This generic name was proposed for an ovoid seed from St Étienne described as Polypterospermum Renaultii[869]; hexagonal in section with six deep and sharp flanges at the angles alternating with six secondary ridges distinguished by their blunter edges and slightly smaller depth (fig. 495, A). Without further anatomical details it is hardly possible to say whether or not the species represents a well-defined generic type, but it is not improbable that a fuller knowledge would confirm Brongniart’s institution of a new generic designation. The seeds described by Arber[870] and Kidston[871] respectively as Radiospermum ornatum and Polypterospermum ornatum are transferred to Polygonocarpus on the ground that they afford no evidence of anatomical characters of the Polypterospermum type.
Brongniart[872] established this genus for some unusually large seeds (fig. 496, A) from the Coal Measures of France reaching a length of 11–12 cm. and resembling in form and size a hen’s egg. Petrified examples have been described by Brongniart and Renault[873], and Oliver[874] has given a lucid statement of the more interesting features of this Permo-Carboniferous genus.
In the separation of nucellus and integument Pachytesta agrees with Stephanospermum and Trigonocarpus. There is a double series of vascular bundles in the outer part of the testa or exotesta (fig. 497, ex) consisting of a spongy tissue bounded externally and internally by sclerotic layers: the exotesta is succeeded by a broader endotesta of spongy consistency which Renault suggested may have served as a floating mechanism, and this is intimately associated with the nucellus by means of grooves engaging with short ridges (fig. 497, nr) on the surface of the nucellar tissue. The exotesta is divided into three valves by radial extensions of the sclerotic tissue (b, c, fig. 497; at a the exotesta is shown in an unsplit condition). In connexion with each radial plate are two curved plates of fibrous tissue which extend towards the grooves in the endotesta (fig. 497, t): as Oliver suggests, these plates may have served a mechanical purpose for the support of the bulky nucellus. The peripheral region of the nucellus is supplied by several vascular bundles (fig. 497, n) from the tracheal chalazal disc. The exotesta is regarded by Oliver as corresponding to the sarcotesta and sclerotesta of other seeds, and the grooves in the endotesta interlocking with the nucellar ridges are compared with the interlocking of nucellus and canopy in Lagenostoma, as also with the ruminated endosperm of Torreya.
We have no definite information as to the plants which bore Pachytesta, but it is probable that they were members of the Medulloseae. Grand’Eury[875] believes Pachytesta to be the seed of Alethopteris Grandini, though this view requires confirmation. This author figures several Pachytesta seeds attached in two oblique rows to a comparatively slender axis which may be the rachis of a large compound frond[876]. Renault and Zeiller[877] have figured specimens of P. gigantea and P. incrassata from Commentry which afford a good idea of the form of these large seeds; the genus is recorded also from Gard, Blanzy[878], and other localities. Kidston[879] described a large oval seed, blunt at each end, from the Middle Coal Measures of Lancashire as Carpolithus Wildii which he thinks may be allied to Pachytesta, but adds, ‘I do not think we are justified in placing mere impressions of plants in genera, whose distinctive characters are derived from their internal organisation, unless there is conclusive evidence to show their identity.’ On this specimen Arber[880] has founded a new genus Megalospermum, but as the type-specimen shows no distinctive features other than large dimensions it would seem preferable to retain the more general designation Carpolithus[881].
Another example of a seed that may be generically identical with Pachytesta is that described by Lesquereux from North American Coal Measures as Rhabdocarpus Mansfieldi and more recently recorded by White from Missouri as Rhabdocarpus (Pachytesta) Mansfieldi[882]. There is little doubt that this and other species of seeds preserved as impressions are examples of Pachytesta but, as Kidston says, it is preferable to reserve the name for specimens showing anatomical features.
The genus Stephanospermum, founded by Brongniart[883] on petrified specimens from French Stephanian beds, affords a good example of a radiospermic seed without ribs differing in certain well-marked characters from such seeds as Lagenostoma, Physostoma, and Conostoma, notably in the complete separation of the nucellus from the integument and in the possession of a nucellar vascular system. The descriptions by Brongniart and Renault[884] have been extended by the thorough investigations of Oliver[885].
This species is represented by ellipsoidal seeds, 10 mm. long with a maximum breadth of 4–4·5 mm., circular in transverse section. The integument consists of a thick sclerotesta lined internally by a thinner soft layer, the endotesta (fig. 494, A, sc, e), and in all probability covered by an outer flesh or sarcotesta as in Trigonocarpus, though this tissue is not preserved and is omitted from the section shown in fig. 494, A. The sclerotesta is composed of a broad outer band of thick-walled palisade cells—the contracted contents of which are a striking feature in the silicified seeds (fig. 493, F); these are succeeded by an inner zone of longitudinal fibres. In the structure of the palisade-layer Stephanospermum bears a close resemblance to the sporocarp wall of Pilularia[886]. In the apical region the shell forms a circular ridge surrounding a peri-micropylar trough, a character expressed by the term ‘crown-seed[887]’ employed by Grand’Eury: the trough sometimes contains partially destroyed tissue that may be a remnant of a sarcotesta. A Stephanospermum seed, with its apical core and surrounding depression with remnants of some partially destroyed tissue, may be compared with a fruit of Attolia speciosa (Palmae) in which a tuft of fibrous tissue picked out by decay from the mesocarp surmounts the conically pointed apex of the harder interior of the fruit-wall. The base of the sclerotesta is perforated by a vascular strand which expands into a tracheal disc, td, fig. 494, A, at the base of the megaspore from which a continuous mantle 2–3 cells broad, of short spiral and scalariform tracheids (fig. 493, G), spreads over the whole of the nucellus immediately below the nucellar epidermis as far as the lower part of the sides of the pollen-chamber: this mantle forms the floor of the large domical pollen-chamber excavated out of the nucellar cone (fig. 494, A, pc). The fact that in sections of older seeds the tracheal floor shows signs of splitting and disorganisation led Oliver to conclude that in the living seed the tracheids underwent a gradual disintegration prior to fertilisation, thus allowing the passage of the antherozoids to the egg-cells (fig. 494, A, a). The presence of a continuous tracheal sheath instead of separate vascular bundles is a special feature in which Stephanospermum differs from Trigonocarpus and other Palaeozoic seeds as well as from those of recent Cycads; as Oliver suggests, ‘the apparent perfection of the vascular mantle in Stephanospermum may have proved an obstacle to further development[888]’ and was not retained by the more successful types. In its tracheal sheath Stephanospermum resembles the seeds of Ginkgo. The nucellar cone is prolonged as a beak into the micropyle formed by the tubular integument. The megaspore occupies the central portion of the seed and in the course of its development it compressed the megasporangium (nucellus) to such an extent that little more than the epidermis remains: there is a definite megaspore-membrane surrounding the prothallus-tissue in which there were probably two archegonia (fig. 494, A, a)[889]. The nucellus stands free within the integument from which it is separated by a space (s, fig. 494, A). Microspores are frequently met with in the pollen-chamber in which they matured after their introduction through the micropyle, probably with the aid of a drop of mucilage: they are large oval bodies with an average size of 160μ × 100μ, some reaching over 200μ, in length and, as Renault was the first to point out, within a finely granulate exospore there are several thin-walled cells: this statement was not accepted by some authors but Prof. Oliver’s researches have amply confirmed it, and an examination of the original preparations convinced me that Renault had correctly described the structural features. Oliver shows that there are about 20 cells within each microspore regularly arranged as rows of five wedge-shaped elements with their pointed ends directed towards the centre, and he thinks that these cells may have undergone further division to produce sperm-mother-cells, though there is not such good evidence of this as in the differently constituted microspores of Stephanospermum caryoides. No trace of pollen-tubes was found and it is probable that the antherozoids were liberated by the rupture of the delicate prothallus-tissue. The tracheal sheath afforded an adequate means of water-supply to the pollen-chamber and this enabled the motile antherozoids to reach the archegonia.
This species, founded on a specimen from ., is a larger seed (15·5 × 12·5 mm.) and more globular than S. akenioides; there is a shorter micropylar beak and a less developed peri-micropylar trough. As in S. akenioides the nucellus is free from the base and the two species conform to the same general type of construction. The microspores are distinguished by the presence of only two internal cells which do not occupy the whole of the spore-cavity but are surrounded by a large peripheral cell comparable with the tube-cell in recent microspores, though there is no proof that a tube was formed: in the case of S. akenioides Oliver considers that fertilisation was not assisted by the production of a pollen-tube. The two cells by further division gave rise to a secondary cell-complex consisting of at least eight antherozoid-mother-cells. No antherozoids have been found in the microspores but it is possible that two small bodies, 17μ × 15μ, met with in a pollen-chamber may represent the nuclei of sperms. Their small size differentiates them from the much larger male gametes of Cycads and from the supposed sperms of Physostoma and Lagenostoma.
We have no proof as to the nature of the plant which bore seeds of the Stephanospermum type, but it is significant that the specimens occur in close association with fragments of Alethopteris and Myeloxylon, a circumstance that favours the view, based on the resemblance of these seeds to Trigonocarpus, that Stephanospermum is the seed of a member of the Medulloseae.
The type-species Polylophospermum stephanense Brongn.[892], founded on partially preserved material from Grand’ Croix, is a narrow hexagonal seed 15 mm. long. Additional facts as to the structural features have been contributed by Oliver[893]. The testa is differentiated into an inner shell (sclerotesta) and an outer flesh (sa, fig. 495, C, D): the sclerotesta has six prominent, fissured, ribs, one at each angle, and between these are six solid and less prominent secondary ribs. Oliver states that there is an outer series of vascular bundles in the sarcotesta, one bundle immediately external to each secondary rib (fig. 495, C, v). In the presence of two kinds of ribs and in the relation of ribs to tracheal strands Polylophospermum agrees with Trigonocarpus Parkinsoni. Strands of short tracheids supply the peripheral region of the nucellus and, as in Stephanospermum, reticulate elements extend as far as the floor of the large pollen-chamber. There was probably no lateral union between nucellus and integument. A striking feature is the prolongation of the testa at each end of the seed to form an open chamber surrounding the micropylar beak and the seed-base (fig. 495, D, s, s): the apical chamber, though relatively more prominent, is comparable with that in Stephanospermum, while the basal chamber recalls that in the seed named by Scott and Maslen Trigonocarpus Oliveri[894] but subsequently removed by Salisbury[895] from that genus. There is no evidence as to the parent-plant of Polylophospermum, but it may be assumed to have been a Pteridosperm, probably one of the Medulloseae.
Our knowledge of this peculiar genus is derived from Brongniart’s description of the type-species Codonospermum anomalum[897] (fig. 506, B, C) from St Étienne and from Renault’s account of C. olivaeforme[898]. The seeds are globular or ellipsoidal and reach a length of 2 cm.: the testa is differentiated into an outer flesh and a sclerotesta and has usually eight ribs. The most striking feature is the division of the seed into two regions, an upper portion containing the nucellus and megaspore and a lower portion in the form of an empty chamber that probably served as a float (fig. 498, A, s). Externally the upper half or seed-proper forms a depressed cupola with eight slightly developed ribs, separated by a circular transverse constriction from the basal chamber (fig. 498, B). There is a fairly large pollen-chamber, pc, in which Renault found multicellular microspores in C. olivaeforme. In C. anomalum 16 vascular bundles (fig. 498, B, v) surround the central region of the seed probably in the peripheral tissue of the nucellus; these bundles unite in the chalazal region to form a strand that passes up the centre of the empty chamber. In C. anomalum the testa of the lower half has eight ribs and corresponding with each rib is a strand of fibrous tissue (fig. 498, B, f). In C. olivaeforme the testa is thicker than in C. anomalum and the lower region of the seed is smooth and circular in section.
Impressions of Codonospermum are described[899] from Commentry, from the Loire—the Gard district[900], and by Zeiller[901] from Blanzy. There is no decisive evidence as to the parent-plant, but some French authors[902] regard the frequent association of Codonospermum with leaves of Dolerophyllum as significant.
Brongniart instituted the genus Aetheotesta[903] for an incomplete seed (A. subglobosa) from Grand’ Croix, and Renault[904] subsequently founded the species A. elliptica on much better material. The testa of the elliptical seed, 3 cm. long, consists of an outer region very thin on the flanks but highly developed at the apex and base which forms a sarcotesta characterised in the apical region by large radially disposed spaces, fig. 498, C, s; the sclerotesta, composed of harder tissue, is prolonged as an apical beak. There is a large pollen-chamber, pc, in which Renault found multicellular microspores (320μ–400μ). In the chalazal region the main vascular supply forms a cup-like investment, v, in the basal portion of the nucellus, and at a higher level this breaks up into several nucellar strands. Renault thinks that Aetheotesta is the seed of Dolerophyllum, but there is no proof of any connexion. The presence of large spaces in the sarcotesta may be interpreted, as Renault suggests, as evidence of adaptation to dispersal by water.
Brongniart[905] instituted this genus[906] on an incomplete transverse section of a seed from Grand’ Croix, 8 mm. in diameter, which he called Eriotesta velutina, characterised by a ribbed and probably octagonal testa bearing numerous elongated hairs over the whole surface. The material is, however, too meagre to throw any light on the important features of the seed.
The generic name Gaudrya[907] was proposed for two petrified seeds from the Gard coal-field briefly described as G. trivalvis, the type-species, and G. lagenaria. The testa of the former species shows signs of splitting along three equidistant lines; it consists of a sarcotesta enclosing an endotesta with six longitudinal ribs: Grand’Eury speaks of lacunae between the endotesta and nucellus which he regards as spaces in a tissue which made the seed buoyant and facilitated dispersal by water. It is not clear whether the nucellus and integument were originally connected or laterally free: the lacunae may be the remains of an inner flesh and not spaces in a spongy tissue. The genus is radiospermic and characterised by a long micropyle and a broad pollen-chamber. On the available evidence it is difficult to assign the specimens to their systematic position, but the genus is probably a member of the Trigonocarpales.
Platyspermic seeds for the most part belonging to Cordaitalean plants and agreeing in their plan of organisation, even more closely than the seeds of the Trigonocarpales, with those of recent Cycads. The nucellus is free laterally from the integument and there is a series of vascular bundles in the nucellus. The testa consists of an outer flesh, the sarcotesta, which may reach a considerable thickness, a shell or sclerotesta, and in some genera an endotesta. In seeds preserved as impressions the sarcotesta often gives them a winged appearance (Samaropsis). The pollen-chamber is relatively small, resembling in this respect the chamber in recent Cycads. A central prolongation of the prothallus-tissue in the form of a blunt column or ‘tent-pole’ is a characteristic feature; the same feature occurs in seeds of the Lagenostomales, but in the Cordaitales it resembles more nearly the ‘tent-pole,’ as it was called by Hirase, in recent Cycadean seeds and in Ginkgo. The presence of two vascular strands in the sarcotesta in the principal plane of the seeds is a character shared with the seeds of Cycas: the position and course of these bundles are useful characters for distinguishing different types within the group. The microspores are multicellular.
Genera. Cardiocarpus; Cyclospermum; Cycadinocarpus; Rhabdospermum; Mitrospermum; Diplotesta; Leptocaryon; Taxospermum; Compsotesta. These generic names are all used in the following pages for seeds known to possess certain anatomical features; there are also included in the Cardiocarpales the genera Samaropsis, Cordaicarpus, and Rhabdocarpus, but it is proposed to limit their use to specimens which furnish no anatomical data and cannot therefore be assigned with equal confidence to a section of seeds based on definite morphological characters. It is certain that some at least of the seeds described under these names would, if preserved as petrified specimens, be included in one or other of the genera named above.
There is ample proof that some of these seeds were borne on Cordaitean plants and that the group as a whole represents the seeds of the Cordaitales[908]. It is, however, certain that some Platyspermic seeds were produced by Pteridosperms. No little confusion has been caused by the employment of the same generic names for petrified seeds and for casts and impressions affording no evidence as to similarity in anatomical characters. With a view to avoid the risks necessarily entailed by following this practice it is suggested that a clearer distinction should be drawn between genera based primarily on structural features and form-genera. The following notes on the genera Cardiocarpus, Cordaicarpus, Cyclocarpon, Cycadinocarpus, Jordania, and Samaropsis, may serve to illustrate some of the difficulties connected with the terminology of Palaeozoic seeds.
Cardiocarpus. Brongniart[909] proposed the name Cardiocarpon in 1828 for Upper Carboniferous seeds described as compressed lenticular, cordiform or reniform ‘fruits’ with an acute apex: in his later work[910] he recognised their true morphological nature and gave an account of some exceptionally well-preserved examples from Grand’ Croix. Brongniart in common with other authors believed Cardiocarpus seeds to belong to Cordaitean plants, a view that in several cases is based on conclusive evidence. The specimen represented in fig. 501, D, illustrates the characteristic form of a cast of a Cardiocarpus seed, and the sections shown in fig. 501, A and B, are from an identical or a very closely allied species. The generic characters are: (i) the presence of a narrow flattened border or wing surrounding a platyspermic nucule, (ii) the cordiform base and more or less pointed apex, (iii) the differentiation of the testa into a sarcotesta and sclerotesta free from the nucellus except at the base, (iv) the ‘tent-pole’ prolongation of the prothallus (fig. 510, A, b) and the presence of a fairly large pollen-chamber, pc, (v) the occurrence of two sets of vascular bundles, an inner nucellar series and two double vascular strands (fig. 500, A, v, B) which are given off from the main supply before it reaches the sclerotesta. The term Cardiocarpus as used by Brongniart signifies a type of seed possessing both certain anatomical and external characters. The proposal is to restrict the generic appellation to seeds exhibiting definite structural features agreeing in essentials with Cardiocarpus sclerotesta and C. drupaceus.
Cordaicarpus. This name was first employed by Geinitz[911] in the form Cordaicarpon, the type-species being C. Cordai (fig. 502, C) from the Coal Measures of Germany, a seed referred by the author of the genus to Cordaites principalis but, as Kidston has pointed out, there is evidence that this correlation may be incorrect: there is, however, no doubt as to its Cordaitean parentage. Specimens included in this genus agree closely with species of Cardiocarpus, but they are usually described as being distinguished by the absence of a flat border and by a more rounded and less cordate base. In the example of Cordaicarpus Cordai shown in fig. 502, C, and in other species assigned by authors to this genus there is a narrow border and the form of the base is an inconstant character. As Kidston[912] and other authors point out, there are no definite and constant characters by which to distinguish Cardiocarpus from Cordaicarpus as regards the form of the seeds preserved as casts or impressions. A further account of Cordaicarpus is given on a later page.
Cordaispermum. This designation was formerly adopted by Renault[913] for seeds having the form and anatomical features of Cardiocarpus which there is good reason for attributing to Cordaites or to some allied genus. In view of the fact that the majority of the seeds under consideration are undoubtedly Cordaitean there is no need to employ this additional generic name.
Cyclocarpon. Fiedler[914] instituted this term for seeds previously described by Berger as Cardiocarpon emarginatum (fig. 502, B, now included in Samaropsis) and added a new species Cyclocarpon nummularium. Brongniart[915] subsequently described the structure of two species, C. tenue and C. nummulare, which he referred to Fiedler’s genus, and Bertrand[916] has shown that these differ from the genus Cardiocarpus in the recurrent course of the bundles given off from the chalazal strand as in the genus Rhabdospermum (cf. fig. 501, E). There are no good grounds for retaining the designation Cyclocarpon for casts and impressions, as the specimens so named are indistinguishable from impressions referred to Cordaicarpus. The generic name Cyclocarpus, retained by Bertrand only for C. tenuis and C. nummularis simply for anatomical reasons, is now altered to Cyclospermum on the ground that the designation Cyclocarpon has been used for impressions affording no information with regard to anatomical features.
Cycadinocarpus. Renault[917] transferred to this genus Brongniart’s species Cardiocarpus augustodunensis on the ground that the vascular system exhibits in a greater degree than the other types included by Brongniart in Cardiocarpus a resemblance to that in recent cycadean seeds. Bertrand[918] confirms Renault’s account and retains Cycadinocarpus augustodunensis as a species worthy of generic distinction. A short account of this seed is given on a later page.
Jordania. This name was given by Fiedler[919] to compressed ovate-cordate seeds characterised by a broad membranous border bearing a superficial resemblance to the seeds of Bignonia. The type-species, J. bignoniodes (fig. 502, I), from the Coal Measures of Saarbrücken has the form usually associated with the designation Samaropsis, and as the latter term is generally adopted there are no adequate reasons for the retention of Jordania. The name Jordania has also been applied, by Schenk[920], to fossil Dicotyledonous wood and was previously used by Boissier[921].
Samaropsis. Goeppert[922] defined Samaropsis as ‘Fructus samaroideus membranaceus, compressus, margine alatus, monospermus.’ The type-species, S. ulmiformis, from the Permian of Brenau, is a small seed with a broad wing or border, but a better example of Samaropsis is figured by Goeppert as the wing of an insect[923]. Examples of the genus are shown in figs. 502, A–H; 503; 504. Seeds included in Samaropsis differ from those referred to Cordaicarpus in the presence of a broader and more clearly defined border which in some cases, as in the genus Mitrospermum (fig. 494, K, L), undoubtedly represents a lateral wing-like extension of the sarcotesta. In some instances the wing may be a tangentially expanded integument comparable with the perianth of Welwitschia, and in some Jurassic seeds referred by Heer[924] to Samaropsis the lateral appendages are probably true wings. It is advisable to restrict the designation Samaropsis to Palaeozoic seeds. Nucules deprived of the broad border would be referred to Cordaicarpus as usually employed for impressions. The generic name Samaropsis serves a useful purpose as a distinctive term for platyspermic seeds preserved as casts or impressions characterised by the possession of a wide border or wing broader than in typical examples of Cordaicarpus. The specimen represented in fig. 499 affords a good illustration of the difference between Samaropsis and Cordaicarpus. In this specimen the border clearly consists of two portions, an inner narrower border (black in the drawing) and an outer more delicate portion; the former is the impression of the sclerotesta and the outer represents the fleshy sarcotesta which in the living seed may have formed a wing. If, as often happens, the seed were preserved with the narrow border only it would be assigned to Cordaicarpus, many species of which are undoubtedly incomplete Samaropsis seeds.
The seeds described by Lindley and Hutton as Cardiocarpon acutum (fig. 444, p. 171) have been made by Arber[925] the type of a new genus Cornucarpus, the distinguishing feature being the triangular form and the apical horns of the wing. The seeds figured by Arber[926] from the Kent coalfield as Cornucarpus acutus are, however, not identical with the type of Lindley and Hutton, which has the characters of Samaropsis. Samaropsis is widely distributed in Permo-Carboniferous rocks in Europe and North America and is recorded also from India[927] (fig. 504), China[928], South Africa[929] (fig. 503), South America[930] (fig. 502, F, G) and Australia[931]. Some seeds of this form were certainly borne on Cordaitean plants (cf. fig. 480, A), but seeds of similar type have been found in organic connexion with the foliage of Pteridosperms (figs. 442, 445, pp. 167, 172). The Permian ‘cone-scales’ bearing seeds described by Geinitz as Cardiocarpon triangulare[932], represented by well preserved impressions in the Dresden Museum, appear to be of the Samaropsis type: the same author referred some Samaropsis seeds to the Conifer Walchia, but Weiss[933] dissents from this correlation as the seeds often occur in beds in which Walchia is not represented. Renault states that the seeds of the fertile shoot described by him as Cycadospadix Milleryensis[934] from Autun [= Strobilites Milleryensis (Ren.)] closely resemble Samaropsis fluitans Daws. as figured by Weiss. The suggestion by White[935] that Samaropsis seeds were borne on fertile leaves of Gangamopteris adds a further difficulty to the use of the generic characters of Samaropsis as criteria of systematic position. The Permian seeds figured by Goeppert[936] as Oreodoxites Martianus are possibly specimens of Samaropsis.
The designation Samaropsis, though usually restricted to Palaeozoic species, is applied by some authors to ‘winged’ seeds from Mesozoic strata; but as some of the Jurassic seeds[937] so named appear to have true wings like those of some recent Conifers it is advisable to adhere to the more limited use of the name.
It is safe to assert that many Samaropsis seeds agreed generally in structure with Cordaitean seeds such as the petrified examples described by Brongniart as Cardiocarpus. The species Mitrospermum compressum[938] is an example of a petrified seed having the external features of Samaropsis.