This generic title I propose to restrict to petrified seeds exhibiting the characters described by Brongniart[939] and more recently by Bertrand[940] in C. sclerotesta and C. drupaceus. In general organisation seeds of this generic type agree with those of recent Cycads and with the seed of Ginkgo biloba, but there are certain distinguishing features. An important character is afforded by the course and place of origin of the lower vascular strands from the main supply at the base of the seed. The outer vascular system consists of two bundles given off from the main strand, before it reaches the sclerotesta, which pass up the sarcotesta (fig. 500, B). In Rhabdospermum, on the other hand, the corresponding bundles arise at a higher level and form recurrent strands which penetrate the sclerotesta before passing up the fleshy part of the integument (cf. fig. 501, E).
Cardiocarpus sclerotesta Brongniart. The testa is differentiated into an inner shell and an outer sarcotesta (fig. 501, A; the sclerotesta is shown in black); there is a well developed pollen-chamber (pc) and below this the prothallus-tissue is prolonged as a blunt and short tent-pole, b, as in Gingko and in several fossil seeds. On each side of the apical tent-pole the slightly shrunken prothallus shows two small archegonia, a, which in the relatively small size and spherical form of the egg-cells resemble those of Ginkgo. In transverse section (fig. 501, B) the seed is bi-convex and at each end of the major axis the sclerotesta forms a small keel. There are two sets of vascular bundles concerned in the supply of material to the ovule; a lower pair of bundles given off from the central strand in the sarcotesta (fig. 500, B) which pass to the apical region in the inner tissues of the sarcotesta in the principal plane (fig. 500, A, v), and an inner set of bundles that pass up the peripheral tissue of the nucellus.
The species described by Brongniart as Cardiocarpus (Cyclocarpus) tenuis and C. nummularis have been removed by Bertrand[941] from Cardiocarpus on the ground that the integumental bundles pursue a course like that in Rhabdospermum; it is now referred to the new genus Cyclospermum[942].
As stated on a previous page Bertrand[943] re-establishes the generic name Cyclocarpus, founded on impressions without reference to anatomical characters, for two petrified seeds from St Étienne described by Brongniart as Cardiocarpus tenuis and Cyclocarpus nummularis[944]. These types differ from Rhabdospermum in the absence of an apical snout but agree with that genus in the steeply descending course of the vascular strands in the basal region of the seed. As stated on a previous page, the name Cyclospermum is proposed as a substitute for Cyclocarpus because of the employment of the latter term for impressions.
Cycadinocarpus augustudunensis (Brongniart[946]). In the possession of two sets of vascular bundles this type agrees with Cardiocarpus, but the more internal strands pass up on the inner face of the sclerotesta without penetrating into the nucellus, a feature in which Cycadinocarpus agrees with the majority of recent cycadean seeds: the outer bundles are given off from the main supply after it has entered the sclerotesta and not before as in Cardiocarpus; they follow an oblique course in the sclerotesta and emerge into the sarcotesta at the shoulders of the basal curve of the seed. As in Cardiocarpus and Rhabdospermum the two outer bundles lie in the principal plane of the seed. There is a pollen-chamber at the apex of the nucellus and the latter tissue is prolonged as a tent-pole which engages with the micropyle. In the absence of data as to the course of the vascular bundles in the chalazal region it would not be possible to distinguish between this genus and Cardiocarpus.
It is proposed to restrict the name Rhabdocarpus[947] to impressions and casts of seeds of the type represented by R. tunicatus as figured by Berger[948] and reproduced in fig. 506, K, the term Rhabdospermum being applied to seeds of similar form in which are shown certain distinguishing anatomical features. Rhabdocarpus tunicatus Berger is a species founded on a specimen from the Coal Measures of Silesia characterised by an outer carbonised testa prolonged apically as a blunt snout and, as seen in fig. 506, K, covering an apparently ribbed nucule, but the ‘ribs’ are due to the presence of fibrous strands and are not ribs in the ordinary sense. The outer tissue shows numerous longitudinal striations due presumably to the presence of fibrous elements in the sarcotesta like those shown in the petrified seed represented in fig. 501, C. The genus is defined by Berger as follows: ‘Semina ovata vel elliptico-oblonga secundum longitudinem parallele nervosa vel tenuissime striata, putamine (interdum deficiente) instructa.’ As used by Berger and many other authors Rhabdocarpus includes a miscellaneous collection of seeds often differing widely from the type-species. Many of the examples correctly referred to Berger’s genus are platyspermic though a bilateral symmetry is by no means always clear. Renault and Zeiller[949] in their definition of Rhabdocarpus include bilateral symmetry as a characteristic feature and speak of the seeds as oblong or oval with a pointed or truncate apex and a rounded base. Impressions of Rhabdocarpus differ from those of Cardiocarpus or Cordaicarpus in their more elongate form, always longer than broad, and in the absence of a basal sinus. The seeds found attached to Neuropteris pinnae and, in external features, agreeing with many specimens included in Berger’s genus, have been transferred by P. Bertrand[950] and Arber[951] to a new genus Neurospermum[952]. Arber[953] in his recent revision of British seeds proposes to restrict the name Rhabdocarpus to platyspermic seeds having a ‘large unsymmetrical nucule enclosed in a large unsymmetrical wing or sarcotesta,’ that is to forms symmetrical in one plane. In this category he includes Rhabdocarpus tunicatus Berg. (fig. 506, K) and R. subtunicatus[954] Grand’Eury, but it is not clear on what grounds Berger’s species is spoken of as symmetrical in only one plane: in the species R. Lilleanus Arb.[955] the symmetry is hardly sufficiently well defined to rank as a generic character. In the case of the Neuropteris seeds the apical snout is slightly curved, thus giving them an unsymmetrical appearance (cf. fig. 422, p. 114). The Carboniferous and Permian seeds usually referred to Rhabdocarpus are transferred by Arber to his genus Platyspermum[956], a designation for which it is proposed to substitute Nathorst’s genus Holcospermum[957]. In seeds preserved as more or less flattened impressions it is practically impossible in many cases accurately to determine the symmetry: as fig. 506, A, shows, casts indistinguishable from some examples of Platyspermum are radially symmetrical. Brongniart[958] extended the original definition of Rhabdocarpus to include certain anatomical characters, and these have been more fully defined by Bertrand[959]. It is for seeds showing these anatomical features that the name Rhabdospermum is now proposed. This course is followed on the ground that it is advisable to avoid confusion between petrified specimens and impressions which in spite of superficial resemblance may not be closely related. In some cases it is practically certain that an impression of the Rhabdocarpus type is generically identical with a seed of similar form showing the anatomical structure of Rhabdospermum, but unless identity is established a distinct terminology is preferable. The use of the generic name Rhabdospermum carries with it an implication of platyspermy, but under Rhabdocarpus may be included seeds which are radiospermic and platyspermic. Some seeds agreeing with Rhabdospermum are referred by Grand’Eury[960] to Poroxylon, and it is probable that Rhabdospermum like Cardiocarpus is a Cordaitean seed. On the other hand Rhabdocarpus may well include species, apart from those transferred to Neurospermum, that belong to Pteridosperms. A species, Rhabdocarpus Oliveri, recently described by Kidston[961] from the Staffordshire coalfield is an example of a radiospermic seed which may be assigned to a Pteridosperm. The type-specimen is an ovate seed 4 cm. long and 2 cm. broad agreeing in form and surface-features with Rhabdocarpus as already defined, but the evidence it affords of internal structure is insufficient to determine its position with regard to genera founded on anatomical characters. Other examples of Rhabdocarpus are described by Lesquereux[962] and White[963] from American Coal Measures, by Grand’Eury[964] from the Loire, by Renault[965] from Autun, and by many other authors.
The platyspermic seeds included in this genus agree in size and form with impressions assigned to Rhabdocarpus and as regards the main features conform anatomically to Cardiocarpus; they were probably borne on Cordaitean plants. Fig. 501, C, represents a longitudinal section of the species Rhabdospermum cyclocaryon described by Brongniart as Rhabdocarpus cyclocaryon: the sarcotesta is particularly well developed in the apical region; at the apex a portion of the micropyle is seen at m and near the nucellus are pieces of the sclerotesta shown in black. The presence of anastomosing fibres near the surface is a characteristic feature: these, as Bertrand points out, do not form a hypodermal tissue in the strict sense as they may be separated by some of the thin-walled parenchyma of the sarcotesta from the epidermis. The sclerotesta is only partially preserved but the inner portion forms a dark line enclosing the nucellus, the superficial tissue of which is separated from the shrunken prothallus represented by the almost spherical dotted region: the remains of an archegonium are seen at a (fig. 501, C) and the characteristic tent-pole apex of the prothallus is shown at b. While in shape and in the general plan of organisation Rhabdospermum agrees with Cardiocarpus, the vascular system in the chalazal region constitutes a distinguishing feature. In Rhabdospermum the main vascular strand passes through the sclerotesta, e, e′, fig. 501, E, before giving off two bundles which bend back (‘faisceaux récurrents’), traverse the shell, and then pass up the sarcotesta in correspondence with the feebly developed lateral keels as far as the micropyle, while in Cardiocarpus (fig. 500, B) the bundles are given off before the main strand reaches the sclerotesta. Similar recurrent bundles occur also in Mitrospermum (fig. 494, K)[967].
Mitrospermum compressum (Williamson). Mrs Arber[968] proposed the name Mitrospermum[969], suggested by the peculiar form of the seed-base, as a substitute for Cardiocarpon for Williamson’s species C. compressum[970] from the Lower Coal Measures of Lancashire. The seed is platyspermic and there is some evidence that it split into two valves along the principal plane (the longer axis of the section, fig. 494, L). The diagrammatic and partially restored longitudinal section reproduced in fig. 494, K, shows the main features: a sarcotesta, sa, covers the surface of the testa as a thin layer except at the edges of the flattened sides where it forms a wing-like border; preserved as an impression the seed would be assigned to Samaropsis. The sclerotesta, sc, has a pointed apex which surrounds the lower third of the micropyle and a broad base perforated by the chalazal vascular strand. There was probably a narrow inner flesh as in Trigonocarpus and recent Cycadean seeds (fig. 494, K, cf). The nucellus was free from the integument except at the base, as in Trigonocarpus and Stephanospermum (fig. 494, K, n): internal to the shrivelled remains of the inner flesh there was a nucellar tapetum surrounding the megaspore. Details as to the pollen-chamber are lacking though there are indications that it resembled that of some species of Cordaitean seeds. The main vascular supply passes through the sclerotesta and then forms a low cushion of short reticulate elements below the base of the nucellus from which two bundles are given off (fig. 494, K, v) in the principal plane. The course of the bundles which pierce the sclerotesta led Mrs Arber to remove this seed from Cardiocarpus, as recently defined by Bertrand[971], since in that genus the integumental bundles have their origin below the sclerotesta. In the course of the vascular bundles Mitrospermum is intermediate between Rhabdospermum and Taxospermum.
This genus is founded on a detached seed, but its resemblance to undoubted Cordaitean species favours its attribution to that group though, as Mrs Arber points out, some markedly platyspermic seeds are known to have been borne on fern-like fronds and Mitrospermum may belong to some genus of Pteridosperms.
Diplotesta Grand’Euryana Brongniart. The generic name Diplotesta, suggested by Grand’Eury, was given by Brongniart[972] to a Grand’ Croix seed which he compared with that of the Conifer Cephalotaxus. The type-specimen is elliptical and platyspermic (fig. 495, H, p. 322), and differs from Taxospermum in the cordate form of the seed-cavity, also in the more restricted union of nucellus and testa. The testa is differentiated into a sarcotesta (sa) and sclerotesta, and the latter forms two feebly developed keels in the principal plane: a characteristic feature shared with Mitrospermum is the splitting of the shell into two equal valves (fig. 495, F). Fig. 495, H, shows the contracted cylindrical nucellus and the pollen-chamber: the sarcotesta (sa) is only partially preserved. Diplotesta differs from Cardiocarpus in the course of the integumental vascular bundles which are of the type illustrated by Rhabdospermum, Taxospermum, and Cyclocarpus, but from these genera it is distinguished by the dehiscence of the shell, also by its form and certain anatomical features as described by Brongniart and Bertrand[973].
In this genus Bertrand includes Brongniart’s species Sarcotaxus avellana (fig. 495, F), a correlation suggested by the latter author.
Brongniart[974] founded this genus for a single species, Leptocaryon avellana, represented by a Grand’ Croix specimen 12 × 10 mm., which he believed to be related to Taxus. Leptocaryon differs from Taxospermum in the structure of the testa, but resembles it in external features. Bertrand[975] in his revision and extension of Brongniart’s account says that the sections throw no light on the nature of the vascular supply, and it is therefore impossible to form a satisfactory opinion as to the relationships of the genus. Renault[976] referred this genus to the Cordaitales, but we have no evidence as to the parent-plant.
Brongniart[977] gave this name to a small elliptical seed, 15×9 mm., recalling in external features the seed of Taxus, a genus to which he believed the Grand’ Croix species to be related. The type-species Taxospermum Grüneri (fig. 495, G) has a comparatively thin testa characterised by the absence of a sub-chalazal pad formed by the swelling of the sclerotesta. The nucellus is attached by a broad base to the testa and the two regions appear to be connected for a short distance on the flanks[978]; in this feature the seed is comparable with that of the Conifer Torreya and differs from the other platysperms, Diplotesta, Rhabdospermum, etc. Bertrand[979] states that the main vascular strand extends from the hilum to the chalaza before giving off the two opposite bundles which ‘follow the floor of the shell-cavity, and on reaching the flanks traverse the shell obliquely from below upwards[980].’ In the course and position of the integumental bundles Taxospermum differs from Cardiocarpus, Cycadinocarpus, and Rhabdospermum. In this genus Bertrand includes Sarcotaxus angulosus Brongn. and S. olivaeformis Brongn.[981]
Compsotesta Brongniarti Bertrand. The generic name Compsotesta[982], though adopted by Brongniart for some incomplete specimens from Grand’ Croix, was not published either by him or Renault: it has recently been revived by Bertrand[983] in his account of the anatomical details of Brongniart’s sections. This seed appears to be closely allied to the polypterous forms Ptychotesta and Hexapterospermum: the testa consists of a sarcotesta differentiated into two zones the outer of which contains vascular bundles in correspondence with the ribs, and an inner shell. There is a nucellar vascular supply and the nucellus is free on the flanks as in Stephanospermum and Trigonocarpus.
The characters of this Permian and Carboniferous genus have already been described: the name has reference only to superficial features especially the samara-like ‘wing,’ and connotes no special anatomical features.
Samaropsis fluitans (Dawson). The species described by Dawson[984] as Cardiocarpum fluitans from Carboniferous strata in Nova Scotia, is represented by oval seeds with a fairly broad border usually showing an apical notch. Fig. 502, A, is a copy of Dawson’s figure: the apparent absence of an apical sinus in the ‘wing’ is probably due either to an error in interpretation or to some imperfection in the specimen. As fig. 502, A, shows, the type-specimens are far from satisfactory, and it may be that they are not specifically identical with the more complete specimens from European strata referred to Dawson’s species. Zeiller[985] points out that seeds of this species vary considerably in size, but there is always in well-preserved examples a bifid beak at the apex. Seeds of similar form though not specifically identical are described from the Coal Measures of Missouri as Cardiocarpon (Samaropsis) Branneri Fairch. and White[986]. Good examples of S. fluitans are figured by Weiss[987] from the Coal Measures of Saarbrücken and the species is widely distributed in Upper Carboniferous beds generally.
Samaropsis bicaudata Kidston. This species (fig. 502, E) originally described[988] from Lower Carboniferous rocks in Scotland as Cardiocarpus bicaudata and subsequently assigned to Samaropsis, is characterised by a greater development of the flat wing-like border which is divided into two long tapering basal lobes. Seeds of similar form are figured by Lesquereux[989] from Pennsylvania as Cardiocarpus (Ptilocarpus) bicornutus.
Samaropsis (Samarospermum) moravica (Helmhacher). This type[990] (fig. 502, H) is characterised by the great length of the wing-like border and on that account it was transferred by Arber to a new genus. It was originally described by Helmhacher from the Permian of Moravia as Jordania moravica and the type-specimen has been refigured by Zeiller[991] who records the species from Upper Carboniferous and Permian rocks in France: it is recorded also from several localities in Germany[992]. Seeds figured by Potonié[993] from the Permian of Thuringia as Samaropsis Crampii (Hartt) are undoubtedly examples of S. moravica: the true S. Crampii has recently been well illustrated by Dr Stopes[994] from the Westphalian of New Brunswick. The species is recorded by Arber[995] from the Kent coalfield.
The seed represented in fig. 502, B, from the Lower Coal Measures of Kilmarnock, Scotland[996], affords a good example of the genus: the species was originally described by Berger as Cardiocarpon emarginatum and it was on this type that Fiedler founded the genus Cyclocarpon[997]. It has been referred by many authors to Cardiocarpon and might be regarded as a type intermediate between Cordaicarpus, as used in this chapter, and Samaropsis, though the breadth of the border is more in keeping with the latter designation. The figured specimen is 1·6 cm. long and 1·4 cm. broad; the nucule has a slightly cordate base and shows several faint converging ribs which are too inconspicuous to be represented in a natural-size drawing. A narrow median groove in the apical region shows the position of a vascular strand. The species is recorded from several countries: similar though specifically distinct seeds, described by Dawson as Cardiocarpon cornutum, have recently been re-described by Dr Stopes[998] from the Westphalian of New Brunswick where they occur in association with the leaves of Cordaites Robbii Daws.
This species was originally described by Andrews[999] from the Coal Measures of Ohio: the specimen shown in fig. 502, K, was sent to Dr Kidston by Mr Claypole. The whole seed is 5 cm. wide and 3·5 cm. in depth; it is characterised by a short and relatively broad nucule surrounded by a very broad and flat border showing faintly marked radially disposed lines and in places some irregularly distributed pits. The apex is emarginate and there is a broad and deep sinus in the sarcotesta in the chalazal region. The seed resembles Samaropsis alata Kidst.[1000] and S. Baileyi (Daws.)[1001] but it differs from these in the greater breadth of the ‘wing’ and in the form of the nucule.
The seed described by Fiedler[1002] as Jordania bignonioides (fig. 502, I) agrees closely with S. alata Kidst. but is probably specifically distinct.
White[1003] described this species (fig. 502, G) from Permo-Carboniferous rocks in Brazil (Rio Grande do Sul) as Cardiocarpon barcellosum. The nucule is said to be cordate but, as seen in the figure, there is no clear indication of a basal sinus: the presence of a relatively broad ‘wing,’ as White says, entitles the seed ‘to a place in the Samaropsis section of the genus.’ The author of the species compares it with seeds described from Westphalian rocks in Ohio[1004] and Pennsylvania[1005]. It is interesting to find a type which is common in both Europe and North America in the western portion of Gondwana Land. Seeds of similar form are recorded also from India, South Africa, and Australia[1006].
This Brazilian species from the same beds is described by White as Gangamopteris (Samaropsis) Seixasi[1007]: it is characterised by a small ovate nucule 8–10 mm. long and 5 mm. broad, in some specimens surrounded by a complete ‘wing’ extending above the apex and below the base, giving the seed an appearance similar to that of Samaropsis (Samarospermum) moravica while sometimes, as in the example shown in fig. 502, F, the broad border is preserved only at the sides. These seeds are abundant in the Santa Catharina beds, where they were discovered by Dr Esdras do Prado Seixas, in association with leaves of Gangamopteris, and White thinks that they were borne on the fertile leaves of that genus which he has named Arberia[1008]. Although there is as yet no proof of a connexion between Gangamopteris and seeds of this or any other type it is almost certain that it was a seed-producing plant.
The seed on which this species is founded was discovered by Mr T. N. Leslie in the Ecca beds (Permo-Carboniferous) of Vereeniging, South Africa, a locality from which the same geologist has obtained leaves of Cordaites, Psygmophyllum, Glossopteris and other genera[1009]. In the slightly cordate base and tapered apex (fig. 503) the nucule agrees closely with those of European examples, but the Vereeniging type is distinguished by its larger dimensions and by the wider border indicating a thick sarcotesta continued basally into a stout stalk. The apex is emarginate and a median rib marks the position of a vascular strand. There is no evidence as to the nature of the parent-plant.
Prof. Zeiller[1010] described this species as Cardiocarpus indicus from the Karharbari (Lower Gondwana) beds of India. An examination of the type-specimens enables me to confirm the accuracy of the original account. The platyspermic seed is 5·5 cm. long and 4·5 cm. broad; a cordate nucule is enclosed by a flat border similar to that of Samaropsis Leslii but narrower especially on the sides of the nucule. At the apex there is a deep sinus extending to the nucule, and at the base a fairly broad band of carbonaceous matter shows the position of the chalazal vascular strand. The seed is characterised by its large size and by its almost orbicular form: it occurs as a detached specimen in beds containing Cordaites (Noeggerathiopsis) and Glossopteris.
A species from Arkansas described by Lesquereux[1011] as Cardiocarpus ingens [= Cordaicarpus ingens (Lesq.)] affords another example of a large seed similar to C. indicus and, except in its more orbicular form, to Samaropsis Leslii.
An examination of the type-specimen from the Calcutta Museum enables me to amplify the original description in an important particular. The species was found in Lower Gondwana (Katharbari) beds in India and referred by Feistmantel[1012] to the genus Carpolithes. Arber[1013], who tentatively employed the generic name Cardiocarpus in place of Carpolithes, suggests that the seeds may be radiospermic, as Feistmantel’s drawings show only a very narrow border to the nucule. The specimen reproduced in fig. 504 was figured by Feistmantel without any indication of a definite sarcotesta or wing, but as seen in the drawing the ovate cordate sclerotesta is surrounded on one side and at the base by an outer envelope: this is clearly seen at the apex where it shows a rounded termination sloping downwards towards the micropyle precisely as in Samaropsis indica (Zeill.). The border is narrow at the sides and broader at the base as in S. Leslii. The seed is 4·5 cm. long and 2·3 cm. broad, differing from S. indica in its rather smaller size and in the slightly narrower nucule. Though there is no decisive evidence as to the parent-plant the occurrence of a specimen of this species partially covered by a scale-leaf of a type[1014] very similar to that which is generally recognised as belonging to Glossopteris suggests the possibility that the seeds may belong to that genus. Scale-leaves of Glossopteris are described in Volume ii., but it may be added here that leaves similar in form to those from India, Australia, and elsewhere are figured by Geinitz[1015] from the Altai Mountains as Trigonocarpus? actaeonelloides: the specimens are represented with the basal scar at the apex.
In view of the fact that the generic names Cardiocarpus, Cordaicarpus, and other designations have been applied to casts and impressions which cannot be distinguished by any constant or important feature it is proposed to adopt the name Cordaicarpus for platyspermic seeds, preserved as casts or impressions, having a comparatively narrow border enclosing an ovate or cordate-ovate nucule; the base is either rounded or cordate. The choice between Cordaicarpus and Samaropsis depends on the breadth of the border. Cordaicarpus, though more suggestive of a Cordaitean alliance, may in some cases be a Pteridosperm seed.
Cordaicarpus Cordai (Geinitz). Lenticular seeds more or less orbicular or broadly ovate (fig. 502, C, D), often slightly cordate at the base of the nucule and with a broadly acute apex: the border is narrow or sometimes hardly represented as in the seeds described by Berger as Rhabdocarpus ovoides, a species similar to but more oval than Cordaicarpus Cordai. The latter species[1016] occurs in several coalfields in Britain, France, Germany, and elsewhere. Fig. 502, C, shows a good example from the Middle Coal Measures of Yorkshire, 9 mm. long by 8 mm. broad; on the very slightly cordate nucule are several faint ribs converging towards the base and apex and between them fine striations, characters too indistinct to be reproduced in the natural-size drawing. The flat border represents the sclerotesta. The seed shown in fig. 502, D, from the Westphalian series of Warwickshire belongs to the same species or is a closely allied type: the faint suggestion of reticulation on its surface might be regarded as a reason for referring it to C. areolatus Boul.[1017], a form characterised by a reticulation on the testa, described by Zeiller[1018] and other authors. This reticulation is, however, in some cases at least, formed by crumpling and splitting of the superficial carbonised film into more or less regular meshes: the figured specimen occurs with several other seeds of the same type, most of which have a smooth surface. Dr Kidston tells me that a recent critical examination of seeds in his collection leads him to regard some specimens (e.g. fig. 502, D) previously referred by him to C. Cordai as identical with Carpolithes membranaceus Goepp.[1019]