PSILOTALES.

The two recent genera Psilotum and Tmesipteris are usually spoken of as members of the family Psilotaceae which is included as one of the subdivisions of the Lycopodiales. It is probable, as Scott[36] first suggested, that these two plants are more nearly allied than are any other existing types to the Palaeozoic genus Sphenophyllum.

We may give expression to the undoubted resemblances between Tmesipteris and Psilotum and the Sphenophyllales by including the recent genera as members of that group, originally founded on the extinct genus Sphenophyllum; this is the course adopted by Thomas[37] and by Bower[38]: or we may emphasise the fact that these two recent genera differ in certain important respects from Lycopodium and Selaginella by removing them to a separate group, the Psilotales. The latter course is preferred on the ground that the inclusion of Psilotum and Tmesipteris in a group founded on an extinct and necessarily imperfectly known type, is based on insufficient evidence and carries with it an assumption of closer relationship than has been satisfactorily established.

The genus Tmesipteris ([fig. 120], A) is represented by a single species T. tannensis Bertr.[39] which usually occurs as an epiphyte on the stems of tree-ferns in Australia, New Zealand, and Polynesia. Psilotum, with two species P. triquetrum Sw. ([fig. 118]) and P. complanatum Sw., flourishes in moist tropical regions of both hemispheres, growing either on soil rich in organic substances or as an epiphyte. Both genera are considered to be more or less saprophytic.

Fig. 118. Psilotum triquetrum (½ natural size).

  1. Synangium.
  2. Sporophyll after removal of the synangium. (M.S.)

Psilotum. The common tropical species P. triquetrum ([fig. 118]) is characterised by an underground rhizome which forms a confused mass of dark brown branches covered with filamentous hairs as substitutes for roots and gives off erect repeatedly forked aerial shoots. In P. complanatum[40] the habit is similar to that of the more abundant and better-known species, but the pendulous shoots are characterised by their broader and flatter form. In both species the function of carbon-assimilation is performed by the outer cortex of the green branches, as the small size of the widely-separated foliage leaves renders them practically useless as assimilating organs.

The sporophylls consist of a short axis terminating in two small divergent forks and bearing on its adaxial surface a trilocular or in rare cases a bilocular synangium ([fig. 118], A and B). The walls of the loculi are composed of several layers of cells and dehiscence takes place along three lines radiating from the centre of the synangium. Professor Thomas[41] has recorded “fairly numerous instances in Psilotum of a second dichotomy of one branch of the first fork, or, less frequently, of both branches”: instead of one synangium subtended by the two slender leaflets of the forked sporophyll-axis, there may be two synangia and three leaf-lobes or three synangia and four leaf-lobes. The occurrence of both these abnormalities in Psilotum and Tmesipteris shows a decided tendency in the Psilotales to a repeated dichotomy of the sporophylls[42].

A single stele[43] with a fluted surface occupies the axis of an aerial shoot ([fig. 119], A); the axial region is occupied by a core of elongated mechanical elements (s), which may occasionally extend to the periphery of the xylem and break the continuity of the band of scalariform tracheae ([fig. 119], A, a). The tracheae form the arms of an irregularly stellate stele and each arm is terminated by protoxylem elements ([fig. 119], B, px). The rays of the xylem cylinder, which may be as many as six or eight in the upper part of the aerial shoots, become reduced in number as the rhizome is approached, assuming a diarch structure near the junction. In the rhizome the xylem forms an approximately triangular group of tracheae without any core of mechanical elements. Three to four layers of parenchyma succeeded externally by an ill-defined phloem ([fig. 119], A, p) surround the xylem and a fairly distinct endodermis ([fig. 119], A and B, e) encloses the whole. To Mr Boodle[44] is due the interesting discovery that in some parts of the rhizome the parenchymatous zone surrounding the scalariform tracheae may become the seat of meristematic activity which results in the production of secondary tracheae often characterised by a sinuous longitudinal course. There is no definite cambium, but the radially disposed tracheae and the adjacent parenchymatous elements clearly demonstrate the secondary nature of the tissue immediately external to the group of primary xylem. Fig. 119, C, drawn from a section kindly supplied by Mr Boodle, shows the secondary xylem elements at x2 associated with radially disposed thin-walled cells abutting on the primary xylem, x1. It is probable that this added tissue may be a remnant of a more extensive secondary thickening characteristic of the ancestors of the recent species. In their manner of occurrence and sinuous course these secondary tracheids bear a resemblance to the secondary xylem of Lepidodendron fuliginosum[45]. The stele of the aerial shoot bears a fairly close resemblance to the vascular axis of Cheirostrobus, and its three-rayed form in the lower portions of the green branches recalls that of the Sphenophyllum stele, except that the axial xylem elements of the Palaeozoic genus are usually represented in Psilotum by mechanical tissue. The cortex consists of three regions ([fig. 119], A), an outer zone of chlorophyllous tissue (a) rich in intercellular spaces succeeded by a band of mechanical tissue (b) which gradually passes into an inner region of larger and thinner-walled cells (c).

Fig. 119.

  1. Diagram of transverse section of aerial shoot of Psilotum triquetrum. a—c, cortex; p, phloem; e, endodermis; s, stereome; x, xylem; a, gap in xylem.
  2. Enlarged view of one of the angles of the xylem shown in A. px, protoxylem.
  3. Part of transverse section of an approximately triangular rhizome stele showing a portion of the metaxylem x1; px, protoxylem elements; x2, secondary xylem.

TMESIPTERIS

The genus Tmesipteris[46] agrees with Psilotum in general habit and in its epiphytic and probably in some degree saprophytic mode of life. Its brown rootless rhizome, which grows among the roots of tree-ferns or rarely in the ground, gives off pendulous or erect shoots reaching a length of two feet and bearing lanceolate mucronate leaves 2–3 cm. long ([fig. 120], A) attached by decurrent leaf-bases. The sporophylls, replacing the upper leaves or occurring in more or less well-defined zones alternating with the foliage leaves, consist of a short axis terminating in a pair of lanceolate lobes and bearing on its adaxial surface an elongated bilocular synangium attached to a very short stalk ([fig. 120], B). Reference has already been made to the divergent opinions as to the morphological nature of the sporophylls or sporangiophores, but recent investigations distinctly favour the view that a sporophyll is best interpreted as a stalked leaf with two sterile laminae and an almost sessile, or in some cases a more obviously stalked, synangium; the whole sporophyll is characterised by the possession of a ventral and a dorsal lobe[47]. The drawings reproduced in [fig. 120], D and F, illustrate some of the frequent variations described by Thomas in plants which he observed in the New Zealand forests. The sporophyll shown in [fig. 120], D and F, has branched twice and bears three synangia.

Fig. 120. Tmesipteris.

The aerial branches of Tmesipteris possess a central cylinder of separate xylem groups in which the protoxylem occupies an internal position ([fig. 120], C and E, px) enclosing an axial parenchymatous region. The cells of a few layers of the inner cortex immediately outside the endodermis are rendered conspicuous by a dark brown deposit. The cortex as a whole is composed of uniform parenchymatous tissue. In the lower part of the aerial shoots and in the rhizome the xylem forms a solid strand without protoxylem elements and conforms more clearly to that of Psilotum.

In this short account of the anatomy of Tmesipteris no mention is made of the effect produced on the stele by the departure of leaf-traces and of vascular stands to supply branches. Miss Sykes[48] in a recently published paper on the genus has shown that the exit of a leaf-trace does not break the continuity of the xylem of the stele, while the exit of a sporophyll-trace is marked by an obvious gap. Evidence is adduced in support of the conclusion that this difference, which at first sight appears to be one of morphological importance, is in reality merely a question of degree and “is due to the earlier preparation for the formation of ‘sporophyll’ than leaf-traces.” Miss Sykes gives her adherence to the view that the “sporophylls” of Tmesipteris are branches and not leaves, but despite the arguments advanced this interpretation seems to me less probable than that which recognises the sporophyll as a foliar organ. Prof. Lignier[49] has pointed out that if Miss Sykes’s conclusion as to the axial nature of the sporophyll in Tmesipteris is accepted, it diminishes the force of the comparison between the sporophylls of that genus and Sphenophyllum as those of the latter can hardly be regarded as other than foliar organs.

Both members of the Psilotales may, as Boodle has suggested, be regarded as descendants of a common parent in which the aerial stems possessed a fluted or stellate cylinder of mesarch xylem. There can be no doubt as to the significance of the morphological resemblances between the Psilotales and the genera Sphenophyllum and Cheirostrobus, but the position of Tmesipteris and Psilotum in the plant-kingdom may probably be best expressed by adopting the group-name Psilotales rather than by transferring the recent genera to the Sphenophyllales. One of the most striking differences between the Psilotales and the genus Lycopodium is in the form of the sporophylls and sporangia; in Lycopodium a single sporophyll bears a unilocular sporangium, but in the Psilotales the sporophyll may be described as a bilobed structure homologous with a foliage-leaf, bearing a sporangiophore which consists of a short stalk terminating in a bilocular or trilocular synangium; the short stalk receives a special branch from the vascular bundle of the sterile portion of the sporophyll[50].

Fossils described by authors as being closely allied to Psilotum.

A search through palaeobotanical literature reveals the existence of a very small number of specimens which have been identified as representatives of the Psilotales. An inspection of the material or published drawings leads one to the conclusion that practically no information of a satisfactory kind is available in regard to the past history of the two southern genera Psilotum and Tmesipteris, which are regarded by some botanists as relics of an ancient branch[51] of pteridophytes.

PSILOTITES, ETC.

In 1842 Münster[52] instituted the genus Psilotites for a small impression of a slender branched axis from Jurassic rocks near Mannheim in Germany which he named Psilotites filiformis; Schimper[53] spoke of the specimens as too doubtful for determination, an opinion with which every botanist would cordially agree. Goldenberg’s species Psilotites lithanthracis[54] from the Saarbrücken coal-field is founded on impressions of axes: some of these are dichotomously branched and bear small oval projections, which may be rudimentary leaves or possibly leaf-scars. More recently Kidston[55] described specimens of branched axes from the Lanarkshire coal-field bearing a row of lateral thorn-like projections under the title Psilotites unilateralis; but these fragments, as Dr Kidston himself admits, are of no botanical value.

In a paper on fossil Salvinias, Hollick[56] mentions Salvinia reticulata, originally described by Heer and by Ettingshausen and S. Alleni Lesq.[57] a Tertiary species, and calls attention to their very close resemblance in form, nervation, and apex to the leaves of the genus Tmesipteris: he refers both species to that genus. The drawings reproduced by Hollick represent leaves with a midrib and numerous anastomosing lateral veins, whereas in Tmesipteris the lamina of the leaf has a midrib without lateral branches. An enlarged drawing of the outlines of the epidermal cells would correspond closely with the small reticulations in the fossil leaves and it may be that there has been some confusion between veins and cell-outlines. In any case there would seem to be no reason for the use of the recent generic name[58].

Among other fossils assigned to the Psilotales we have Marion’s genus Gomphostrobus from the Permian of France and Germany[59]. Marion placed this plant in the Coniferales on the strength of its resemblance to Walchia and Araucaria, but Potonié[60] is inclined to recognise in the leaves and monospermic sporophylls characters suggestive of Lycopodiaceous affinity.

The latter author in 1891[61], in ignorance of Marion’s proposal to adopt the name Gomphostrobus, instituted a genus Psilotiphyllum for the sporophylls of a species originally described by Geinitz[62] as Sigillariostrobus bifidus, but he subsequently adopted Marion’s designation and with some hesitation included the French and German specimens in the Psilotales. As stated elsewhere[63], Potonié’s arguments in favour of his view hardly carry conviction, and it is probably more in accordance with truth to deal with Gomphostrobus in the chapter devoted to the Coniferales.

Psilophyton.

The generic title Psilophyton, instituted by the late Sir William Dawson[64], has become familiar to geologists as that of a Pre-Carboniferous plant characteristic of Devonian and Silurian rocks in Canada, the United States of America, and Europe. From the botanist’s point of view the name stands for miscellaneous remains of plants of different types and in many cases unworthy of record. The genus was founded on impressions of branched axes from the Devonian strata of New Brunswick resembling the rachis and portions of lateral pinnae of ferns or the forked slender twigs of a Lycopod. The type-species Psilophyton princeps Daws. as represented on somewhat slender evidence in Dawson’s restoration, which accompanies the original description of the genus and has since been copied by several authors, is characterised by the possession of a horizontal rhizome bearing numerous rootlets and giving off dichotomously branched aerial shoots with spinous appendages, compared with rudimentary leaves, and terminating in slender branchlets bearing pendulous oval “spore-cases” from their tips. Some of the branchlets exhibit a fern-like vernation. The plant is spoken of by Dawson as apparently a generalised type[65], resembling in habit and in its rudimentary leaves the recent genus Psilotum and presenting points of contact with ferns. Specimens were found in an imperfectly petrified state showing a central cylinder of scalariform tracheae surrounded by a broad cortical zone of parenchyma and fibrous tissue.

Among other species described by the author of the genus we need only mention Psilophyton robustius, characterised by vegetative shoots and “spore-cases” similar to those of the type-species; but, as Solms-Laubach[66] has pointed out, the petrified sections referred by Dawson to P. robustius are of an entirely different anatomical type from that of P. princeps[67].

British fossils from the Old Red Sandstone from the north of Scotland, Orkney and Caithness, originally figured by Hugh Miller and compared by him with algae but more especially with recent Lycopods, were subsequently placed by Carruthers[68] in the genus Psilophyton as P. Dechianum, the specific designation being chosen on the ground that the Scotch specimens are specifically identical with fossils described by Goeppert[69] as Haliserites Dechianus.

Various opinions have been expressed in regard to the nature of the Devonian species Haliserites Dechianus Goepp. with which Carruthers[70] identified Miller’s Old Red Sandstone plant: reference may be made to a paper by White[71] containing figures of dichotomously branched impressions described as species of Thamnocladus which he includes among the algae.

In describing some Belgian impressions of Devonian age as Lepidodendron gaspianum Daws. Crépin[72] states that Carruthers has come to regard the specimens named by him Psilophyton Dechianum as branches of a Lepidodendron; he also quotes Carruthers as having expressed the opinion that the name Psilophyton had been employed by Dawson for two kinds of fossils, some being twigs of Lepidodendron while others, identified by Dawson as the reproductive branches of species of Psilophyton, represent the spore-cases of ferns comparable with Stur’s genus Rhodea[73]. One of the examples figured by Carruthers[74] as P. Dechianum from Thurso (preserved in the British Museum, no. 52636), measuring 34 cm. in length and 8 mm. broad, bears a close resemblance to a fern rhizome covered with ramental scales such as that of a species of Davallia. Other Belgian specimens described by Gilkinet[75] as Lepidodendron burnotense, like Crépin’s species, are no doubt generically identical with some of the Scotch and Canadian fossils placed in the genus Psilophyton, though Penhallow[76] considers that the species Lycopodites Milleri is more correctly referred to Lycopodites than to Psilophyton.

A more recent paper on the Geology of the Perry basin in South-eastern Maine by Smith and White[77] contains a critical summary of the literature on Psilophyton and drawings of specimens. The latter afford good examples of Pre-Carboniferous plant fragments, such as are often met with in various parts of the world, which conform in habit to the New Brunswick specimens made by Dawson the type of his genus.

An examination of material in the Montreal Museum and of Hugh Miller’s specimens in the Edinburgh collection leads me to share the opinion of Count Solms-Laubach that the name Psilophyton has been applied to plants which should not be included under one generic title. As Kidston[78] pointed out, the Canadian species Psilophyton robustius is not generically distinct from British and Belgian specimens referred to Lepidodendron; it may possibly be identical with the Bohemian plants on which Stur founded his genus Hostinella[79]. The Devonian plants described by Stur have since been examined by Jahn[80] who regards them as vascular plants, and not as algae to which Stur referred them; he mentions two species of Psilophyton but gives no figures.

The “spore-cases” of Dawson may be found to be the microsporangia or perhaps the small seeds of some pteridosperm; the forked axes with a smooth surface and others figured by Miller and by Dawson, with the surface covered with scales suggesting the ramenta of a fern, may be the rachises or rhizomes of filicinean plants. Other specimens may be Lepidodendron twigs, as for example the petrified fragments figured by Dawson as Psilophyton princeps; while the stem identified as P. robustius is most probably that of a Gymnosperm. It is doubtful whether a useful purpose is served by retaining the genus Psilophyton. It was in the first instance instituted on the assumption, which cannot be upheld, that the abundant material in the New Brunswick beds bore a sufficiently close resemblance to the rhizome and aerial branches of Psilotum. Psilophyton has served as a name for miscellaneous plant fragments, many of which are indeterminable. Dr White concludes his account of the genus with the following words[81]:

“The examination of such so-called Psilophyton material as I have seen shows the existence in America of two or more groups, represented by several fairly well-marked species which possess stratigraphical value, and which should be carefully diagnosed and illustrated. It is probable also that additional material throwing light on the structure and relationships of these very remarkable early types of land-plants will be discovered at some locality. The inspection of the material in hand emphasises the need, as was pointed out by Solms-Laubach, for the revision of the material referred by various authors to Psilophyton, together with a thorough re-examination and re-publication of the types.”

Until a thorough re-examination has been made of the Canadian material, with a view to determine whether there exist substantial reasons for the retention of Dawson’s genus, it is undesirable to continue to make use of this name for Pre-Carboniferous fossils which are too incomplete to be assigned with certainty to a definite group of plants. Dr White draws attention to the similarity of some of the Perry basin specimens to Nathorst’s genus Cephalotheca[82] from Devonian rocks of Bear Island in the Arctic regions, a comparison which might be extended to other genera and which serves to illustrate the possibility that many of the specimens labelled Psilophyton may eventually be recognised as examples of well defined generic types belonging to more than one group of plants.