The photograph reproduced in fig. 505 shows the result of chemically treating the carbonised cuticle of a seed of Cordaicarpus Cordai, a method little used as yet in the case of Palaeozoic plants but which may be useful in distinguishing seeds which cannot be satisfactorily separated by microscopic features. The superficial cells have very thick walls and present an appearance similar to that of the sclerous cells in the testas of some petrified specimens.
The larger seed shown in fig. 501, D, from Commentry, described by Renault and Zeiller[1020] as Cardiocarpus sclerotesta, is another example of Cordaicarpus as now defined.
IV. Miscellaneous Seeds.
In this section are included genera based on characters of comparatively little morphological importance; their claim to treatment under a common heading is that we know little or nothing of their anatomical features or of the parent-plants.
i. Ribbed seeds.
The seeds represented by the following genera possess ribs or flanges usually in multiples of three; the transverse section may be triangular, hexagonal, or polygonal. The symmetry is generally radial, but it is difficult to determine whether a slight departure from the radiospermic form is an original feature or the result of pressure. Some of the ribbed seeds with which we are now concerned are no doubt closely allied to Trigonocarpus and should be included in the Trigonocarpales, but others may well be distinct forms.
Genera: Hexagonocarpus, Decagonocarpus, Polypterocarpus, Rhynchogonium, Boroviczia, Diplopterotesta, Musocarpus, Holcospermum.
Hexagonocarpus. Renault.
This generic name is adopted by Renault[1021] for casts from Commentry which, while probably identical with Brongniart’s Hexapterospermum, afford no confirmatory anatomical evidence. The species Hexagonocarpus crassus[1022] is represented by casts or nucules characterised by six well-defined ribs and grooves with a length of nearly 3 cm. In this genus may be included the cast from the Coal Measures of Lancashire described by Williamson[1023] as Hexapterospermum Noeggerathi (fig. 506, H), and another British example is afforded by Hexagonocarpus Hookeri Kidst.[1024], a rare fossil in the South Staffordshire coalfield. It is impossible confidently to assign these ribbed casts to genera founded on petrified specimens as they almost certainly belong to different types, but the employment of the name Hexagonocarpus may conveniently be used for casts or impressions of seeds with six longitudinal ribs differing in their relatively broader and less prominent form from the wing-like flanges of Polypterocarpus (cf. fig. 496, B). The seeds described by Dr P. Bertrand[1025] as Hexapterospermum modestae, and believed by him to belong to a plant with Neuropteris fronds, should be included in Hexagonocarpus as we have no definite knowledge of their anatomical features.
Decagonocarpus. Renault.
This name, proposed by Renault[1026], is applied to seeds without petrified tissues characterised by ten ribs; an example is afforded by Decagonocarpus olivaeformis from the Commentry coalfield, an elliptical seed which bears a close resemblance to Holcospermum sulcatum (fig. 506, A) except in the smaller number of ribs.
Polypterocarpus. Grand’Eury.
Grand’Eury[1027] adopted this generic name for seeds from St Étienne characterised by the presence of three, six, or more deep wings or flanges. The term Pterospermum has recently been proposed by Arber[1028] for a seed from the Coal Measures of Staffordshire, which he names P. anglicum: the type-species of the genus has three deep wings, one from each angle. For the same seed Kidston[1029] proposed the name Tripterospermum ellipticum, but as Arber’s account was published first his specific name has priority. Pterospermum had, however, been previously used for a genus of Sterculiaceae, and partly on this account but mainly because Grand’Eury’s genus Polypterocarpus is available the latter designation is adopted. In Polypterocarpus anglicus (fig. 496, B) the flanges project slightly beyond the apex of the seed and there is a small notch at the base; the nucule is 5 cm. long and 1 cm. broad. While it is not improbable that this seed is generically identical with Brongniart’s Tripterospermum[1030], it is safer, in the absence of structural details, to employ the less committal term. There is no information with regard to the nature of the parent-plants of species of Polypterocarpus. The English seed from the Middle Coal Measures of Derbyshire and the Staffordshire coalfield described by Arber[1031] as Radiospermum ornatum and by Kidston[1032] as Polypterospermum ornatum affords another example of Polypterocarpus as the generic name is here employed.
Rhynchogonium. Heer.
Heer[1033] proposed this generic name for some globose, ovate, or oblong ‘fruits’ from Lower Carboniferous strata in Spitzbergen, including also fragments of ‘leaves’ which without satisfactory evidence he believed to belong to the same plant. The supposed fruits are clearly seeds, and Nathorst regards the ‘leaves’ as portions of a Fern rachis. Heer described four species, but these have since been reduced to two, and indeed it is probable that only one type, Rhynchogonium costatum, is represented. Nathorst[1034] compares Heer’s seeds with a Lower Carboniferous species described by Young[1035] as Trigonocarpum gloagianum, the resemblance of which to the Spitzbergen seeds was pointed out by Kidston. A seed of Rhynchogonium costatum is about the size of a hazel-nut and may reach a length of 21 mm.; it is ovate, with a broad rounded base, and in the upper third is tapered and conical, the sides of the characteristic snout being distinguished from the smooth surface of the rest of the seed by the presence of eight ribs converging towards the apex (506, G). Zalessky[1036] recorded closely allied seeds from Lower Carboniferous beds in Northern Russia, assigning them to a new genus Boroviczia, the type-species being B. Karpinskii; he adduced arguments in favour of Heer’s interpretation of the fossils as fruits but, according to Nathorst, this view has been abandoned. In his recent memoir on the Culm flora of Spitzbergen Nathorst[1037] discusses the morphological nature of Rhynchogonium seeds and describes additional specimens. Without the aid of petrified examples it is hardly possible to determine the true nature of the fossils.
Some specimens of Rhynchogonium sulcatum in Dr Kidston’s collection show the original surface-features: the carbonised integument is divided in the upper region into linear lobes separated from one another at their origin by fairly wide sinuses, a type of integument suggesting comparison with Physostoma. This species was originally described by Lindley and Hutton as Carpolithes sulcata[1038] from Lower Carboniferous rocks at Newhaven in Scotland and has recently been figured by Zalessky[1039] from specimens in the Kidston collection from the Lower Calciferous sandstone of Midlothian. Nothing definite can be said as to the parent-plant, but it is significant that in Midlothian Rhynchogonium sulcatum occurs in a bed full of isolated pinnules of a Cardiopteris almost to the exclusion of any other fossils[1040].
The seed figured by Nathorst[1041] from the Culm of Spitzbergen as Lagenospermum? glandiforme agrees closely with Rhynchogonium and Boroviczia, and it is impossible to decide whether the lobed appearance is due to the presence of a cupule or to the divisions of an integument.
Boroviczia. Zalessky.
The specimens from Lower Carboniferous beds in Russia[1042] on which this genus was founded are perhaps unnecessarily separated from Rhynchogonium; they are ovate and beaked, 10 mm. long with a maximum breadth of 6 mm. The type-species, Boroviczia Karpinskii (fig. 506, I), is represented both by specimens in which the cast is complete and by others in which the apical snout of the integument is split into separate lobes identical in form and apparently in number with those of Rhynchogonium. Nathorst[1043], who retains the generic name and describes two additional species from Spitzbergen, points out that in Boroviczia the tapered apex is more sharply differentiated from the broader basal portion, a difference hardly worthy of generic recognition. If Boroviczia is retained as a genus distinct from Rhynchogonium it should include the seeds described by Young as Trigonocarpum gloagianum.
Diplopterotesta. Nathorst.
Diplopterotesta spitzbergensis (Heer). Heer[1044] included in Samaropsis some seeds, described as Samaropsis spitzbergensis, from Lower Carboniferous strata in Spitzbergen which differ considerably from typical examples of the genus. These have recently been made the type of a new genus Diplopterotesta[1045]: they are platyspermic seeds nearly twice as long as broad, 6–9 × 3·5–5 mm., characterised by a thin sclerotesta expanded throughout the length of the seed into two prominent transversely striated wing-like ribs or flanges (fig. 506, D). At the apex, which is broad in contrast to the pointed basal end of the seed, the sclerotesta forms a crown of eight lobes about the flat summit of the seed-body. As Nathorst points out, this species closely resembles the Carboniferous seeds figured by Grand’Eury[1046] as Polypterocarpus, but in the latter type there are more than two wings. Nathorst’s figures, one of which is reproduced in fig. 506, D, suggest a difficulty in determining the number of the flanges, which would seem to be more than two, but this appearance is regarded by Nathorst as misleading and he believes that except at the apex there are only two wings.
Musocarpus. Brongniart.
Brongniart[1047] proposed this name for two species, M. prismaticus and M. difformis, from French Coal Measures, but gave no description of the specimens. The type-species, described from the Loire by Grand’Eury as Musocarpus prismaticus[1048], is an ovate seed nearly 3 cm. long with six longitudinal ribs, three being more prominent than the others, and characterised by a ribbed prolongation of the base of uniform diameter spoken of as a carpophore, which was apparently articulated to the lower part of the seed-proper from which it was easily detached by a natural absciss-layer. Nothing is known of the structure or of the affinity of the genus. Some specimens in Dr Kidston’s collection from Westphalian beds in Lanarkshire are closely allied to or perhaps identical with M. prismaticus. In the presence of a distinct basal region Musocarpus resembles Codonospermum[1049], though without any knowledge of the anatomy of the former type it is impossible to say whether or not this resemblance has any morphological importance.
Holcospermum. Nathorst.
Holcospermum sulcatum (Sternberg). The cast reproduced in fig. 506, A, affords a good example of a form of seed recorded under several generic names and not uncommon in Upper Palaeozoic rocks in Europe and North America, which in most cases cannot be assigned to a genus implying the possession of certain anatomical characters. This type was figured by Sternberg[1050] from the Coal Measures of Radnitz as Carpolites sulcatus. Some ‘fruits’ collected on the beach near Newhaven, Midlothian, from the Calciferous Sandstone series, are figured by Lindley and Hutton[1051] as Carpolithes sulcata, but as already stated these have been transferred[1052] to the genus Rhynchogonium. Several authors have referred specimens of the type now included in Holcospermum to Rhabdocarpus, but that genus, as stated on a previous page, is restricted to seeds agreeing in form with R. tunicatus. Renault[1053] instituted the genus Colpospermum for a partially petrified seed from Commentry agreeing externally with Carpolites sulcatus Sternb. and regarded by him as specifically identical, characterised by longitudinal ribs which represent folds of the testa, the intervening grooves being occupied by an irregular reticulum formed by occasionally anastomosing smaller ribs. The generic name Colpospermum should therefore be reserved for ribbed seeds showing the anatomical features described by Renault and Zeiller: its application to Sternberg’s species is inadvisable on the ground that we have no information with regard to the morphological nature of the ribbing. More recently Arber[1054] has proposed the name Platyspermum, a name previously applied to a Cruciferous plant, for Stephanian and Permian seeds formerly assigned to Berger’s genus Rhabdocarpus, which are symmetrical in two planes. In this genus he includes Platyspermum sulcatum and among other species P. Kidstoni founded on a seed originally identified by Kidston[1055] as Rhabdocarpus multistriatus Sternb. which, though probably a distinct species, is of the same general type as C. sulcatus Sternb.
The cast represented in fig. 506, A, from the Middle Coal Measures of Yorkshire is 3·5 cm. long and has 18 regular longitudinal ribs: it is radially symmetrical and does not conform in this respect to Arber’s definition of Platyspermum. In many cases, e.g. the flattened seed figured by Kidston as Rhabdocarpus multistriatus, it is impossible to determine the symmetry of the seed. The old generic name Carpolites, Carpolithes or Carpolithus has been used in a very wide sense and does not connote any well-defined features; it should be reserved, in the form Carpolithus as used by Linnaeus, for seeds that cannot be assigned to a systematic position or which do not exhibit any distinctive characters worthy of emphasis by the institution of a special name. The generic term Holcospermum, recently proposed by Nathorst[1056], is a suitable name for this type of seed; as defined by the author, it includes seeds that may be radiospermic or platyspermic. Specimens of the type-species, H. dubium[1057],from the Lower Carboniferous beds of Spitzbergen, agree closely in size and shape with some examples of Rhynchogonium and Boroviczia, but they are distinguished by prominent ribs extending the whole length of the cast. In some seeds similar to H. sulcatum the ribs form sharp ridges, but the difference between rounded and sharp ridges is often determined by the method of preservation: a specimen deprived of its outer flesh would form a cast more sharply ribbed than a seed in which the sarcotesta had been moulded on to the ribbed sclerotesta. On the other hand some ribs are formed by blunt sclerotestal folds as in Colpospermum: both types of ribbing are included in the genus Holcospermum.
ii. Other Genera.
Malacotesta. Williamson.
This generic name was instituted by Williamson[1058] for a petrified seed from the Coal Measures of Lancashire which he named Malacotesta oblonga: the type-specimen is a small seed 6–7 mm. long characterised by a thick fleshy integument. The structure is however insufficiently known to admit of a satisfactory comparison of the imperfectly preserved specimen figured by Williamson with other types.
Thysanotesta. Nathorst.
Nathorst[1059] founded this genus on a single seed from the Lower Carboniferous plant-beds of Spitzbergen which exhibits features sufficiently distinctive to justify its recognition as the type of a separate genus.
Thysanotesta sagittula Nathorst. The type-specimen is a long and narrow seed, 20 mm. long, ovate below and prolonged apically into a slender beak, 8 mm. in length, which probably represents a micropylar tube; the beak bears numerous stiff hairs (fig. 506, F). The seed closely resembles a carpel of Erodium without the horizontal part of the awn. There is no evidence as to the nature of the parent-plant but the species affords an interesting example of a Palaeozoic seed apparently adapted for wind-dispersal, or possibly the bristles may have served the same purpose as in the fruits of Erodium. Attention is called elsewhere[1060] to the frequent resemblance of Palaeozoic seeds such as Thysanotesta to fruits of recent flowering plants.
Carpolithus. Linnaeus.
Nathorst[1061] has pointed out that this generic name was employed by Linnaeus for fossil fruits: in the form Carpolites (Sternberg) or Carpolithes[1062] it has been widely used and differently defined by authors, usually in a comprehensive sense including fossil seeds from both Palaeozoic and Mesozoic strata which cannot be assigned to a definite position in the plant-kingdom. It is desirable to adopt some designation for seeds from strata of different ages which do not exhibit any features sufficiently distinctive to justify the creation of a special genus. By employing such a name as Carpolithus, without attempting to define its characters within prescribed limits, for casts or impressions of seeds which are not distinguished by any striking characters and cannot be allocated to any particular section of seed-bearing plants the unnecessary multiplication of generic titles is avoided: when any additional data are obtained differentiating particular types from other forms of Carpolithus the provisional term should be superseded by some distinctive generic name. Among seeds from the Coal Measures there are several examples of both large and small types without any regular ribs or lacking such features as serve to distinguish the genera already described, which are appropriately included in Carpolithus. The two species Carpolithus Wildii Kidst.[1063] and C. bivalvis Goepp.[1064] are two examples of seeds from Upper Carboniferous rocks which it is desirable to refer to this comprehensive genus pending further discoveries as to their morphological features. The Jurassic species C. conicus Lind. and Hutt.[1065] is another type which it has been customary to include in Carpolithus.
(Microspermum. Arber.)
The generic name Microspermum has recently been proposed by Arber[1066] for some Westphalian seeds described by Carpentier[1067] from the North of France as Carpolithes? samaroides and for similar fossils from the Middle Coal Measures of Nottingham. The supposed seeds are small, ovate or pyriform bodies, 5–12 mm. long and 2·5–6 mm. broad, with one extremity broadly rounded and the other acute: one side is keeled, the other grooved, and a large foramen occurs near the broader end and on the grooved surface. Arber emphasises the fact that the specimens are symmetrical in one plane only. As the author of the genus points out the morphological nature of the fossils is not thoroughly established, nor is the parent-plant known. In view of the ill-defined characters of the specimens so far discovered it is hardly desirable to institute a new generic name implying their seed-nature; moreover Microspermum has previously been employed for a genus of Compositae. An examination of specimens leads me to doubt their seed-nature and it would seem more likely that they are foliar organs, possibly bracts which originally bore seeds or sporangia.
An examination of some of the specimens of supposed Permian seeds for which Geinitz[1068] proposed the generic name Guilelmites convinced me that they are inorganic structures, probably nodules in shale smoothed and rounded by slickensiding.