CHAPTER XXXV.
Seeds are abundantly represented as fossils from Carboniferous to Post-Tertiary deposits. The importance of fossil and sub-fossil species in the later geological series has been demonstrated by the investigations of Mr and Mrs Clement Reid and a few other workers in this neglected field. In cases where it is possible to assign seeds to their parent-plants the descriptions of casts, impressions, or petrifactions are added to the account of vegetative organs; but it frequently happens that seeds are preserved only as detached specimens many of which have little or no value as botanical records, while others that afford striking examples of the possibilities of petrifaction as a means of preserving the most delicate structures, are of great importance. In Volume ii. an account was given of such Palaeozoic seed-bearing organs as Lepidocarpon and Miadesmia, and the genera Lagenostoma, Sphaerostoma, and Trigonocarpus are dealt with in this volume under Lyginopteris, Heterangium, and Medullosa. Certain seeds afford some evidence as to the systematic position of the parent-plants though insufficient to warrant more than a surmise as to the nature of the vegetative organs: in several cases it is only from the resemblance of detached seeds to types that on the strength of more or less convincing evidence are referred to definite parent-plants that any conclusions can be drawn with regard to precise systematic position. In view of the occurrence of several different types of seeds that retain their morphological features, but cannot be assigned with any degree of certainty to genera founded on vegetative organs, a special chapter is devoted to a comparative study of selected examples with the object of directing attention to data bearing on evolutionary problems. The chief interest of Palaeozoic seeds to the botanist lies in the facts they contribute towards the elucidation of questions connected with the promotion of the megasporangium and megaspore of the Pteridophyta to the higher stage represented by the integumented megasporangium (nucellus) and single megaspore that in the main fulfil the definition of a seed[790]. ‘With the evolution of the seed,’ as Oliver says, ‘the plant rose at a bound to a higher plane, and this structure in its perfected form has become the very centre of the plant’s existence[791].’ We can as yet form a very partial conception of the successive stages in the adoption of the seed-habit, but since 1855, the year in which Hooker and Binney[792] published their paper on the structure of Trigonocarpus, ample proof has been furnished of the importance of Palaeozoic seeds from the standpoint of affinity between recent Gymnosperms and extinct seed-bearing plants, and of the modus operandi of evolutionary tendencies. A cursory examination of Palaeozoic seeds suffices to demonstrate their resemblance to those of recent Cycads and the seed of Ginkgo biloba; but while it is clearly with these Gymnosperms that the majority of the seeds described in the following pages are most closely allied, the extinct types possess many distinguishing features that throw light on some at least of the factors concerned in the production of the modern type. In many of the Palaeozoic seeds the nucellus stands free within the integument, to which it is attached only in the chalazal region, in contrast to the lateral union between integument and nucellus in the ovules of recent Cycads. It has been suggested by Oliver[793] that the seed of the Conifer Torreya affords a clue to the interpretation of this difference and that the lower part of the seed in Cycads and Torreya represents a later intercalation in the basal portion of the ovule, the ancient type having a free nucellus in contrast to the nucellus of modern seeds which is free only at the apex. It has been objected[794] that there are no adequate grounds for assuming the addition of an intercalated zone or of the elongation of the ovule that this implies, the more probable view being that the lateral union of nucellus and integument represents congenital fusion in the ancestral type, a fusion comparable with that of the coherent petals of a gamopetalous corolla. In the presence of a pollen-chamber most of the Palaeozoic seeds agree with those of recent Cycads, but in the extinct forms it is usually a more highly developed structure. The name pollen-chamber was given by Brongniart[795] to the pollen-containing cavity in the free region of the nucellus in the petrified seeds from St Étienne in ignorance of the use of the same term by Griffith[796] in a posthumous work published in 1852 for the nucellar chamber in Cycas[797]. The genus Stephanospermum (fig. 494, A) illustrates the means by which the pollen-chamber was liberally supplied with water and thus adapted to the requirements of fertilisation by motile gametes. The pollen-chamber and its vascular supply paved the way for siphonogamy, that is the development of a pollen-tube for the more direct transmission of the male sperms. The highly developed mantle of tracheal tissue at the periphery of the nucellus in Stephanospermum, represented on a reduced scale by the separate vascular strands of other seeds, may be compared with the tracheal investment to the nucellus in the recent Dicotyledonous genus Cassytha[798]. The presence of a nucellar vascular system in several Palaeozoic seeds is a feature in which they differ from those of recent Cycads with the exception of Bowenia. The retreat of the vascular supply from the immediate neighbourhood of the pollen-chamber in recent Cycads may, as Oliver points out, be correlated with the evolution of the pollen-tube—the substitution of siphonogamy for zoidiogamy. The diagram reproduced in fig. 492 represents a synthetic type based on such seeds as Stephanospermum and Cardiocarpus which illustrate an arrangement of conducting tissue frequently found in Palaeozoic seeds: the main strand gives off a pair of bundles in the sarcotesta in the principal plane, as in Cardiocarpus[799]; from the tracheal mass in the chalazal region numerous bundles pass up the nucellus as far as the floor of the pollen-chamber. The nature of the vascular supply in this generalised type and in individual genera should be compared with that in the seeds of recent Cycads[800] described in Chapter xxviii.
Recent research has revealed the not unexpected fact that in such Upper Carboniferous petrified seeds as have been investigated—a small proportion of the large number produced in the Palaeozoic forests—there was a remarkable range in the mechanism connected with pollination and the maturation of the microspores. A comparison of the genera Physostoma, Lagenostoma, Conostoma, Trigonocarpus, Stephanospermum and others reveals the occurrence of very different though not unrelated structural features especially in the apical region of the seed. These seeds, including Physostoma, probably the most archaic type, represent a stage in evolution already far removed from the starting-point: the diversity of plan recalls the variety in the form of the chloroplasts in the Green Algae, and in both cases we are in touch with an experimental phase representing a tentative advance towards greater efficiency.
In its differentiation into an outer fleshy region, the sarcotesta, a stony layer, the sclerotesta, and in many cases an inner flesh, the Palaeozoic seeds resemble recent Cycads: in both extinct and modern seeds the balance of evidence would seem to be in favour of attributing a single rather than a double origin to the integument.
Among the numerous types of Palaeozoic seeds are several which invite comparison with the fruits or carpels, apart from the seeds, of Angiosperms. Impressions of Samaropsis seeds (figs. 502, B–K; 503; 504) bear a close resemblance to the laterally expanded fruits of the common Crucifer Thlaspi arvense; the ribbed testa of Hexagonocarpus (fig. 506, H) and other genera recalls the fruit-wall of Alstroemeria; the recently described Lower Carboniferous seed Thysanotesta sagittula Nath. (fig. 506, F) simulates a carpel of Erodium. These and similar instances of a close parallelism in external features between organs that are not homologous, though in themselves of no morphological significance, are at least interesting as illustrating the plasticity displayed by reproductive structures, which in the Palaeozoic period marked a morphological achievement comparable in its importance with the still greater achievement represented by the highly specialised fruits of the modern Flowering plants. The range in form and surface-features of Angiospermous fruits was foreshadowed by Palaeozoic seeds. Structural types and in some cases, superadded to these, features which may reasonably be supposed to have facilitated dispersal had been acquired by the seeds of Palaeozoic plants in forms that in a much later period were adopted by fruits even to a greater degree than by seeds. Characters useful in seed-dispersal, that are now shared by fruits and seeds, are illustrated by the fleshy and possibly edible seeds of extinct Gymnosperms, the plumes and hairy beak of Gnetopsis (fig. 494, E) and Thysanotesta (fig. 506, F) suggestive of feathery stigmas and other appendages. The lacunar sarcotesta of Aetheotesta, the thick endotesta of Pachytesta (fig. 497), and the air-chamber of Codonospermum (fig. 498), are strictly comparable with aids to buoyancy in fruits of existing Flowering plants. The mucilage-hairs and superficial cells in Physostoma (fig. 494, I) and Stephanospermum may be compared with the thick mucilaginous investment of the megaspores of recent water-ferns and with similar tissues of some Angiospermous seeds.
The bionomics of Palaeozoic plants is a subject worthy of more serious attention than it has so far received. The search for morphological characters that may have facilitated the wanderings of widely distributed genera and species and a closer investigation of physiological-anatomical problems presented by the vegetative organs of petrified plants would not only extend our knowledge of the morphology of ancient types but would stimulate comparative study and, incidentally, relieve the dullness of pure description. It may be argued that we should first establish a more solid foundation by further observations on living plants; but even at the risk of allowing speculation too free a hand the attempt is worth making, and it may be urged that, as in phylogenetic enquiries so in other branches of botany, facts obtained from plants of other ages may serve to supply deficiencies in knowledge based only on existing forms. One of the difficulties inseparable from the study of fossil plants, namely the identification of impressions and casts with specimens exhibiting anatomical characters, is particularly well illustrated by seeds. The description of a genus based on mere external form may sometimes be extended without great risk of error to include species founded on anatomical characters, but on the other hand, there are many instances in which—despite a general resemblance in form and size between petrifactions and impressions—lack of evidence of generic identity requires the employment of distinctive names. The determination of impressions is, as Lesquereux recognised, ‘subject to a great deal of uncertainty,’ and many of the genera founded on external features are purely artificial and include species that have no essential features in common. Moreover in the case of petrified specimens the apparent absence of an external fleshy layer is often due to destruction before preservation: as Solms-Laubach[801] points out, it is obviously impossible to be certain as to the number of integumental layers in seeds that are not well preserved in all their parts. Goeppert founded a new genus, Acanthocarpus, on a Permian seed described as A. xanthioides[802], because of the occurrence of spinous processes attached to an obcordate kernel: these apparent spines are in all probability the remains of a very imperfectly preserved sarcotesta. The preservation of the central portion of a seed, that is the seed-cavity with the enclosing shell, conveniently called the nucule, has often led to an unnecessary multiplication of generic terms. Other examples of confusion resulting from different states of preservation are quoted in the accounts of some of the selected types.
Williamson in 1877 pointed out that we learn from the large number of different kinds of Palaeozoic seeds that ‘there were in the Carboniferous forests many gymnospermous stems clothed with foliage of which we have not yet discovered any traces, probably because these Gymnosperms did not flourish upon the low swampy grounds which were the homes of the great mass of the coal-producing plants[803].’ Prof. Zeiller[804] has also drawn attention to the numerical excess of seeds over vegetative organs. This discrepancy has to a large extent been explained by the discovery that many of the supposed Ferns were seed-bearing plants, and a further explanation is suggested by the superiority of seeds over stems and leaves in their adaptation to dispersal by water.
In 1874 Brongniart[805] described several petrified seeds from material discovered by Grand’Eury in the St Étienne coal-field, and seven years later his descriptions were republished[806], with the addition of several beautifully executed drawings, as a posthumous volume edited by his distinguished pupil Renault. Williamson’s researches supplied much additional information, and in recent years the more detailed study of French and English seeds by Bertrand and particularly by Oliver and his pupils has further emphasised the interest and importance of this field of work. Brongniart proposed a two-fold classification of French seeds: (i) bilaterally symmetrical seeds, more or less flattened in section, which he believed to be Cordaitean; (ii) radially symmetrical seeds, circular in transverse section: the latter group he considered to be less closely allied to recent types. The employment of the terms Platyspermeae and Radiospermeae, proposed by Oliver[807] for Brongniart’s divisions, serves a useful purpose if due regard is paid to the adequacy of the evidence as to symmetry and if it is recognised that this classification cannot be rigidly employed in all cases. It was pointed out by Brongniart that the occasional occurrence of tricarinate seeds of Ginkgo (fig. 631) and Taxus is an exception to the general rule of bilateral symmetry: seeds of Cycas are normally bilateral, but radially symmetrical forms also occur[808]. The genus Conostoma (fig. 494, B) represents an intermediate type which, though almost radially symmetrical, exhibits a slight tendency towards platyspermy. Evidence recently brought forward by Nathorst[809] renders probable a connexion of a presumably radiospermic seed Lagenospermum Arberi[810] with the Lower Carboniferous fronds Adiantites bellidulus Heer, and this furnishes an interesting illustration of the association of both platyspermic and radiospermic seeds with the same generic type of foliage. While retaining Radiosperm and Platysperm as convenient descriptive terms, I have not adopted them as group-designations on the ground that they do not in themselves serve as trustworthy criteria of relationship. Attention is called by Salisbury[811] to the occurrence of bilaterally and radially symmetrical fruits among British Carices and to a similar mixture in the family Polygonaceae.
The acquisition of more detailed and accurate knowledge of Palaeozoic seeds led to an extension of the two-fold division of Brongniart and Oliver which is based on such characters as the position of the vascular tissue in relation to the integument and nucellus, the form of the pollen-chamber, and other features. The division Lagenostomales has been instituted for Lagenostoma and some other Radiosperms connected by certain important characters: these seeds may be referred to the Pteridospermeae though it is only in the case of Lagenostoma, and to a less extent Sphaerostoma, that a correlation between vegetative organs and seeds has been rendered sufficiently probable to justify an assumption of generic identity. The name Trigonocarpeae[812] has recently been used for a section of Radiosperms represented by Trigonocarpus, Stephanospermum, and other genera. Although the genus Stephanospermum, as Oliver[813] says, may be regarded as the type-genus of a group of seeds, it is more fitting, as the same author[814] insists, to adopt a divisional term based on the generic name of the much more widely spread and more familiar Trigonocarpus. For the sake of uniformity in nomenclature it is proposed to adopt the name Trigonocarpales instead of Trigonocarpeae to rank with Lagenostomales.
The Platyspermeae comprise such seeds as Cardiocarpus, Mitrospermum, and Rhabdospermum, genera characterised by well-marked anatomical features and probably Cordaitean; it has, however, been shown that typical Platysperms were also borne on leaves of Pteridosperms and, as Mrs Arber[815] says, the notion that every member of the Platyspermeae was necessarily a Cordaitean seed has been discredited by the discovery of the seeds of Aneimites (Wardia) and Pecopteris Pluckeneti[816]. For general purposes it is hardly necessary to adopt the subdivisions of the Lagenostomales used by Oliver and Salisbury[817], though as facts accumulate we shall no doubt be able to make further advances towards a natural system of classification. The following three divisions of Permo-Carboniferous seeds include genera founded on petrified specimens and thus afford valuable morphological data. The groups Lagenostomales and Trigonocarpales include types belonging to closely related plants, a relationship clearly expressed in the seed-characters.
I. Lagenostomales.
The seeds included in this group are for the most part Radiosperms, but in its slightly developed bilateral symmetry Conostoma oblongum is a type transitional between Radiosperms and Platysperms. The testa may be ribbed and the ribs vary in number. The nucellus (megasporangium) is united to the integument not only at the base but laterally as far as the shoulder of the seed up to a level corresponding to the base of the pollen-chamber (lagenostome) as in all recent Cycads and in the majority of Conifers. The seeds proper apart from the cupule are supplied with a single set of vascular bundles; there is no vascular tissue in the nucellus, a feature no doubt correlated with the fusion of nucellus and integument[818]. The free portion of the integument is more or less deeply lobed or, in Lagenostoma, it forms a pyramidal canopy of fused lobes enclosing the lagenostome. The presence of a tapetal zone surrounding the megaspore is believed to be a feature characteristic of the group[819]. The testa, wholly or partially ribbed, is relatively thinner than in the Trigonocarpales and Cardiocarpales, and in its differentiation agrees less closely with the testa of recent Cycadean seeds. In Lagenostoma and possibly in other genera a loose sheath or cupule surrounded the ovule, while in Gnetopsis a similar envelope enclosed two to four seeds.
The microspores are multicellular and smaller than those of Trigonocarpales, the average dimensions (Conostoma, Physostoma, Lagenostoma) being 67μ × 52μ.
Genera. Physostoma; Conostoma; Sphaerostoma; Lagenostoma; Gnetopsis.
Lagenostoma may safely be referred to Lyginopteris, and Sphaerostoma with but little risk of error to Heterangium: the parent-plants of the other genera are unknown, but all may be regarded as the seeds of Pteridosperms and probably of genera more nearly allied to the Lyginopterideae than to the Medulloseae. The genus Lagenospermum, recently instituted by Nathorst[820], is dealt with in Chapter xxxi.
Physostoma. Williamson.
Physostoma elegans Williamson.
The generic name Physostoma[821] was applied by Williamson[822] to a seed from the Lower Coal Measures of Lancashire which he named P. elegans; he afterwards described it as Lagenostoma physoides, but the original name has been revived by Oliver[823] to whom our knowledge of this type is chiefly due. The specimens figured by Williamson[824] as Sporocarpon ornatum also belong to Physostoma elegans. The seeds are circular in section, approximately 6 mm. long with a maximum diameter of 2 mm. The testa has about 10 longitudinal ribs passing in the apical region into a ring of free lobes or tentacles surrounding and considerably overtopping the nucellar apex: these tentacles take the place of a micropylar tube (fig. 494, I; fig. 493, D) and are a feature ‘in which this seed differs from all other known seeds, fossil or recent[825].’ A single vascular strand passes through the chalazal region and divides into 10 bundles, one to each rib and tentacle. The single integument consists of a few layers of cells, those of the epidermis being prolonged into clavate mucilaginous hairs, fig. 494, I, h, that may reach a length of ·5 mm. and in the living seed almost covered the whole surface of the testa, being especially long on the ribs and tentacles. There is no special development of sclerous tissue, the vascular bundles, v, being embedded in parenchyma in the inner portion of the integument. The nucellus is represented by a zone rich in secretory cells, and internal to this is a tapetum. Integument and nucellus are coalescent up to the apical region where the former splits into 10 tentacles. The nucellar apex has the form of a tall dome surrounded by a bell-shaped pollen-chamber (fig. 494, I, pc; fig. 493, C, D, c) into which it projects like the base of a wine-bottle. The circular opening of the pollen-chamber overtops the roof of the dome formed of the secretory tissue of the nucellus and the carbonised remains of the tapetum: this dark band surrounds the large megaspore-cavity (fig. 494, I). Physostoma is the only member of the Lagenostomales in which the megaspore projects into the free nucellar apex: in other genera intercalary growth has produced a more or less prominent plinth, the name given to the free portion of the nucellus between the megaspore and the pollen-chamber. Williamson[826] described the mammillated apex of the nucellus as pushed up into the base of the lagenostome which ‘looks like a bladder half full of fluid resting upon and overhanging the end of a soda-water bottle’: it was this appearance that suggested the name Physostoma. The section reproduced in fig. 493, D, shows in the centre the limiting tissue of the nucellus surrounded by the pollen-chamber, c, and external to this are the tentacles with their groups of long hairs: the vascular bundles are represented by spaces in the more internal small-celled tissue (see also fig. 494, I). A characteristic feature is the presence of a tapetum or megaspore-jacket[827] in the nucellus: immediately internal to the vascular bundles stretching from the chalaza to the apex of the megaspore-cavity is a layer of delicate cells with secretory sacs, and this is succeeded by a broad black layer of rather larger cells, a tissue which was probably in full activity in a younger stage of development.
A comparable tapetal layer is described by Lang[828] in the ovule of Stangeria: the majority of the sporogenous cells surrounding the megaspore become disintegrated and are absorbed, but the outermost zone forms a more definite tapetal layer: as already suggested, this tissue in Physostoma may be a group-character. No archegonia have been found, but in a few cases some of the delicate prothallus-tissue occurs in the interior of the seed. Microspores are often abundant in the pollen-chamber (fig. 493, C, c); in one seed 80 are recorded. The occurrence of so many microspores suggested to Oliver that insect-agency may have been responsible for the precision in pollination that is greater than one would expect in anemophilous plants. The spores are smaller than those of Lagenostoma (55μ × 45μ) and in several of them the remains of a cellular tissue are preserved (fig. 494, N), also some sub-reniform bodies (fig. 494, M) similar to those described as spermatozoids by Dr Benson in Lagenostoma (fig. 408, D).
The most interesting features of Physostoma are: the absence of a continuous micropylar tube and its replacement by a circle of integumental lobes; the apical prolongation of the nucellar apex into the pollen-chamber, and the presence of long mucilaginous hairs on the integument. The large pollen-chamber is a character which distinguishes Physostoma from Conostoma and its form is very different from that in Lagenostoma.
The tentacles of the integument and the form of the nucellar apex are features consistent with Oliver’s view that Physostoma is the most primitive of Palaeozoic seeds though, as Burlingame[829] says, the elaborate form of the encasing envelope marks a considerable advance beyond the earliest type of megasporangium integument.
A new type of Physostoma has been briefly described by Gordon[830], without a specific designation, from the Lower Carboniferous beds of Pettycur (Fife): it was found in association with Heterangium and Sphaerostoma ovale.
We have no knowledge of the plant to which Physostoma belonged, but the general plan of organisation of the seed points to a near relationship to Lagenostoma and presumably, as regards the parent-plant, to a genus related to Lyginopteris.
Conostoma. Williamson.
This name[831], suggested by the funnel-like pollen-chamber or lagenostome, was applied by Williamson[832] to some seeds from the Lower Coal Measures of Lancashire and Yorkshire and from the Lower Carboniferous beds of Burntisland. The Burntisland seeds, referred by Williamson to two species, have recently been united and described by Miss Benson as Sphaerostoma ovale[833]. The English species has been thoroughly investigated by Oliver and Salisbury[834] who have also described a second species, C. anglo-germanicum, from the Coal Measures of Lancashire and Germany.
Conostoma oblongum Williamson.
This rare type is represented by approximately cylindrical seeds with an average length of 5 mm. and a maximum breadth of 2·3 mm. borne on a relatively stout stalk and tapering to a blunt apex characterised by a canopy of six short lobes (fig. 494, B, C) in marked contrast to the long tentacles of Physostoma. In the basal region the integument has six prominent ribs which soon die out when traced upwards: a transverse section through the body of the seed is hexagonal (fig. 494, D), the angles corresponding to the basal ribs, and there is a slight tendency to platyspermy. The testa has an epidermal mucilaginous layer which becomes exfoliated through the lifting-up of the cuticle by the underlying mucilage: below this, at the apex of the seed, is a cap of fleshy tissue (fig. 494, B, sa) which, it is suggested, may have had a secretory function in connexion with a drop-mechanism in pollination like that in recent Conifers. No microspores have been found in the pollen-chamber. The epidermis, called by Oliver and Salisbury the blow-off layer (fig. 494, B, m), together with the cap of soft tissue constitute a feebly developed sarcotesta. A sclerotesta consisting of a palisade-layer and a fibrous hypoderm extends over the main body of the seed below the epidermis; it forms the basal ribs and increases considerably in breadth at the apical region to form a sclerous cone penetrated by six strands of parenchyma enclosing vascular bundles (fig. 494, D) which pass up from the conducting tissue immediately external to the nucellus. The nucellus is coalescent with the integument, as in Physostoma, as far as the level of the domical free apex of the nucellus where the tapetal tissue that lines the seed-cavity passes across the almost flat top of the central region originally occupied by the megaspore. In some sections prothallus-tissue was found with an apical ‘tent-pole’ protuberance. A striking feature of Conostoma is the mechanism for the reception of the microspores. The free part of the nucellus consists of the plinth, a broad tapering region originally filled with parenchyma but in most cases represented only by its epidermis: the plinth, p, is seen in fig. 494, B, to be two-storeyed, the upper and narrower storey being a space formerly filled by a pad of tissue suspended from the floor of the superposed lagenostome (pollen-chamber)[835]. The greater development of the domical plinth is a feature in which Conostoma differs from Physostoma. At the apex of the plinth and resting on a slight depression is a small lagenostome, bowl-shaped in section, and like the pollen-chamber of Lagenostoma, formed as the result of enzyme-action on the apical papilla of the nucellus (fig. 494, B, B′, l′). The mouth of the lagenostome engages with the micropylar tube by a projecting flange (fig. 494, B′, f) of tissue lining the micropylar canal and by a second flange (f′) at the base of the lagenostome where the roof of the plinth (fig. 494, B′, p) bends downwards and inwards. The walls of the lagenostome are formed by strong cells with thickening bands giving them the appearance of tracheids (l′), but the floor of the lagenostome is made of thinner cells which become disorganised, allowing the microspores to fall into the large plinth-cavity below (p, fig. 494, B), an arrangement comparable with the two-storeyed pollen-chamber of Bowenia[836] and, to a less extent, with the micropyle of the Conifer Tsuga. The microspores are multicellular and ellipsoidal measuring 75μ × 65μ.
The species Conostoma anglo-germanicum agrees closely with C. oblongum in general form and organisation, but it has eight ribs, four more prominent than the others, and differs also in other minor characters from the rather shorter seeds of the type-species. Conostoma differs from Lagenostoma in the absence of the tubular prolongation of the lagenostome, the micropyle being like that in recent Gymnosperms. In Conostoma the tracheid-like elements of the lateral wall of the lagenostome are a characteristic feature, and no evidence has been found of the existence of a central core of tissue such as occupies the centre of the seed-apex in Lagenostoma. The long hairs of Physostoma are represented in Conostoma by the much smaller mucilaginous cells of the epidermis and in Lagenostoma by the less closely united mucilage-cells of the superficial layer of the testa.
Sphaerostoma. Benson.
As already pointed out in Chapter xxix. where this genus is described as probably the seed of Heterangium, there is a fairly close general resemblance between Sphaerostoma and Lagenostoma. In the presence of free apical lobes the former genus resembles Conostoma, and while agreeing with Lagenostoma in its annular pollen-chamber it is peculiar in the retention of an epidermis over the roof of the pollen-chamber: as in Lagenostoma the seed is enclosed by an outer integument or cupule.
Lagenostoma. Williamson.
An account of this type of seed is included in the description of Lyginopteris[837]. The more striking peculiarities are exhibited by the pollen-chamber and the free region of the integument: the annular pollen-chamber (fig. 493, D, c; fig. 409) surrounds a central nucellar cone and is prolonged upwards as a tube engaging with the micropyle in contrast to the form of the pollen-chamber and the absence of a tubular prolongation in Conostoma. The tentacles of Physostoma and the short apical lobes of Conostoma are replaced by an apical cone formed by the coalescence of the integument containing nine cavities originally filled with parenchyma (figs. 409; 493, B). The presence of a cupule is a characteristic feature of young seeds, but from negative evidence in the case of most other seeds it is unsafe to assume that the cupule of Lagenostoma is an exceptional feature. The nucellus and testa are united as far as the shoulders of the seed as in the seeds of recent Cycads and in contrast to their lateral independence in Trigonocarpus, Stephanospermum, and other genera.
Gnetopsis. Renault.
This generic name was given by Renault[838] to some small petrified seeds from the Stephanian of Grand’ Croix and to impressions from the Commentry coal-field which he believed to belong to some Gnetaceous plant. Saporta and Marion[839] and other authors have accepted these seeds as evidence of the existence of Palaeozoic Gnetales: it has, however, been shown[840] that Gnetopsis has no claim to such relationship and is a type of seed closely allied to Conostoma. Renault described three species, afterwards adding three from another locality[841]; the genus is recorded also from Commentry[842] and Gard[843]. More recently Depape and Carpentier[844] have described examples from the Westphalian of Valenciennes which they place in the Pteridosperms in accordance with the conclusion of Oliver and Salisbury. Gnetopsis has also been discovered by Mr Hemingway in the Middle Coal Measures of England[845] (fig. 494, H).
Gnetopsis elliptica Renault.
The seeds of this species, slightly oval in section, occur in groups of 2–4 in a cupular investment (fig. 506, E, p. 359) described by Renault as an ovary but correlated by Oliver and Salisbury with the cupule which surrounds the single ovule of Lagenostoma. The cupule is lined with hairs similar to those on the wall of the cupule of Lagenostoma. A characteristic feature of the French seeds is the presence of three or four long plumes of hairs at the apex (fig. 494, E, F). As seen in fig. 494, E, a small lagenostome (pollen-chamber) rests on the roof of a broad plinth precisely as in Conostoma, and four vascular bundles, corresponding to the six bundles in Conostoma, pass into the apical cap of sclerous tissue enclosed by a sarcotesta, sa (fig. 494, E, G). A ‘tent-pole’ prolongation (fig. 494, E, t) occurs at the apex of the prothallus. Renault described a portion of the integument as consisting of lacunar tissue which Oliver and Salisbury homologise with the superficial mucilaginous layer of Conostoma: this is seen above the sclerotesta in the apical region of fig. 494, E, sa.
Gnetopsis anglica Kidston MS.
This species (fig. 494, H) is represented by seeds from the Middle Coal Measures near Barnsley, Yorkshire, 4 mm. long with apical appendages at least 2·2 cm. in length and probably four in number. The appendages do not show the hairs which form a prominent feature in the French specimens, but this is probably the result of imperfect preservation: there are indications of hairs on other specimens in Dr Kidston’s Collection. The type-specimen, in Dr Kidston’s Collection, was generously lent to me for examination.
Gnetopsis, while agreeing with Conostoma in the more important features, is distinguished by the apical plumes, the very slight development of a tent-pole prolongation of the nucellar apex (fig. 494, E, t), the smaller number of vascular bundles, and by the presence of an enclosing cupule (fig. 506, E). In its slight departure from radial symmetry Gnetopsis forms a transition between the Radiospermeae and the Platyspermeae. It is undoubtedly the seed of a Pteridosperm, but nothing is known as to the nature of the vegetative organs of the parent-plant.
II. Trigonocarpales.
In this group are included radially symmetrical seeds for the most part belonging to members of the Medulloseae. The peripheral zone of the nucellus is supplied with vascular tissue and the nucellus is free within the integument except at the base; it is superior and not semi-inferior[846] as in recent Cycads and in Lagenostomales. The ovule of Myrica Gale, in which the nucellus stands free within the single integument, affords an interesting parallel to seeds of this class in contrast to the usual Angiospermous type with a laterally coalescent nucellus. In Myrica Gale[847] the vascular supply is confined to the integument. There is a comparatively broad pollen-chamber and in some types the lateral tissue of the nucellus is prolonged as a tube within the micropyle. The usually ribbed testa is differentiated into an outer flesh (sarcotesta), a sclerotesta, and probably in most cases an endotesta or inner flesh. The ribs of the sclerotesta are in the majority of genera in multiples of three and in position correspond to the outer ring of vascular bundles. The presence of lacunar tissue in the sarcotesta of several genera may be correlated with dispersal by water. The apical region of the integument is not lobed but extends as a longer or shorter micropylar tube above the summit of the nucellus. In the differentiation of the testa, the form of the pollen-chamber, and in some other features, the seeds of this group present a general agreement with those of recent Cycads.
The microspores are multicellular and larger than those of the Lagenostomales: in Stephanospermum akenioides they measure 160μ × 100μ[848] while in Aetheotesta[849] they reach 360μ × 290μ.
Genera. Trigonocarpus; Tripterospermum; Ptychotesta; Hexapterospermum; Polypterospermum; Pachytesta; Stephanospermum; Polylophospermum; Codonospermum; Aetheotesta; Eriotesta; Gaudrya.
Trigonocarpus. Brongniart.
A description of the morphological features of Trigonocarpus Parkinsoni and T. shorensis is given in the chapter on Medullosa (p. 117), as there is good evidence that they are the seeds of that genus. There is considerable difference in size and to some extent in the form of casts included in Trigonocarpus and, in the absence of anatomical data, it is hardly possible to determine the actual systematic position of many of the specimens so named. Dr Arber[850] has recently proposed a new generic name Schizospermum for casts very like those of T. Parkinsoni, but distinguished by the splitting of the shell into three valves, a character which leads him to conclude that it is the external surface which is preserved and not a mere cast of the seed-cavity. It is, however, more probable that the specimens are casts of a split sclerotesta. In Trigonocarpus pusillus[851] the shell is divided into three valves, the dividing lines being marked by greatly reduced ribs, and in T. schizocarpoides Grand’Eury[852], a species that may not be a true Trigonocarpus, there is also evidence of splitting. Arber points out that the species Rhabdocarpus Boschianus Berg. is founded on a Trigonocarpus from which the outer flesh has disappeared leaving the shell as the external covering. Trigonocarpus seeds are widely distributed in Carboniferous and Permian rocks in Europe and North America: from the latter continent Newberry[853] has described several different forms that afford good examples of the abundance and variety of the genus. Some of the specimens included by Newberry[854] in Trigonocarpus are probably distinct generic types: his species T. multicarinatus may be identical with the ribbed cast shown in fig. 506, A. The casts described by Lindley and Hutton and by other authors as T. Dawesi[855] are nearly 5 cm. long, and if these are correctly included in the genus they point to the occurrence of seeds much larger than the type-species. The French species Trigonocarpus pusillus[856] Brongn., one of the smallest Palaeozoic seeds, from 6·5 to 15 mm. long, differs from Trigonocarpus Parkinsoni and T. shorensis in the absence of prominent ribs and in the much feebler development of the sarcotesta. Specimens of the German type T. sporites Weiss, believed by some authors to be megaspores, were described by Zeiller[857] from Valenciennes as seeds: these are from 2·5 to 3·5 mm. long and have three small ribs. Zeiller quotes the presence of cell-outlines on the surface as evidence of their seed-nature, but it may be that this feature represents a sculpturing of the exine of a spore. Typical Trigonocarpus seeds agree in several morphological characters with those of recent Cycads. They differ in the lack of a lateral union between nucellus and integument; the presence of nucellar tracheids, though a feature shared with Bowenia, distinguishes them from the majority of recent Cycadean seeds. In the comparatively long and fleshy micropylar tube a seed of Encephalartos Lehmanni presents a fairly close resemblance to a Trigonocarpus. Salisbury has pointed out that the three species T. Parkinsoni, T. shorensis, T. pusillus form a consecutive series illustrating the gradual disappearance of the secondary ribs that form a prominent feature in T. Parkinsoni; but for a comparison of these with other types of fossil and recent seeds the reader is referred to Salisbury’s summary[858].
Tripterospermum. Brongniart.
The seed on which this genus was founded by Brongniart[859] is clearly very closely related to Trigonocarpus and, as Oliver[860] says, the distinguishing character described by the author of the genus is unimportant. Brongniart describes the type species, T. rostratum, as characterised by the presence of three prominent wings composed of a testa differentiated into an inner hard tissue and an outer lacunar tissue. It is, however, hardly possible to say whether the outer soft tissue was originally flattened in the form of ‘wings’ or pressed down on to the harder shell. Renault[861] notes the association of seeds that he refers to this genus with the leaves of Dorycordaites, but apart from the improbability of any connexion between Tripterospermum and Cordaites, Renault’s seeds are too imperfect to demonstrate their identity with Brongniart’s genus. Kidston[862] has described an impression of a three-winged seed from the coal-field of Staffordshire as Tripterospermum ellipticum, a form described on page 357 as Polypterocarpus anglicus (fig. 496, B).
Ptychotesta. Brongniart.
The type-species of this genus[863], Ptychotesta tenuis[864], about 3 cm. long, is characterised by six very prominent flanges or wings formed by the fissured or folded sclerotesta (fig. 495, B). There is no information as to the vascular supply or other anatomical details. It is not at all improbable that there is no real distinction between this genus and Brongniart’s genus Hexapterospermum.
Hexapterospermum. Brongniart.
In this genus Brongniart[865] included two species, Hexapterospermum stenopterum and H. pachypterum: the shell is hexagonal in transverse section, each angle being prolonged as a narrow flange. In one of the sections figured by Brongniart (fig. 495, E) the ribs are not fissured: this is said to be a feature distinguishing Hexapterospermum from Ptychotesta, but the occurrence of a fissured rib in another section suggests that in the structure of the ribs there is no essential difference between the two genera. In Ptychotesta pachypterum the testa is prolonged at the chalazal end as in Polylophospermum, and it is possible that there is no generic difference. Williamson described a cast from the Coal Measures of Lancashire as Hexapterospermum [= Hexagonocarpus] Noegerrathi[866] (fig. 506, H), but in the absence of anatomical characters it is preferable to avoid the use of Brongniart’s term and to assign them to Renault’s genus Hexagonocarpus[867]. Similarly the seeds referred by Dr P. Bertrand[868] to Hexapterospermum may appropriately be included in the genus Hexagonocarpus.