardly distinguishable from the seed-bearing leaves known as Pecopteris Pluckeneti.

The position of Psaronius may be best expressed by assigning it to a separate family, the Psaronieae, as advocated by Stenzel, and by regarding it as one of the many instances of a generalised type which in the sum of its characters approaches most nearly to the Marattiaceae.