The specimen on which the diagrammatic drawing reproduced in fig. 419 is based has been investigated by Weber and Sterzel[277] and by Solms-Laubach[278]. The figure is a slightly simplified version of that given by Weber and Sterzel; it represents the stem of Medullosa Leuckarti as a transparent object, the two lower transverse sections, B and C, being seen in perspective through the longitudinal faces. The steles are shaded obliquely in the longitudinal sections, and in the three transverse sections, A, B, C, the primary xylem (partial pith) is black and the enclosing secondary vascular tissue radially shaded. The whole block is 9 cm. in length and 6 cm. broad. Only a part of the axial region is shown internal to the peripheral snake-rings and in it are the star-rings S, S, b, and c. Outside the main steles is the narrow cortex R and portions of leaf-bases I–IV. The lowest section, C, shows part of a peripheral snake-ring with a slight swelling at f on its inner side which, as seen in sections B and A, foreshadows the separation of the star-ring S and the consequent break in the continuity of the snake-ring (d, e, sect. B). In section A the gap is closed: in the longitudinal section between B and A the star-ring S is seen to form two branches, a and b, the branch a closing the gap between d and e in section B. These sections demonstrate the formation of a star-ring from the main peripheral stele and the formation of additional star-rings by branching.
Numerous vascular bundles destined for the leaves are scattered in the cortex. The course of the decurrent leaf-base I is shown on the longitudinal faces, its boundary being marked by crowded stereome strands (of the Myeloxylon Landrioti type); other leaf-bases are represented by II, III, and IV.
In habit Medullosa Leuckarti differs from such a type as M. stellata in its relatively shorter and stouter stem and in the shorter internodes.
i. Leaves. It has already been stated that in some cases petioles occur in organic connexion with Medullosan stems, notably in M. anglica and M. Leuckarti: in the exceptionally rich collection in the Chemnitz Museum, which forms a fitting memorial of the work of the late Prof. Sterzel, there is a stem of M. Leuckarti bearing large petioles of the type known as Myeloxylon radiatum. The occurrence of vascular bundles in the cortex of other species of stem identical with those in the attached petioles points to a uniform type of leaf-structure so far as regards the petioles and rachises of Medullosa. While it is clearly unnecessary to distinguish by a special generic title the petrified portions of fronds known to belong to certain species of stems, the frequent occurrence of detached petioles necessitates some distinctive term. The name employed is Myeloxylon: the genus was instituted by Brongniart in 1849 for Cotta’s species Medullosa elegans the petiolar nature of which was suspected by Binney in 1872.
| 1832. | Medullosa elegans Cotta. |
| 1865. | Stenzelia Goeppert. |
| 1876. | Myelopteris Renault[279]; |
| 1877. | Aulacopteris Grand’Eury[280]. |
There is a very close agreement in general anatomical structure between the numerous specimens of Myeloxylon from the Permian strata of Saxony and France and the Coal Measures and Millstone Grit of England[281]; the genus is also recorded from the Upper Carboniferous of Kansas[282]. Two well-defined types instituted by Renault are, however, readily distinguished by the form of the hypodermal stereome strands. Myeloxylon may be defined as follows: Oval or cylindrical branched axes, reaching a diameter of 15 cm., bearing pinnae having the characters of Alethopteris, Neuropteris, Odontopteris, and some other genera that were formerly classed as Ferns. Below a single-layered epidermis, in which stomata have been recognised, occur a few layers of parenchyma: this superficial tissue, which is rarely preserved, is succeeded by a hypodermal region consisting of parenchymatous tissue and numerous vertical groups of narrow thick-walled fibres arranged as radial plates or circular, oval, or reniform strands (the Sparganum type of cortex). In the hypoderm as in the ground-tissue generally secretory canals, often accompanied by stereome, are a characteristic feature. The vascular system is represented by a considerable number of collateral bundles scattered through the ground-tissue and especially abundant in the outer region: the bundles sometimes assume a more or less regular disposition in concentric circles. Each bundle consists of a small group of xylem tracheids, for the most part spiral or scalariform, though reticulately pitted elements are by no means rare, with a single protoxylem group on the outer face next the phloem (fig. 420, B, C). As a rule the xylem is wholly centripetal, but occasionally the exarch structure becomes mesarch by the occurrence of a few centrifugal tracheids. The phloem, rarely preserved (fig. 420, B), consists of narrow sieve-tubes with parenchyma, and the bundle as a whole is often partially enclosed by a sheath of fibres.
Superficially the anatomical structure is similar to that of the petioles of Angiopteris or Marattia, and both Williamson[283] and Renault placed Myeloxylon in the Ferns; but the collateral form of the vascular bundles, the position of the protoxylem, and the arrangement of the hypoderm tissues, are cycadean features.
This type is characterised by the radially elongated stereome of the hypoderm. Prof. Zeiller[284], who has given a very clear and concise description of Myeloxylon, is disposed to regard Cotta’s Medullosa elegans as a specifically distinct form on the ground that there are two concentric zones of stereome in the hypoderm; but this feature is shown only in one of Cotta’s figures, and Weber and Sterzel[285] point out that a doubting of the hypoderm zone may be caused by accidental juxtaposition of two faulted pieces of peripheral tissue. The drawing reproduced in fig. 420, A, shows the structural plan of an unusually large petiole from the Permian of Autun: a portion of the outer tissue is seen in fig. 418, A. The vascular bundle, fig. 420, C, from a Millstone Grit specimen[286], shows the centripetal nature of the xylem and fragments of phloem in the outer half of the bundle, with imperfectly preserved fibres abutting on the xylem. The characteristic hypoderm is shown also in fig. 418, A; the double xylem strand on the left illustrates a common feature caused by the branching of vascular bundles. Several secretory canals are scattered in the ground-tissue. The pinnules of Myeloxylon radiatum, or at least of some specimens, have been shown by Renault to be of the Neuropteris type.
In this species the distinguishing feature is the occurrence of the hypodermal stereome in the form of circular, oval, or reniform strands in place of the radial plates of M. radiatum. It is this form of petiole that was borne by the stems of Medullosa anglica and M. Leuckarti. In M. anglica the pinnules (fig. 420, D) are of the Alethopteris type, almost certainly A. lonchitica. Renault and Zeiller have described French specimens of Myeloxylon Landrioti bearing pinnules like those of Alethopteris aquilina and A. Grandini.
The occurrence of Myeloxylon petioles in the New World was recorded by the late Prof. Penhallow[288] who founded this species on some imperfectly petrified specimens from Upper Carboniferous strata at Topeka, Kansas. Enough material was available to show the Myeloxylon characters, but the preservation is too imperfect to admit of a complete diagnosis. The hypodermal stereome shows a tendency to form tangentially extended strands in place of the more circular or radially elongated groups in the European species.
In addition to Alethopteris, Neuropteris (including Cyclopteris) and Odontopteris fronds, which are known to possess rachises with the Myeloxylon features, there is reason to believe that the Permian Callipteris fronds and possibly some of the older Taeniopteris leaves may also belong to Medullosa[289]. It is, however, unsafe to assume that the occurrence of Myeloxylon petioles necessarily denotes the existence of Medullosa. The French stem Colpoxylon aeduense Brongn.[290] bore leaves with the same general anatomical features as those of a typical Myeloxylon, and there can be little doubt that other genera of the Medulloseae also possessed fronds constructed on the same plan as those known to have been borne by Medullosa. An interesting illustration of an injured organ that had produced a wound-cambium is afforded by a Myeloxylon petiole from the Coal Measures described by Mr Holden[291].
Reference was made in volume ii. to the reasons which led to the removal of several genera of Carboniferous and Permian fronds from the Filicales to the Pteridosperms, and in Chapter xxix. of the present volume certain species of Sphenopteris are described as the foliage of Lyginopteris and Heterangium. It is with such genera as Neuropteris, Alethopteris, Linopteris and others that we are now more especially concerned, as they represent some of the types of leaves borne by Medullosa and other members of the Medulloseae. The absence of any specimens among the large number of these common genera bearing undoubted sporangia aroused suspicion as to the correctness of the generally accepted view that these fern-like fossils were the leaves of Palaeozoic Ferns. Subsequently the suspicion based on negative evidence was confirmed by researches into the anatomical structure of the leaf-bases, petioles, and fragments of pinnae attached to and associated with stems of Medullosa. It is only in a few cases that actual organic connexion between reproductive organs and Medullosan leaves has been demonstrated, but from such facts as are established it is safe to make the general statement that stems of Medullosa—a generic term that undoubtedly includes plants which, had we a fuller knowledge of them as complete individuals, would be assigned to more than one generic type—possessed fronds simulating in habit those of certain Ferns with some of the pinnae bearing seeds often of considerable size and in all cases of complex structure, agreeing in many respects with those of existing Cycads, while other fronds, or in some cases it may be other pinnae, bore microsporangia similar in form to the sporangia of Ferns.
Several examples of supposed fertile specimens of Neuropteris are recorded in palaeobotanical literature, but it was not until 1887 that any satisfactory specimen was discovered. In that year Kidston[293] described a specimen of Neuropteris heterophylla from the Lower Coal Measures of Scotland in which slender forked branchlets bear small bodies at their tips some of which appear to represent four-valved organs (fig. 421, D), though the imperfect state of preservation renders impossible any definite pronouncement as to their structure. To the specimen are attached a few sterile pinnules, showing that it is a portion of a frond of N. heterophylla characterised by the substitution of reproductive organs for pinnules. The subsequent discovery of seeds attached to pinnae of the same species afforded strong presumptive evidence, almost amounting to proof, of the microsporangial nature of the Scotch specimen. For this specimen, although no precise diagnosis is possible, Dr P. Bertrand[294] has proposed the generic name Neurotheca. In 1911 the Abbé Carpentier[295] described some small ovoid bodies, 1–1·5 mm. long, from the Coal Measures of France arranged in groups of 4 to 6 and in some cases said to be borne on a slender pedicel which he found in association with N. heterophylla and compared with sporangia described by Lesquereux from the Coal Measures of Arkansas as Sorocladus stellata[296]. These supposed microsporangia have recently been assigned by Bertrand[297] to Sphenophyllum.
In 1899 Zeiller[298] instituted the name Potoniea for some peculiar fertile leaves found in the Coal Measures of Heraclea consisting of a branched axis with cuneate segments, 7–10 × 6–8 mm., bearing numerous fusiform bodies, 1–1·5 mm. long, at the upper edge (fig. 421, A). These marginal bodies he regarded as sporangia and expressed the opinion that Potoniea may be the fructification of some form of Neuropteris, Mariopteris or Alethopteris. Carpentier[299] afterwards described similar though rather larger examples from the Pas-de-Calais coal-field as microsporophyll fragments of some Pteridosperm: he stated that similar specimens had been found by Kidston in England. In a later work Carpentier[300] described the sporangia as crowded in groups (fig. 421, B) in the substance of the thick lamina of Potoniea, and he connected the fertile segments with Neuropteris gigantea, N. pseudogigantea Pot. and Linopteris obliqua (Bunb.). Similar specimens are said to have been found in Holland and Silesia. Bertrand[301] also records the association of Potoniea with Neuropteris gigantea and N. pseudogigantea: he describes some specimens as belonging to N. gigantea Sternb. while others, distinguished only by small differences, he attributes to N. pseudogigantea. Kidston[302] has recently drawn attention to the inconstancy of the characters mentioned by Potonié as distinguishing features of N. pseudogigantea, and he shows good cause for referring the examples so named to N. gigantea. The fertile lamina is almost orbicular in surface-view and attached to a slightly excentric pedicel; the microsporangia are borne on the lower surface and probably in groups as described by Carpentier.
Neuropteris Carpentieri Kidston. Kidston[303] has recently described some fertile leaflets under this name from the Westphalian series of South Staffordshire which he identifies with French examples referred by Carpentier[304] to Potoniea adiantiformis Zeill. The fertile pinnules are thick and sub-cyclopteroid in form; the upper surface bears densely packed, narrow and long, microsporangia, 4 × 0·5 mm., containing more or less spherical microspores 45–60μ in diameter; the ventral face of the lamina on the removal of the spores shows several strong veins. With these are associated sterile pinnules of the Neuropteroid type, and examples are described intermediate between the sterile and fertile leaflets. Kidston believes the specimens to be microsporophylls of some species of Neuropteris, but as the material does not suffice for identification with any known species a new name is proposed. It is pointed out that in some states of preservation the leaflets resemble Goeppert’s Permian species Dictyothalamus Schrollianus[305].
No specimens of Alethopteris fronds have so far been described that afford any information as to the nature of the microsporangia, and we have no means of knowing whether they were borne on naked pedicels as in Neuropteris heterophylla, or on modified pinnules as in N. gigantea.
Zeiller in 1888[306] described some fertile pinnules of Dictyopteris Schützii Roem. from Commentry bearing two rows of long sporangia: he subsequently transferred this species to the genus Linopteris[307] and expressed the opinion that the sporangia occur singly and not in groups as he originally believed. Zeiller compares the fertile pinnules with the type Crossotheca. Bertrand[308], as the result of examining similar specimens, has suggested that the fringe of pendulous bodies regarded by Zeiller as sporangia may be tooth-like lobes of modified pinnules which served to protect microsporangia borne on the lower surface of the lamina. The nature of the impressions is not clear, though there is little doubt that they are microsporophylls. The fertile pinnae of Linopteris obliqua (Bunb.) described by Carpentier[309] and Bertrand[310] closely resemble the microspore-bearing organs which have been referred to Neuropteris gigantea; they have the characters of Potoniea and consist of oval laminae similar to the sterile pinnules but about half their size: the lamina was attached excentrically to a slender stalk (fig. 421, C, E) and traversed by numerous occasionally anastomosing veins. No actual microsporangia have been discovered in organic connexion with the lamina.
Kidston’s discovery of undoubted seeds attached to pinnae of Neuropteris[311] marked an important step in our more exact knowledge of the morphology of Medullosan sporophylls. Specimens from the ironstone balls (Coal Measures) of Coseley near Dudley showed seeds attached to portions of pinnae bearing pinnules of Neuropteris heterophylla. The seeds are approximately 3 cm. long and from 1·10 to 1·40 cm. broad; oblong and gradually tapering from the middle to a slightly curved and obtuse apical snout (fig. 422). The outer surface shows numerous longitudinal ribs which no doubt represent hypodermal fibres. The preservation of the seeds, which appear to be circular in section, is not such as to render possible a description of structural features. In their asymmetrical form the seeds agree with the genus Platyspermum as recently defined by Arber, but Kidston’s specimens are in all probability radiospermic. Kidston compares the Neuropteris seeds with Rhabdocarpus tunicatus as figured from the Commentry coalfield[312] and with specimens from Gard named by Grand’Eury R. subtunicatus[313].
Additional proof of the occurrence of seeds on Neuropteris fronds is furnished by examples from the Coal Measures of Holland described by Kidston and Jongmans[314]: these seeds are of the same general type as those from Coseley but nearly twice as large, and they were borne at the tips of a dichotomously branched pedicel of Neuropteris obliqua. Grand’Eury in 1904[315] recorded the association of radiospermic seeds with Neuropteris fronds though no case of actual attachment was found. It is, however, noteworthy that he speaks of the frequent association with Neuropteris of seeds characterised by six or rarely twelve longitudinal keels, a feature recalling the sclerotesta of Trigonocarpus and allied seeds. Until petrified specimens are available it is impossible to refer the seeds of Neuropteris to a generic type founded on structural features: the seeds described by Kidston are, as he says, very similar in external characters to species assigned to Rhabdocarpus, and there can be little doubt as to the generic identity of the Neuropteris seeds and some of the impressions referred to Rhabdocarpus which are characterised by a similarity in form, an apical snout that gives an asymmetrical appearance to the specimens and the presence of numerous longitudinal striations[316]. It is, however, by no means certain that these seeds possessed the morphological features of Rhabdocarpus as described by Brongniart in petrified examples from St Étienne[317].
The seeds of Neuropteris may, as Kidston suggests, agree anatomically more closely with Pachytesta[318], a type that Grand’Eury associates with Alethopteris fronds. With a view to avoid the danger of incorrectly identifying petrified specimens and impressions that cannot be proved to belong to the same generic type, I have suggested the restriction of the name Rhabdocarpus[319] to seeds that do not furnish evidence as to internal structure, and the employment of the designation Rhabdospermum for seeds that conform to those described by Brongniart as Rhabdocarpus. The seeds of Neuropteris may be of the Rhabdospermum type or they may agree anatomically more closely with Trigonocarpus or Pachytesta; they are members of either the Trigonocarpales or the Cardiocarpales, probably the former group. Dr P. Bertrand[320] assigns to Neuropteris gigantea some seeds of the Hexapterospermum (= Hexagonocarpus[321]) type which occur in association with cupule-like organs. These supposed cupules resemble the Potoniea leaflets with microsporangia also referred to the same species of Neuropteris; they are characterised by a laciniate edge and may be compared with the Indian fossil described by Zeiller as Ottokaria bengalensis[322] (fig. 433). Dr Arber[323] and Dr P. Bertrand[324] have independently proposed the generic name Neurospermum for the seeds of Neuropteris heterophylla and N. obliqua in preference to Rhabdocarpus: the former author speaks of the seeds of N. heterophylla as Neurospermum Kidstoni. The generic term Neuropterocarpus used by Grand’Eury in 1904[325], though not defined by him, has priority and avoids the adoption of a new designation for seeds attached to Neuropteris fronds. In his definition of Neurospermum Arber makes no reference to the obliquity of the apical snout that is clearly shown in fig. 422. There is no evidence that Neuropterocarpus possessed a cupular investment comparable with that of Lagenostoma. Additional instances of the association of seeds with Neuropteris fronds are recorded by Renier from the Belgian Coal Measures, also by Bertrand and Chodat from France: Renier found seeds associated with N. Schlehani Stur and with the same species of frond Bertrand found impressions of oval ribbed seeds. Prof. Chodat[326] has figured some fragments of Neuropteris pinnules referred to N. auriculata Brongn. from the Stephanian of France which show small seed-like bodies apparently in organic connexion with the lamina; but the specimens are too imperfect to afford any satisfactory evidence as to the nature of the reproductive organs.
Lotsy[327] has expressed the opinion that the bodies attached to Neuropteris pinnae described by Kidston and other authors as seeds may possibly be vegetative buds, but if this were the case one would expect to find some evidence of the bud-nature in some at least of the specimens that have already been found.
It would seem that the microsporophylls of Neuropteris were constructed on different plans, some being of the type described by Zeiller and other observers as Potoniea, while others bore sporangia on pinnae without any accompanying laminae; but our knowledge of the latter form represented by Kidston’s specimen of Neuropteris heterophylla (fig. 421, D) is very incomplete. On the other hand the seeds appear to have been characterised by features suggesting a close affinity to Trigonocarpus and pointing to membership of the same family.
Although no specimens have been discovered showing actual connexion between fronds and seeds, it is practically certain that Alethopteris leaves, or at least some species of the genus, bore seeds of the Trigonocarpus type. The association of Trigonocarpus Parkinsoni with Alethopteris lonchitica is too frequent to be fortuitous and there is further evidence afforded by certain anatomical resemblances. In France other species of Alethopteris, e.g. Alethopteris Serlii and A. Grandini, occur in association with Pachytesta[328], a large seed similar to Trigonocarpus, and Trigonocarpus is found in the Pas-de-Calais coal-field with Alethopteris Serlii.
The generic name Trigonocarpum was instituted by Brongniart in 1828[329] for ovoid longitudinally ribbed ‘fruits’ from Upper Carboniferous strata, the type-species being named T. Parkinsoni. The generic name is often altered to Trigonocarpon: Williamson[330], who adopted this form, states that Brongniart substituted Trigonocarpon for Trigonocarpum in his Tableau[331], but in that work the original termination is used, the form Trigonocarpon, probably the result of a slip, appearing only in the index. In his later work on seeds Brongniart adopted the name Trigonocarpus, and in recent years this has been widely employed. Among other species named by Brongniart are two previously referred by Sternberg to Palmacites. Several examples of Brongniart’s genus were described by Lindley and Hutton, and in their description of T. Noeggerathii the statement is made that a fractured specimen demonstrated that ‘the fossil in its ordinary state is an interior part divested of fleshy covering’[332]; this suspicion of the true nature of the nut-like fossils was afterwards proved correct by the investigations of Hooker and Binney[333] and by the later work of Williamson. The specimens on which the genus was founded are casts of seed-cavities and it is in this state that the seeds are usually preserved, often in large numbers, in the sandstones of the Coal Measures, as in the block shown in fig. 423 from the famous quarry at Peel near Bolton, Lancashire. Another type of preservation is represented by the seeds figured by Lindley and Hutton as Carpolithes alata[334], but the generic identity of the two states was not recognised until the discovery of petrified material afforded the clue. Figs. 424, 1, 425 illustrate the appearance of Trigonocarpus when preserved as a carbonised impression showing a thick fleshy envelope enclosing an oval kernel with a hard wall prolonged upwards as a longer or shorter micropyle. Casts of the seed-cavity are represented in figs. 423; 424, 2, 3. The surface of these casts occasionally shows one or more short cylindrical projections which are probably extensions of the sand or mud into holes formed in the testa by boring insects. The view that Trigonocarpus seeds are ‘obviously Palm fruits’ was not accepted by Hooker and Binney who inclined to regard them as the seeds of Conifers and compared them especially with the similar nuts of Ginkgo seeds (cf. fig. 631, C). It was Mr Wild[335] who was first struck by the association of Trigonocarpus and the petioles of Medullosa (Myeloxylon) and by some resemblances in structure between the testa and the hypoderm of the petioles; though, as Scott and Maslen[336] point out, the agreement is not so close as Wild believed, his view of a possible connexion between the reproductive and vegetative organs has been confirmed. Williamson extended our knowledge of the genus by his account of Trigonocarpus olivaeformis Lind. and Hutt., a form that is specifically identical with T. Parkinsoni Brongn. This author also drew attention to the close resemblance between Brongniart’s three genera Trigonocarpus, Hexapterospermum, Tripterospermum and expressed doubts as to the possibility of founding specific differences on casts of the Trigonocarpus type without the evidence of anatomy. Our knowledge of the structure of Trigonocarpus has in recent years been considerably extended by the researches of Oliver, Scott and Maslen, and Salisbury.
The seeds of this species like all examples of the genus are radiospermic, that is radially symmetrical in contrast to the flattened or platyspermic seeds. The complete seed is elongate oval in form when preserved as an impression (fig. 425, A) and reaches a length of 4–5 cm.: the casts of the seed-cavity are ovoid and provided with three prominent ridges (fig. 424, 2, 3). The testa forms a thick covering differentiated into three regions, an outer flesh or sarcotesta, a sclerous shell or sclerotesta, and an inner flesh. Transverse sections show that the sclerotesta has three sharp longitudinal keels with corresponding furrows on the inner face, and between each pair of main ribs are 2–3 less prominent ridges, usually 12 in all (fig. 426). The sarcotesta consists of thin-walled parenchyma passing externally into a more lacunar tissue with a palisade-like hypoderm: the sclerotesta consists of thick cells which interlace and form an efficient protective shell. Both the sarcotesta and sclerotesta are continued into the apical region as the wall of the long micropyle, the sarcotesta being prolonged beyond the sclerotesta at the apex of the integument[338]. The micropyle is triangular in section and may exceed in length the whole seed (figs. 425; 426, A). Its form as seen in transverse section (fig. 426, B) suggests the presence of wings: this appearance may be deceptive and due to pressure or, more probably, it represents an original feature. The seed-body, that is the portion enclosed by the integument, consists of the nucellus, represented by a few crushed layers of cells, bounded by a well-defined epidermis; the nucellus is separated from the integument from the base of the seed upwards, an important feature in which this and some other Palaeozoic seeds differ from Lagenostoma and the seeds of recent Cycads which are characterised by an integument adnate to the nucellus up to the level of the shoulder; the seeds of the Conifer Phyllocladus afford an example of separation of integument and nucellus as in Trigonocarpus. The innermost layer of the nucellus consists mainly of tracheal tissue investing the large megaspore (fig. 426, A, C, m) which is preserved as a contracted membrane detached from the nucellus after the death of the seed. At the summit of the nucellus is a relatively small pollen-chamber (fig. 426, A, Pc) like a broad and low cupola bearing a terminal beak which extended some distance into the micropylar tube. No microspores have been found in this species, but Oliver[339] records the occurrence of multicellular microspores in Trigonocarpus pusillus. The pedicel of the seed had a central strand of sclerous tissue penetrated by a concentric vascular bundle which gives off six strands to supply the sarcotesta (fig. 426, C, v) and then passes into the nucellus where it forms a tracheal sheath (fig. 426, A, nt) surrounding the lower part of the megaspore and at a higher level breaks up into anastomosing strands of tracheids which reach up to the plane of insertion of the pollen-chamber.