Palaeobotany
From LoveToKnow 1911
PALAEOBOTANY. In the present article the subject of vegetable palaeontology is treated from a botanical point of view. The science of botany is concerned with the vegetable kingdom as a whole, and not merely with the flora now living. The remains of the plants of former periods, which have come down to us in the fossilized state, are almost always fragmentary, and often imperfectly preserved; but their investigation is of the utmost importance to the botanist, as affording the only direct evidence of the past history of vegetable organisms. Since the publication of the Origin of Species the general acceptance of the doctrine of evolution has given a vastly increased significance to palaeontological data. The determination of the course of descent has now become the ultimate problem for the systematist: this is an historical question, and the historical documents available are the remains of the ancient organisms preserved in the rocks. The palaeobotanist thus endeavours to trace the history of plants in the past, with the hope of throwing light on their natural affinities and on the origin of the various groups. His investigations must embrace not only the comparative morphology and anatomy of fossil plants, but also their distribution over the earth's surface at different periods - a part of the subject which, besides its direct biological interest, has obvious bearings on ancient climatology and geography.
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Preservation
Before considering the results of palaeobotanical research, some account must be given of the way in which the evidence is presented, or, in other words, of the modes of preservation of vegetable remains. These fall under two main heads. On the one hand, there is the mode of preservation which gives rise to casts, moulds and generally impressions, exhibiting the superficial features of the specimen. The great majority of vegetable fossils are of this kind, and the term incrustation is used as a general term to cover all such methods of fossilization. On the other hand, there are specimens in which the tissues of the plant have been permeated by some mineral in solution, which, subsequently setting hard, has fixed and preserved the internal structure, often with astonishing perfection of detail. This second method of fossilization is termed petrifaction. In the case of incrustation the whole substance of the fossilized specimen - e.g., a stem of Sigillaria - may be replaced by mineral matter, such as sandstone or shale, giving a cast of the whole, on the outer surface of which the external markings, such as the bases of leaves and the scars left by their fall, are visible in their natural form. Usually the original organic substance remains as a thin carbonaceous layer forming the surface of the cast, but sometimes it has entirely disappeared. The surrounding matrix will of course show the mould of the cast, with its elevations and depressions reversed. In the case of thin, flat organs such as leaves, the whole organ may be spread out in the plane of stratification, leaving its impress on the overlying and underlying layers. Here there has not necessarily been any replacement of organic by inorganic material; the whole leaf, for example, may remain, though reduced to a carbonaceous film. In such carbonaceous impression not only are the form and markings, such as venation, perfectly preserved, but something of the actual structure may remain. The cuticularized epidermis, especially, is often thus preserved, and may be removed by the use of appropriate reagents and examined microscopically. If sporangia and spores are present they also may persist in a perfectly recognizable form, and in fact much of our knowledge of the fructification of fossil Ferns and similar plants has been derived from specimens of this kind.
In many cases internal casts have been formed, some large cavity, such as a fistular pith, having become filled with mineral substance, which has taken the impress of the surrounding structures, such as the wood. The common casts of Calamites are of this nature, representing the form of the hollow medulla, and bearing on their surface the print of the nodal constrictions and of the ridges and furrows on the inner surface of the wood. The whole organic substance may have been removed, or may persist merely as a thin carbonaceous layer. Mistakes have often arisen from confusing these medullary casts with those of the stem as a whole.
Although some information as to minute structure may often be gleaned from the carbonaceous coating of impressions, the fossils preserved by petrifaction are the main source of our knowledge of the structural characters of ancient plants. The chemical bodies which have played the most important part as agents of petrifaction are silicic acid and calcium carbonate, though other substances, such as magnesium carbonate, calcium sulphate and ferric oxide have also been concerned, either as the chief constituents of petrifac tions, or mixed with other bodies. A large number of the most important remains of plants with structure preserved are silicious; this is the case, for example, with the famous French Permo-Carboniferous fossils of St Etienne, Autun, &c., which in the hands of Brongniart, Renault and others have yielded such brilliant scientific results. At a more recent horizon, the silicified specimens of the Mesozoic Gymnosperms from Great Britain, France, and especially North America, are no less important. Calcified specimens are especially characteristic of the British Carboniferous formation; their preservation is equally perfect with that of the silicified fossils, and their investigation by Witham, Binney, Williamson and others has proved no less fertile. In the Coal Measures of England and of certain German and Austrian districts (e.g. Langendreer in Westphalia; Ostrau in Moravia), calcareous nodules, crowded with vegetable fragments of every kind, occur in certain mines embedded in the substance of the coal and representing its raw material in a petrified condition.. Even the most delicate tissues, such as cambium and phloem, the endosperm of seeds, or the formative tissue of the growing-point, are frequently preserved cell for cell, both in calcareous and silicious material. As a rule, the petrified remains, all-important for the revelation of structure, are fragmentary, and give little idea of the habit or external characters of the plants from which they were derived. Hence they must be brought into relation with the specimens preserved as casts or impressions, in order to gain a better conception of the plant as a whole. This is often a difficult task, and generally the fragmentary nature of practically all vegetable fossils is the chief hindrance to their investigation. Owing to this, it has become the common practice of palaeobotanists to give distinct generic names to detached parts of plants which may even have belonged to one and the same species. Thus the roots of Sigillaria are called Stigmaria, detached leaves Sigillariophyllum, and the fructifications Sigillariostrobus; the name Sigillaria applies to the stem, which, however, when old and partly decorticated has been called Syringodendron, while its woody cylinder has often been described under the name Diploxylon. This naming of portions of plants, however objectionable, is often not to be avoided; for detached organs constantly have to be described long before their relation to other parts is established - which, indeed, may never be accomplished. For example, the form and structure of Stigmaria have long been well known; but it is seldom possible to determine whether a given Stigmaria belonged to Sigillaria, Lepidodendron or some other genus. The correct piecing together of the fragmentary remains is one of the first problems of the palaeobotanist, and the gradual disappearance of superfluous names affords a fair measure of the progress of his science. The recent advance of fossil botany has depended in a very great degree on the study of petrified specimens with their structure preserved; so far, at least, as the older strata are concerned, it is, as a rule, only with the help of specimens showing structure that any safe conclusions as to the affinities of fossil plants can be arrived at.
The subject of coal (q.v.) is treated elsewhere. Here it need only be said that the masses of vegetable substance, more or less carbonized and chemically altered, of which coal is composed, frequently contain cells and fragments of tissue in a condition recognizable under the microscope, as for example spores (sometimes present in great quantities), elements of the wood, fibres of the bark, &c. These remnants, however, though interesting as revealing something of the sources of coal, are too fragmentary and imperfect to be of any botanical importance. In lignite, on the other hand, the organized structure is sometimes excellently preserved. In the Wealden of Belgium, for example, specimens of Ferns and Coniferae occur, in the form of lignite, which can be sectioned, like recent plants, with a razor, and exhibit an almost unaltered structure.
I.-Palaeozoic The present section is concerned with the botany of the Palaeozoic age, from the oldest rocks in which vegetable remains have been found up to the close of the Permian period. The Glossopteris flora of India and the southern hemisphere, the age of which has been disputed, but is now regarded as for the most part Permo-Carboniferous, is, however, dealt with in the succeeding section, in connexion with the Mesozoic floras. The various groups of plants represented in the Palaeozoic rocks will first be considered in systematic order, after which some account will be given of the succession and distribution of the various floras during the period.
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In dealing with the plants of such remote epochs, the relative importance of the various groups, so far as they are known to us, is naturally very different from that which they assume at the present day. There is no evidence that the Angiospermous flowering plants, now the dominant class, existed during the Palaeozoic period; they do not appear till far on in the Mesozoic epoch, and their earlier history is as yet entirely unknown. On the other hand, fern-like seed-plants, known as Pteridosperms, and Gymnosperms belonging almost entirely to families now extinct, were abundant, while the Pteridophyta attained a development exceeding anything that they can now show. Among the lower classes of plants we have scarcely any knowledge of Palaeozoic Bryophyta; Fungi were probably abundant, but their remains give us little information; while, even among the Algae, which are better represented, well characterized specimens are scanty.
With few exceptions, the remains of Palaeozoic Algae are of comparatively little botanical interest. A vast number of " species ". have been described, but, as has been said, " by far the Algae greater number of the supposed fossil Algae have no claim to be regarded as authentic records of this class of Thallophytes " (Seward, 1898). The investigations of Nathorst, Williamson and others have shown that a very large proportion of the casts and impressions attributed to Algae had in all probability a totally different origin. Some represent the tracks or burrows of worms, crustaceans or other animals; others, the course of rills of water on a sandy or muddy shore; others, again, the marks left on the bottom by bodies drifted along by the waves. In cases of doubt, evidence may be obtained from traces of organic structure, from the presence of carbonaceous matter, or, as Zeiller has pointed out, by the remains of animals such as Bryozoa being attached to the cast, showing that it represents a solid body and not a mere cavity or furrow. Evidence from traces of organization is alone ' conclusive; the presence of carbonaceous matter, though a useful indication, may be deceptive, for the organic substance may have been derived from other sources than the body which left the impression. The mere external form of the supposed Algae is rarely so characteristic as to afford satisfactory evidence of their nature. Some of the better-attested examples, among which are a few of considerable interest, may now be considered. Of Cyanophyceae, as we should expect, the Palaeozoic remains are very doubtful. ' Gloioconis, found by Renault in a coprolite of Permian age, was regarded by him as a Cyanophycean allied to Gloeocapsa; this may be so, but the argument drawn from the absence of nuclei, considering the extreme rarity of recognizable nuclei even in the best preserved fossil tissues, can hardly be taken seriously. Girvanella, found in Cambrian, Ordovician and Silurian rocks, as well as in later deposits, appears to have played a part in the origination of oolitic rock-structure. It consists of minute interwoven tubular filaments, and has been variously interpreted as possibly representing the sheaths of a Cyanophycean Alga, and as constituting a Siphoneous thallus of the type of the Codieae. The non-cellular order Siphoneae is fairly well represented in Palaeozoic strata, especially by calcareous verticillate forms referable to the family Dasycladeae; the separate tubular joints of the articulated thallus, bearing the prints of the whorled branches, are sometimes cylindrical (Arthroporella, Vermiporella, &c.), sometimes oval (Sycidium) or spherical (Cyclocrinus). These forms, and others like them, go back to the Silurian and Ordovician; while Gyroporella, from the Permian, is another fairly characteristic Siphoneous type. There can be no doubt that the verticillate Siphoneae, a group much isolated among recent organisms, are among the most ancient families of plants. The gigantic Nematophycus, to be described below, has been regarded as having Siphoneous affinities. Little trace of Confervaceae has been found; Confervites chantransioides, apparently consisting of branched cellular filaments, may perhaps represent a Cambrian Confervoid. Cladiscothallus, from the Culm of Russia, in which the filaments are united to form hemispherical or globular tufts, has been compared by Renault to a Chaetophora. This is one of the somewhat doubtful Algae occurring in boghead coal or torbanite, a carbonaceous rock the nature of which has been much disputed, in the law courts as well as in scientific literature. The boghead of Scotland, Autun and New South Wales is regarded by Renault and Bertrand as mainly composed of gelatinous Algae (Piles and Reinschia), having a hollow, saccate thallus formed of a single layer of cells. It may appear surprising that a body containing 65% of carbon should be so largely made up of gelatinous Algae in a comparatively little altered condition, but the material is rich in bitumen, which seems to have replaced the water contained in the organisms when alive. It has recently been stated, however, that the supposed Algae are in reality the megaspores of Vascular Cryptogams. Scarcely anything is known of Palaeozoic Florideae; Solenopora, ranging from the Ordovician to the Jurassic, resembles, in the structure of its thallus, with definite zones of growth, Corallinaceae such as Lithothamnion, and may probably be of the same nature. A branched filamentous organism from the Lower Carboniferous of Scotland, described by Kidston under the name of Bythotrephis worstoniensis, shows some remains of cellular structure, and may probably be a true Alga, resembling some of the filamentous Florideae in habit.
Apart from the multitude of supposed fossil Algae described as " Fucoids " but usually not of Algal nature, and never presenting determinable characters, very little remains that can be referred to Palaeozoic Brown Algae. The most striking of all fossil Algae, however, Nematophycus, may possibly be a Phaeophycean. The first species of the genus, Nematophycus Logani, was discovered by Dawson in 1856 in the Lower and Middle Devonian of Canada, and was described by him as a Conifer under the name of Prototaxites. Carruthers, however, in 1872 established its Algal nature, and gave it the more appropriate name of Nematophycus. In N. Logani the stem, which is found in a silicified state, may be as much as 3 ft. in diameter. The tissue is made up of large, unseptate, occasionally branching tubes, with an undulating vertical course, among which much smaller tubes are irregularly interwoven. Radially placed gaps in the tissue (at first erroneously interpreted as medullary rays, but subsequently more aptly compared to the air-spaces of large Algae) contain very sparse hyphae, which here branch more freely than elsewhere. The concentric rings of growth, which form a characteristic feature, are due to periodic variations in the size of the larger tubes. Transverse septa have occasionally, but rarely, been detected in the smaller hyphae. Penhallow maintains that these smaller tubes arise as branches from the larger, but other observers have failed to confirm this. In N. Storriei, from the Silurian (Wenlock) of South Wales, described by Barber, there is no sharp differentiation of the two kinds of tubes; they are rarely observed to branch, except in the gaps, which in this species are not radially directed. In N. Ortoni (Penhallow), from the Devonian of Canada, the tubes are quite uniform, and there are no spaces or concentric rings. The tubes have their cavity dilated at intervals, and Penhallow has therefore compared them with the trumpet-hyphae of Laminariaceae, but no transverse septa are anywhere visible. Several other species have been described. Carruthers compared the usually non-cellular structure of Nematophycus with that of Siphoneae such as Halimeda, while recognizing the points of resemblance to Laminariaceae (e.g. Lessonia) in the dimensions of the stem and its concentric rings of growth. Later writers, influenced by the occasional occurrence of transverse walls in the smaller hyphae, have laid more stress on Laminariaceous affinities. The existence of these gigantic Algae in Palaeozoic times, attested by such well-preserved specimens, is a fact of great interest, though their systematic position is still an open question. Pachytheca, a spherical organism, usually about the size of a small pea, found in rocks of Silurian and Devonian age, has been much investigated and discussed, without any decisive light having been thrown on its nature. It was once regarded as connected with Nematophycus (with which it sometimes occurs in association), possibly as its fructification. For this view however, there is no evidence, though the tissues of the two fossils are somewhat similar. Pachytheca is formed of cellular filaments resembling those of a Cladophora, irregularly interwoven in the central region, radiating towards the periphery, and often forked. In one case the spherical thallus was found seated in a cup-like receptacle. There can be little doubt of the Algal nature of the fossil, but beyond this it is impossible at present to carry its determination.
On the whole, it cannot be said that the Palaeozoic remains have as yet thrown much light on the evolution of the Algae, though we may not be prepared to maintain, with Zeiller, that plants of this class appear never to have assumed a form very different from that which they present at the present day.
The first evidence for the existence of Palaeozoic Bacteria was obtained in 1879 by Van Tieghem, who found that in silicified vegetable remains from the Coal Measures of St Etienne Bacteria. the cellulose membranes showed traces of subjection to butyric fermentation, such as is produced at the present day by Bacillus Amylobacter; he also claimed to have detected the organism itself. Since that time a number of fossil Bacteria, mainly from Palaeozoic strata, have been described by Renault, occurring in all kinds of fossilized vegetable and animal debris. The supposed Micrococci present little that is characteristic; the more definite, rod-like form of the Bacilli offers a better means of recognition, though far from an infallible one; in a few cases dark granules, suggestive of endospores, have been found within the rods. On the whole, the occurrence of Bacteria in Palaeozoic times - so probable a priori - may be taken as established, though the attempt to discriminate species among them is probably futile.
Fungi were no doubt abundant among Palaeozoic vegetation. In examining the tissues of fossil plants of that epoch nothing is more common than to meet with mycelial hyphae in and among the cells; in many cases the hyphae are septate, showing that the higher Fungi (Mycomycetes), as distinguished from the more algoid Phycomycetes, already existed. An endophytic Fungus referred to the latter group (Peronosporites antiquarius, W. Smith) bears very definite terminal, or intercalary, spherical vesicles, which may probably be regarded as reproductive organs - either oiigonia or sporangia. A minute Fungus bearing sporangia, found by Renault in the wood of a Lepidodendron, and named by him Oiichytrium Lepidodendri, is referred with much probability to the Chytridineae. Conceptacles contaning Spores, and strongly suggesting the Chytridineous Fungus Urophlyetis, have recently been found, in petrified material, on the leaves of an Alethopteris, which appears to have undergone decay before fossilization set in. Small spores, almost certainly those of Fungi, are very common in the petrified tissues of Palaeozoic plants. Spherical sacs, bearing forked spines, described by Williamson under the name of Zygosporites, are frequent, usually in an isolated state.
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Professor Seward, however, has found a Zygosporites in situ, terminating an apparently fungal hypha: he suggests a possible comparison with the mould Mucor. Bodies closely resembling the perithecia of Sphaeriaceous Fungi have often been observed on impressions of Palaeozoic plants, and may probably belong to the group indicated. Professor F. E. Weiss has obtained interesting evidence that the symbiotic association between roots and Fungi, known as " Mycorhiza," already occurred among Carboniferous plants. The few and incomplete data which we at present possess as to Palaeozoic Fungi do not as yet justify any inferences as to the evolution of these plants. The writer is not aware of any evidence for the occurrence of Palaeozoic Lichens.
The important class of the Bryophyta, which, on theoretical grounds, is commonly regarded as more primitive than the. Pteridophyta, is as yet scarcely represented among known fossils of Palaeozoic age. In the Lower Carboniferous of Scotland Mr Kidston has found several specimens of a large dichotomous thallus, with a very distinct midrib; the specimens, referred to the provisional genus Thallites, much resemble the larger thalloid Liverworts. Similar fossils have been described from still older rocks. In one or two cases Palaeozoic plants, resembling the true Mosses in habit, have been discovered; the best example is the Muscites polytrichaceus of Renault and Zeiller, from the Coal Measures of Commentry. In the absence, however, both of reproductive organs and of anatomical structure, it cannot be said that there is at present conclusive evidence for the existence of either Hepaticae or Musci in Palaeozoic times.
Our knowledge of the Vascular Cryptograms of the Palaeozoic period, though recent discoveries have somewhat reduced their relative importance, is still more extensive than of any. other class of plants, and in fact it is here that the dophyta evidence of Palaeontology first becomes of essential importance to the botanist. They extend back through the Devonian, possibly to the Silurian system, but the systematic summary now to be given is based primarily on the rich materials afforded by the Carboniferous and Permian formations, from which our detailed knowledge of Palaeozoic plants has been chiefly derived.
In addition to the three classes, Equisetales, Lycopodiales and Filicales, under which recent Pteridophytes naturally group themselves, a fourth class, Sphenophyllales, existed in Palaeozoic times, clearly related to the Horsetails and more remotely to the Ferns and perhaps the Club-mosses, but with peculiarities of its own demanding an independent position. We further find that, whereas the Ferns of the present day form a well-defined and even isolated class, this was not the case at the time when the primary rocks were deposited. A great group of Palaeozoic fossils, showing evident affinity to Ferns, has proved to consist of seed-bearing plants allied to Gymnosperms, especially Cycads. This important class of plants will be described at the beginning of the Spermophyta under the name Pteridospermeae. The arrangement which we shall adopt for the Palaeozoic Pteridophyta is therefore as follows: I. Equisetales. II. Sphenophyllales. III. Lycopodiales. IV. Filicales. We must bear in mind that throughout the Palaeozoic period, and indeed far beyond it, vascular plants, so far as the existing evidence shows, were represented only by the Pteridophyta, Pteridosperms and Gymnosperms. Although the history of the Angiosperms may probably go much further back than present records show, there is no reason to suppose that they were present, as such, amongst the Palaeozoic vegetation. Consequently, the Pteridophytes, Gymnosperms and their allies had the field to themselves, so far as regards the higher plants, and filled places in nature which have now for the most part been seized on by families of more modern origin. Hence it is not surprising to find that the early Vascular Cryptograms were, beyond comparison, more varied and more highly organized than their displaced and often degraded successors. It is among the fossils of the Palaeozoic rocks that we first learn the possibilities of Pteridophytic organization.
I. Equisetales. - This class, represented in the recent flora by the single genus Equisetum, with about twenty species, was one of the dominant groups of plants in Carboniferous times. The Calamarieae, now known to have been the chief Palaeozoic representatives of the Horsetail stock, attained the dimensions of trees, reaching, according to Grand' Eury, a height of from 30 to 60 metres, and showed in all respects a higher and more varied organization than their recent successors.
Their remains occur in three principal forms of preservation. (I) carbonaceous impressions of the leafy branches, the fructifications and other parts; (2) casts of the stem; these are usually internal, or medullary casts, as described above. Around the cast the organic tissues may be represented by a carbonaceous layer, on the outer surface of which the external features, such as the remains of leaves, can sometimes be traced. More usually, however, the carbonaceous film is thin, and merely shows the impress of the medullary cast within; (3) petrified specimens of all parts - stem, roots, leaves and fructifications - showing the internal structure, more or less perfectly preserved. The correlation of these various remains presents considerable difficulties. Casts surrounded by wood, with its structure preserved, have sometimes been found, and have established their true relations. The position of the branches is shown both on casts and in petrified specimens, and has helped in their identification, while the petrified remains sometimes show enough of the external characters to allow of their correlation with impressions. Fructifications have often been found in connexion with leafy shoots, and the anatomical structure of the axis in sterile and fertile specimens has proved a valuable means of identification.
In habit the Calamarieae appear to have borne, on the whole, a general resemblance to the recent Equisetaceae, in spite of their enormously greater bulk. The leaves were constantly in whorls, and were usually of comparatively small size and of simple form. In the oldest known Calamarian, however, Archaeocalamites (Devonian and Lower Carboniferous), the leaves were repeatedly forked. There is evidence that in some, at least, of the Calamarieae the leaves of each verticil were united at the base to form a sheath. The free lamina, however, was always considerably more developed than in the recent family; in form it was usually linear or narrowly lanceolate. Different genera have been founded on leaf-bearing branches of Calamarieae; apart from Archaeocalamites, already mentioned, and Autophyllites (Grand' Eury), in both of which the leaves were dichotomous, we have Annularia, Asterophyllites and Calamocladus (in Grand' Eury's limited sense), with simple leaves. In some species of Annularia the extremely delicate ultimate twigs, bearing whorls of small lanceolate leaves, give a characteristic habit, suggesting that they may have belonged to herbaceous plants; other Annulariae, however, have been traced with certainty into connexion with the stems of large Calamites. In Asterophyllites., the generic distinction of which from Annularia is not always clear, the narrow linear leaves are in crowded whorls, and the ultimate branches distichously arranged; in the Calamocladus of Grand' Eury - characteristic of the Upper Coal Measures - the whorls are more remote, and the twigs polystichous in arrangement. In all these groups a leaf-sheath has been recognized.
The distribution of the branches on the main stem shows considerable variations, on which genera or sub-genera have been founded by C. E. Weiss. In Archaeocalamites, which certainly deserves generic rank, the branches may occur on every node, but only in certain parts of the stem; the ribs of successive internodes do not alternate, but are continuous, indicating that the leaves were superposed. Using Calamites as a generic name for all those Calamarian stems in which the ribs alternate at the nodes, we have, on Weiss's system, the following sub-genera: Stylocalamites, branches rare and irregularly arranged; Calamitina, branches in regular verticils, limited to certain nodes, which surmount specially short internodes; Eucalamites, branches present on every node. These distinctions can be recognized on petrified specimens, as well as on the casts, but their taxonomic value is somewhat doubtful. In many Calamites there is evidence that the aerial stem sprang from a horizontal rhizome, as in the common species C. (Stylocalamites) Suckowi; in other specimens the aerial stem has an independent, rooting base.
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The anatomical structure of all parts of the plant is now known, in various Calamarieae, thanks more especially to the work of Williamson in England and of Renault in France. The stem has a structure which may be briefly characterized as that of an Equisetum with secondary growth in thickness (fig. I, Plate). The usually fistular pith is surrounded by a ring of collateral vascular bundle, (see Anatomy Of Plants, and Pteridophyta), each of which, with rare exceptions, has an intercellular canal at its inner edge, containing the disorganized spiral tracheae, just as in the recent genus. The cortex is often preserved; in certain cases it was strengthened by hypodermal strands of fibres, as in Equisetum. It is only in the rare cases where a very young twig is preserved that the primary structure of the stem is found unaltered. In all the larger specimens a broad zone of wood, with its elements in radial series, had been added. This secondary wood, in the true Calamites (Arthropitys, Goeppert), has a simple structure comparable to that of the simplest Coniferous woods; it is made up 4 FIG. i. - Calamites. Part of transverse section of a young stem, showing pith, vascular bundles with secondary wood, and cortex. (x about 40.) From a photograph (Scott, " Studies "). FIG. 5. - Sphenophyllum insigne. Transverse section of stem, showing triangular primary wood, secondary wood, remains of phloem, and primary cortex. (x about 30.) From a photograph (Scott, " Studies"). FIG. 22. - Lyginodendron oldhamium. Transverse section of stem, showing the pith containing groups of sclerotic cells, the primary xylem-strands, secondary wood and phloem, pericycle and cortex. U1 - U', leaf-traces, numbered according to the phyllotaxis, It 5 belonging to the lowest leaf of the five; ph, a group of primary phloem; pd, periderm, formed from pericycle. (x 3.) A FIG. 31. - Cordaianthus Penjoni. - A, Male catkin in longitudinal section: a, axis; b, bracts; c, d, filaments of stamens, bearing the pollen-sacs (e and f) at the top; v, apex of axis. (x 6z.) B, Stamens more highly magnified: g, vascular bundle of filament; e, pollen-sac after dehiscence. (x 23.) After Renault (Scott, "Studies"). FIG. 4. - Palaeostachya pedunculata. Fertile shoot, bearing numerous cones and a few leaves. (3 of nat. size.) After Williamson (Scott, " Studies"). B entirely of radial bands of tracheides interspersed with medullary rays. The pitting of the tracheides is more or less scalariform in character, and is limited to the radial walls. In favourable cases remains of the cambium are found on the outer border of the wood, and phloem is also present in the normal position, though it does not seem to have attained any considerable thickness. In the old stems the primary cortex was replaced by periderm, giving rise to a thick mass of bark. The above description applies to the stems of Calamites in the narrower sense (Arthropitys of the French authors), to which the specimens from the British Coal Measures mostly belong. Archaeocalamites appears to have had a similar structure, but in some specimens from the Lower Carboniferous of Burntisland, provisionally named Protocalamites pettycurensis, centripetal wood was present in the stem. In Calamodendron (Upper Coal Measures) the wood has a more complex structure than in Calamites, the principal rays including radial tracts of fibrous tissue, in addition to the usual parenchyma. Arthrodendron (Lower Coal Measures) approaches Calamodendron in this respect. The longitudinal course of the vascular bundles and their relation to the leaves in Calamarieae generally followed the Equisetum type, though more variable and sometimes more complex. The attachment of the branches was immediately above the node, and usually between two foliar traces, as in the recent genus. Where the structure of the leaves is preserved it proves to be of an extremely simple type; the narrow lamina is traversed by a single vascular bundle, separated by a sheath from the surrounding palisadeparenchyma. Stomata of the same structure as in Equisetum have been detected in the epidermis.
The roots (formerly described as a separate genus, Astromyelon) were borne directly on the nodes, not on short lateral branches as in Equisetum. They are of similar structure in all known Calamarieae, the main roots having a large pith, while the rootlets had little or none. The structure is in all respects that typical of roots, as shown by the centripetal primary wood, and the alternation of xylem and phloem groups observable in exceptionally favourable young specimens. A striking feature is the presence of large, radiating intercellular cavities in the cortex, suggesting an aquatic habit. The young roots show a double endodermis, just as in the recent Equisetum. A considerable number of Calamarian fructifications are known, preserved, some as carbonaceous impressions, others as petrified specimens, exhibiting the internal structure. In many cases the cones have been found in connexion with branches bearing characteristic Calamarian foliage. Almost all strobili of the Calamarieae are constructed on the same general lines as those of Equisetum, with which some agree exactly; in most, however, the organization was more complex, the complexity consisting in the intercalation of whorls of sterile bracts, between those of the sporangiophores. In several cases heterospory, unknown among recent Equisetaceae, has been demonstrated in their Palaeozoic representatives.
Four main types of structure may be distinguished among Calamarian strobili.
Calamostachys, Schimper. Here the whorls of peltate sporangiophores alternate regularly with those of sterile bracts, the former being inserted on the axis midway between the latter (fig. 2). The sporangiophores, which are usually half as numerous in each verticil as the bracts, have the same form as in Equisetum, but each bears four sporangia only. The spores are frequently found to be still united in tetrads. In some species, e.g. the British C. Binneyana, numerous specimens have been examined and only one kind of spore observed; here, then, there is a strong presumption that the species was homosporous. In other cases, however, e.g. C. Casheana, Will., two kinds of spore occur, in different sporangia, but on the same strobilus and even on the same sporangiophore. The megaspores, of which there are many in the megasporangium, have a diameter about three times that of the microspores. The abortion of certain spores, which is known to have taken place both in the homosporcus C. Binneyana and in the megasporangia of C. Casheana, may throw some light on the origin of the heterosporous condition. The bracts were sometimes coherent in their lower part (e.g. C. Binneyana), sometimes free (e.g. C. Ludwigi); in all cases their free extremities formed a protection to the fertile whorl above. In some continental species (e.g. C. Grand' Euryi, Ren.) radial membranous plates hung down from each verticil of bracts, forming compartments in which the subjacent sporangio phores were enclosed. The anatomy of the axis is essentially similar to that of a young Calamarian twig, with some variations in detail. Strobili of the Calamostachys type occur in connexion both with Annularia and Asterophyllites foliage.
2. Palaeostachya, Weiss. Here, as in the previous genus, sterile and fertile verticils are ranged alternately on the axis of the cone. The main difference is that in Palaeostachya the sporangiophores, instead of standing midway between the whorls of bracts, are inserted immediately above them, springing, as it were, from the axil of the sterile verticil (fig. 3, A). This singular arrangement has suggested doubts as to the correctness of the current interpretation of the Equisetaceous sporangiophore as a modified leaf Sm ' 'CI= B A (After Renault. Scott, Studies.) FIG. 3.
A, Palaeostachya. Diagrammatic longitudinal section of cone, showing the axis (ax) bearing the bracts (br) with peltate sporangiophores (sp) springing from their axils; sm, sporangia.
B, Archaeocalamites. Part of cone, showing the axis (ax) bearing peltate sporangiophores (sp) without bracts; sm, sporangia.
(cf. Cheirostrobus below). In most other respects the two genera agree; there is evidence for the occurrence of heterospory in some strobili referred to Palaeostachya. The anatomy of the axis is that of a young branch of a Calamite. According to Grand' Eury, the Palaeostachya fructification was most commonly associated with Asterophyllites foliage. The external aspect of a Palaeostachya is shown in fig. 4 (Plate). 3. Equisetum type of strobilus. In certain cases the strobili of Palaeozoic Calamarieae appear to have had essentially the same organization as in the recent genus, the axis bearing sporangiophores only, without intercalated bracts. It is remarkable that fructifications apparently of this kind have been found by Renault in close association with the most ancient of the Calamarieae- Archaeocalamites. In these strobili the peltate scales, like the vegetative leaves of the plant, are in superposed verticils; each appears to have borne four sporangia (fig. 3, B). Other cones, however, namely, those known as Pothocites, have also been attributed on good grounds to the genus Archaeocalamites; they are long strobili, constricted at intervals, and it is probable that the succession of fertile sporangiophores was interrupted here and there by the intercalation of sterile bracts, which may also have been present, at long intervals, in Renault's species. Cones from the Middle Coal Measures, described by Kidston under the name of Equisetum Hemingwayi, but probably belonging to one of the Calamarieae, bear a striking external resemblance to those of recent Equisetum. 4. Cingularia, Weiss. This form of strobilus, from the Coal Measures of Germany, is imperfectly known, and its relation to Calamarieae not beyond doubt. In the lax strobili the sporangiophores, which are not peltate, but strap-shaped, were borne, as C. E. Weiss first showed, immediately below the verticils of bracts, the position thus being the reverse of that in Palaeostachya. The Palaeozoic Calamarieae, though so far surpassing recent Equisetaceae, both in stature and complexity of organization, clearly belonged to the same class of Vascular Cryptogams. There is no satisfactory evidence for attributing Phanerogamic e bn FIG. 2. - Calamostachys. Diagrammatic longitudinal section of the cone, showing the axis (ax) bearing alternate whorls of bracts (br) and peltate sporangiophores (sp) with their sporangia (sm). The upturned tips of the bracts are only shown in every alternate verticil.
| [PALAEOZOIC |
which in structure resembles the vegetative stem in its primary condition, bears numerous verticils of bracts, those of each verticil being coherent in their lower part, so as to form a disc or cup, from the margin of which the free limbs of the bracts arise. The sporangia, which are about twice as numerous as the bracts, are seated singly on pedicels or sporangiophores springing from the upper surface of the bract-verticil, near its insertion on the axis (fig. 6). As a rule two sporangiophores belong to each bract. The sporangium is attached to the enlarged distal end of its pedicel, from which it hangs down, so as to suggest an anatropous ovule on its funiculus. Dehiscence appears to have taken place at the free end of the sporangium; the spores are numerous, and, so far as observed, of one kind only. Each sporangiophore is traversed throughout its length by a vascular bundle connected with that which supplies the subtending bract. This form of fructification appears, from Zeiller's researches, to have been common to several species of Sphenophyllum, but others show important differences. Thus Bowmanites Romeri, a fructification fully investigated by Solms-Laubach, differs from S. Dawsoni in the fact that each sporangiophore bears two sporangia, attached to a distal expansion approaching the peltate scale of the Equisetales. It is thus proved that the sporangiophore is not a mere sporangial stalk, but a distinct organ, in all probability representing a ventral lobe of the subtending bract. The recently discovered species, Sphenophyllum fertile, while resembling Bowmanites Romeri in its peltate, bisporangiate sporangiophores, is peculiar in the fact that both dorsal and ventral lobes of the sporophyll were fertile, dividing in a palmate manner into several branches, each of which constitutes a sporangiophore. Thus the sterile bracts of other species are here replaced by sporangium-bearing organs. In Sphenophyllum majus, where the cones are less sharply defined, the forked bract bears a group of four sporangia at the bifurcations, but their mode of insertion has not yet been made out.
2. Cheirostrobeae. - The family Cheirostrobeae is only known from the petrified fructification (Cheirostrobus pettycurensis) derived from the Lower Carboniferous of Burntisland in Scotland. The excellence of the preservation of the specimens has rendered it possible to investigate the complex structure in detail. The cone is of large size-3.5 cm. in diameter; the stout axis bears numerous whorls of compound sporophylls, the members of successive verticils being superposed. The sporophylls, of which there are eleven or ° ° O 0 ' '00 C ? ?? _ b i 111111,11 1l ???. d10 \ / ,... A A 1 n outer surface. The primary structure f ` is only found unaltered in the 1 1 ,1 ` by means of a cambium set in very Il 111/1/1, 1 1 early, xylem being formed internally and phloem externally in a perfectly normal manner. At the same time a deep-seated periderm arose, by which the primary cortex was soon entirely cut off. The secondary wood in the Lower Carboniferous species, S. insigne, has scalariform tracheides, and is traversed by regular medullary rays, but in the forms from later ' horizons the tracheides are reticulately pitted, and the rays are for the most part replaced by a network of xylem-parenchyma. There are no recent stems with a structure quite like that of Sphenophyllum; so far as the primary structure is concerned, the nearest approach is among the Psiloteae, with which other characters indicate some affinity; the base of the stem in Psilotum forms some secondary wood. The diarch roots of a Sphenophyllum have been described by Renault, who has also investigated the leaves; they were strongly constructed mechanically, and traversed by slender vascular bundles branching dichotomously.
Fructification
Williamson thoroughly worked out, in petrified specimens, the organization of a cone which he named Bowmanites Dawsoni; it was subsequently demonstrated by Zeiller that this fructification belonged to a Sphenophyllum, the cones of the well known species S. cuneifolium having a practically identical structure.
The type of fructification described by Williamson and now named Sphenophyllum Dawsoni consists of long cylindrical cones, in external habit not unlike those of some Calamarieae. The axis, 1 In S. speciosum the leaves in a whorl were of unequal size.
.4x. affinities to any members of the group, and the view, of which Williamson was the chief advocate, that they form a homogeneous Cryptogamic family, is now fully established. II. Sphenophyllales. - The class of Sphenophyllales, as known to us at present, is of limited extent, embracing the two genera Sphenophyllum and Cheirostrobus, which may serve as types of two families within the class. The characters of Sphenophyllum are known with some completeness, while our knowledge of Cheirostrobus confined to the fructification; the former therefore be described first.
i. Sphenophyllum. - The genus Sphenophyllum, of which a number of species have been described, ranging probably from the Middle Devonian, through the Carboniferous, to the Permian or even the Lower Triassic, consisted of herbaceous plants of moderate dimensions. The long, slender stems, somewhat tumid at the nodes, were ribbed, the ribs running continuously through the nodes, a fact correlated with the superposition of the whorled leaves, the number of which in each verticil was some multiple of 3, and usually 6. In the species on which the genus was founded the leaves, as the generic name implies, are cuneate and entire, or toothed on their anterior margina l in other cases they are deeply divided by dichotomy into narrow segments, or the whorl consists of a larger number (up to 30) of apparently simple, linear leaves, which may represent the segments of a smaller number. The different forms of leaf may occur on the same plant, the deeply divided foliage often characterizing the main stem, while the cuneate leaves were borne on lateral shoots. A comparison, formerly suggested, with the two forms of leaf in Batrachian Ranunculi has not proved to hold good; the idea of an aquatic habit is contradicted by the anatomical structure, and the hypothesis that the plants were of scandent growth is more probable. The species of Sphenophyllum have a graceful appearance, which has been compared with that of the trailing Galiums of hedgerows. Branches sprang from the nodes, though perhaps not truly axillary in position. The cones, more or less sharply differentiated, terminated certain of the branches.
The anatomy of the stem of Sphenophyllum, investigated by Renault, Williamson and others, is highly characteristic (fig. 5, Plate). The stem is traversed by a single stele, with solid wood, without pith; the primary xylem is triangular in section, the spiral elements forming one or two groups at each angle, while the phloem occupied the bays, so that the structure resembles that of a triarch root. Two leaf-trace bundles started from each angle of the stele, and forked, in passing through the cortex, to supply the veins of the leaf, or its subdivisions. The cortex was deeply furrowed on its youngest stems; secondary growth (Scott, Studies.) FIG. 7. - Cheirostrobus. Diagram of cone, the upper part in transverse, the lower in longitudinal section. In the transverse section six sporophylls, each showing three segments, are represented.
Sp.a, Section through sterile segPeltate expansions of sporanments. giophores.
Sp.b, Section through sporangio- sm, Sporangia.
phores. v.b. Vascular bundles.
st, Laminae of sterile segments. cy, Stele of axis (Ax).
In the longitudinal section the corresponding parts are shown.
twelve in a whorl, are each composed of six segments, three being inferior or dorsal, and three superior or ventral. The dorsal segments are sterile, corresponding to the bracts of Sphenophyllum Dawsoni, while the ventral segments constitute peltate sporangiophores, each bearing four sporangia, just as in a ax FIG. 6. - Sphenophyllum Dawsoni. Diagram of cone in longitudinal section.
ax, Axis.
br, Bracts.
sp, Sporangiophores, each bearing a sporangium, sm.
br', Whorl of bracts in surface view.
| PALAEOZOIC] |
Calamarian fructification (fig. 7). The great length and slender proportions of the segments give the cone a peculiar character, but the relations of position appear to leave no doubt as to the homologies with the fructification of Sphenophylleae; as regards the sporangiophores, Bowmanites Romeri occupies exactly the middle place between S. Dawsoni and Cheirostrobus. The axis of the cone in Cheirostrobus contains a polyarch stele, with solid wood, from the angles of which vascular bundles pass out, dividing in the cortex, to supply the various segments of the sporophylls. In the peduncle of the strobilus secondary tissues are formed. While the anatomy has a somewhat Lycopodiaceous character, the arrangement of the appendages is altogether that of the Sphenophylleae; at the same time Calamarian affinities are indicated by the characters of the sporangiophores and sporangia.
The Sphenophyllales as a whole are best regarded as a synthetic group, combining certain characters of the Ferns and Lycopods with those of the Equisetales, while showing marked peculiarities of their own. Among existing plants their nearest affinities would appear to be with Psiloteae, as indicated not merely by the anatomy, but much more strongly by the way in which the sporangia are borne. There is good reason to believe that the ventral synangium of the Psiloteae corresponds to the ventral sporangiophore with its sporangia in the Sphenophyllales. Professor Thomas of Auckland, New Zealand, has brought forward some interesting variations in Tmesipteris which appear to afford additional support to this view.
Pseudobornia
Professor Nathorst has described a remarkable Devonian plant, Pseudobornia ursina (from Bear Island, in the Arctic Ocean), which shows affinity both with the Equisetales and Sphenophyllales. The stem is articulated and branched, attaining a diameter of about 10 cm. The smaller branches bear the whorled leaves, probably four in each verticil. The leaves are highly compound, dividing dichotomously into several leaflets, each of which is deeply pinnatifid, with fine segments. When found detached these leaves were taken for the fronds of a Fern. The fructification consists of long, lax spikes, with whorled sporophylls; indications of megaspores have been detected in the sporangia. The discoverer makes this plant the type of a new class, the Pseudoborniales. At present only the external characters are known.
III. Lycopodiales. - In Palaeozoic ages the Lycopods formed one of the dominant groups of plants, remarkable alike for the number of species and for the great stature which many of them attained. The best known of the Palaeozoic Lycopods were trees, reaching loo ft. or more in height, but side by side with these gigantic representatives of the class, small herbaceous Club-mosses, resembling those of the present day, also occurred. Broadly speaking, the Palaeozoic Lycopods, whatever their dimensions, show a general agreement in habit and structure with our living forms, though often attaining a much higher grade of organization. We will first take the arborescent Lycopods, as in every respect the more important group. They may all be classed under the one family Lepidodendreae, which is here taken to include Sigillaria. Lepidodendreae. - The genus Lepidodendron, with very numerous species, ranging from the Devonian to the Permian, consisted of trees, with a tall upright shaft, bearing a dense crown of dichotomous branches, clothed with simple narrow leaves, ranged in some complex spiral phyllotaxis. In some cases the foliage is preserved in situ; more often, however, especially in the main stem and larger branches, S.C. /,???_ A the leaves had been shed, leaving behind them their scars and persistent bases, on which the characteristic sculpturing of the Lepidodendroid surface depends. The cones, often of large size, were either terminal on the'smaller twigs, or, it is alleged, borne laterally on special branches of con- (After Stur. Scott, Studies.) siderable dimensions. At its base the FIG. 8. - Leaf-base of a Lepi- main stem terminated in dichotomous dodendron. roots or rhizophores, bearing numerous rootlets. To these ,underground organs the name Stigmaria is applied; they are not clearly distinguishable from the corresponding parts of Sigillaria. The numerous described species of Lepidodendron are founded on the peculiarities of the leafcushions and scars, as shown on casts or impressions of the stem. The usually crowded leaf-cushions are spirally arranged, and present no obvious orthostichies, thus differing from those of Sigillaria. Each leaf-cushion is slightly prominent; towards its upper end is the diamond-shaped or triangular scar left by the fall of the actual leaf (fig. 8). On the scar are three prints, the central one alone representing the vascular bundle, while the lateral prints (parichnos) mark the position of merely parenchymatous strands. In the median line, immediately above the leaf-scar, is a print representing the ligule, or rather the pit in which it was seated. On the flanks of the cushion, below the scar, are two superficial prints, perhaps comparable to lenticels. In the genus Lepidophloios the leaf-cushions are more prominent than in Lepidodendron, and their greatest diameter is in the transverse direction; on the older stems the leaf-scar lies towards the lower side of the cushion. The genus Bothrodendron, going back to the Upper Devonian, differs from Lepidodendron in its minute leaf-scars and the absence of leafcushions, the scars being flush with the smooth surface of the stem. In the Lower Carboniferous of central Russia beds of coal occur consisting of the cuticles of a Bothrodendron, which are not fossilized, but retain the consistency and chemical composition of similar tissues in recent plants.
The anatomy of Lepidodendron and its immediate allies is now well known in a number of species; the Carboniferous rocks of Great Britain are especially rich in petrified specimens, which formed the subject of Williamson's extensive investigations. The stem is in all cases monostelic; in most of the forms the central cylinder underwent secondary growth, and the distinction between primary and secondary wood is very sharply marked. In L. Harcourtii, however, the species earliest investigated (by Witham, 1833, and Brongniart, 1837), and in one or two other species, no secondary wood has yet been found. The primary wood of Lepidodendron forms a continuous cylinder, not broken up into distinct bundles; its development was clearly centripetal, the spiral elements forming more or less prominent peripheral groups. In the larger stems of most species there was a central pith, but in certain of the smaller branches, and throughout the stem in some species (L. rhodumnense, L. selaginoides), the wood was solid. A single leaf-trace, usually collateral in structure, passed out into each leaf. The primary structure of the stem was thus of a simple Lycopodiaceous type, resembling on a larger scale what we find in the upright stem of Selaginella spinosa. In most species (e.g. L. selaginoides, L. Wunschianum, L. Veltheimianum) secondary growth in thickness took place, and secondary wood was added, (Scott, Studies.) FIG. 9. - Lepidodendron Veltheimianum. Transverse section of stem.
p, Pith, almost destroyed. ph, Phloem and pericycle.
x, Zone of primary wood. br, Stele of a branch.
px, Protoxylem. pd, Periderm.
x 2, Secondary wood. l.b, Leaf-bases.
The primary cortex between stele and periderm has perished. (X41.) in the centrifugal direction, showing l a regular radial arrangement, with medullary rays between the seri s of tracheides (fig. 9). The tissue thus formed often attained a considerable thickness. While .b.
s.c., Scar left by the leaf. v.b., Print of vascular bundle. p,p, Parichnos.
1, Ligule.
a,a, Superficial prints below scar.
primary phloem can be recognized with certainty in favourable cases, the question of the formation of secondary phloem by the cambium is not yet fully cleared up. In the Lepidodendron fuliginosum of Williamson, shown by its leaf-bases to have been a Lepidophloios, the secondary wood is very irregular, and consists largely of parenchyma. The same is the case in Lepidodendron obovatum, one of the few species in which both external and internal characters are known. The occurrence of secondary growth in these plants, demonstrated by Williamson's researches, is a point of great interest. Some analogy among recent Lycopods is afforded by the stem of Isoetes, and by the base of the stem in Selaginella spinosa; in the fossils the process was of a more normal type, but some of its details need further investigation. The cortex, often sharply differentiated into sclerotic and parenchymatous zones, is bordered externally by the persistent leaf-bases. The development of periderm was a constant feature, and this tissue attained a great thickness, consisting chiefly of a phelloderm, produced on the inner side of the formative layer, and no doubt subserving a mechanical function.
The structure of a Bothrodendron has recently been investigated and proves to be identical with that of the petrified stem which Williamson named Lepidodendron mundum. The anatomy is of the usual medullate Lepidodendroid type; no secondary growth has yet been detected in the stem.
The most interesting point in the structure of the leaf-base is the presence of a ligule, like that of Isoetes or Selaginella, which was seated in a deep pit, opening on the upper surface of the cushion, just above the insertion of the lamina. The latter shows marked xerophytic adaptations; the single vascular bundle was surrounded by a sheath of short tracheides, and the stomata were sheltered in two deep furrows of the lower surface.
The cones of Lepidodendron and its immediate allies are for the most part grouped under the name Lepidostrobus. These cones, varying from an inch to a foot in length, according to the species, were borne either on the ordinary twigs, or, as was conjectured, on the special branches (Ulodendron and Halonia) above referred to. In Ulodendron the large circular, distichously arranged prints were supposed to have been formed by the pressure of the bases of sessile cones, though this interpretation of the scars is open to doubt, and it is now more probable that they bore deciduous vegetative branches; in the Halonial branches characteristic of the genus Lepidophloios the tubercles may perhaps mark the points of insertion of pedunculate strobili. The organization of Lepidostrobus is essentially that of a Lycopodiaceous cone. The axis, which in anatomical structure resembles a vegetative twig, bears numerous spirally arranged sporophylls, each of which carries a single large sporangium on its upper surface (fig. io).
lq The sporophyll, usually almost horizontal in position, has an upturned lamina beyond the sporangium, and a shorter dorsal lobe, so that the form of the whole is somewhat peltate. A ligule is present immediately ?` `?
below the lamina, its position 1 showing that the whole of 1 ' »
,? / s h the elongated horizontal pedicel on which the sporangium is seated corresponds to the short base of a FIG. io. - Lepidostrobus. Diagram of vegetative leaf. The spor cone, in longitudinal section. angia, usually of very large ax, Axis, bearing the sporophylls (sph), size compared with those of on each of which a sporangium most recent Lycopods, have (sm) is seated. a palisade-like outer wall, lg, Ligule. and contain either an im The upper sporangia contain numermense number of minute ous microspores; in each of the lower spores or a very small number sporangia four megaspores are shown. of exceedingly large spores (fig. to). It is very doubtful whether any homosporous Lepidostrobi existed, but there is reason to believe that here, as in the closely allied Lepidocarpon, microsporangia and megasporangia were in some cases borne on different strobili. In other species (e.g. in the cone attributed to the Lower Carboniferous Lepidodendron Veltheimianum) the arrangement was that usual in Selaginella, the microsporangia occurring above and the megasporangia below in the same strobilus (diagram, fig. io). The genus Spencerites (Lower Coal Measures) differs from Lepidostrobus mainly in the insertion of the sporangium, which, instead of being attached along the whole upper surface of the sporophyll, was connected with an outgrowth on its upper surface by a small neck of tissue towards the distal end. The spores of this genus are curiously winged, and intermediate in size between the microspores and megaspores of Lepidostrobus; the question of homospory or heterospory is not yet decided. The cones of Bothrodendron and another form named Mesostrobus are in some respects intermediate between Lepidostrobus and Spencerites. A more important deviation from ordinary Lepidostroboid structure is shown by the genus Lepidocarpon, from the English Coal Measures and the Lower Carboniferous of Scotland. In this fructification the organization is at first altogether that of a Lepidostrobus; in each megasporangium, however, only a single megaspore came to maturity, occupying almost the whole of the sporangial cavity (see fig. 12), but accompanied by the remains of its three abortive sister cells. An integument grew up from the superior surface of the sporophyll, completely enveloping the sporangium, except for a narrow crevice left open along the top. In favourable cases the prothallus is found preserved, within the functional megaspore or embryo-sac, and the whole appearance, especially as seen in a section tangential to the strobilus, is then remarkably seed-like (see diagram, fig. II). The seed-like body was detached as a whole from the cone, and in this condition was known for many years under the name of Cardiocarpon anomalum, having been wrongly identified with a true Gymnospermous seed so named a seed are obvious; the which is not tubular, but forms a long crevice, running in a direction radial to the strobilus. Lepidocarpon affords a striking instance of homoplastic modification, for there i s no reason to suppose that the Lycopods were on the line of descent of any existing Spermophyta.
In a male cone, probably belonging to wt Lepidocarpon Lomaxi, the microsporangia are provided with incomplete integuments.
Another case of a " seed-bearing " Lycopod has lately been discovered by Miss Benson in Miadesmia membranacea, a slender Selaginella-like plant from the Lower Coal Measures of Lancashire. The female fructification is in the form of a rather lax strobilus. Each sporophyll bears a megasporangium, attached to its upper surface at the proximal end, containing a single large megaspore (fig. 13).
The megasporangium is enclosed in an a A"° cu integument, which completely envelopes it, leaving only a narrow micropyle at the distal end (fig. 13). The long tentacles of the integument may have served to facilitate pollination. The seed-like character of the organ is even more striking in Miadesmia than in Lepidocarpon. There seems to be no near affinity between these genera, in which the seed-habit must have arisen independently. (Scott, Studies.) Sigillaria. - The great genus Sigillaria, FIG. 12. - Lepidocarpon even richer in " species " than LepidoLomaxii. Sporangium and dendron, ranges throughout the Carbonsporophyll before deveiferous, but has not yet been detected lopment of integument. in earlier rocks. The Sigillariae, like the (X about 12.) Lepidodendra, were large trees, but must cu, Lateral cushions on have differed from those of the previous sporophyll. group in habit, for they appear to have vb, Vascular bundle. branched sparingly or not at all, the w, p, Palisade layer of spor lofty upright shaft terminating, like angium-wall. some modern Xanthorrhaea, in a great wi, Inner layer of wall. sheaf of long, grass-like leaves. The a, Base of sporangium. strobili were stalked, and borne on mg, Membrane of mega the main stem, among the leaves. The spore or embryo-sac. roots, or at least their functional repre sentatives, resembled those of Lepidodendron. The chief distinctive character of Sigillaria lies in the arrangement of the leaf-scars, which form conspicuous vertical series on the surface of the stem.
s FIG. I I. - Lepidocarpon Lomaxii. Diagrammatic section of " seed " in plane tangential to the parent strobilus. sph, Sporophyll.
vb, Its vascular bundle.
i, Integument.
m, Micropylar crevice.
a, Base.
sm, Wall of sporangium.
mg, Membrane of functional megaspore, which is filled by the prothallus, pr.
| PALAEOZOIC] |
by Carruthers. The analogies with chief difference is in the micropyle, In one great division of the genus - the Eusigillariae - the stems are ribbed, each rib bearing a vertical row of leaf-scars; the ribbed Sigillariae were formerly divided into two sub-genera - Rhytidolepis, (From a drawing by Mrs D. H. Scott. Scott, Studies.) FIG. 13. - Miadesmia membranacea. Radial longitudinal section of seed-like organ. (X about 30.) 1, Lamina of sporophyll. lg, Ligules.
vb, Vascular bundle. sm, Sporangium-wall.
v, Velum or integument. m, Membrane of megaspore. t, Tentacles.
with the scars on each rib rather widely spaced, and Favularia, where they are approximated and separated by transverse furrows, each rib thus consisting of a series of contiguous leaf-bases. This distinction, however, has proved to have no constant taxonomic value, for both arrangements may occur on different parts of the same specimen. The species without ribs - Subsigillariae - were in like manner grouped under the two sub-genera Clathraria and Leiodermaria; in the former each scar is seated on a prominent cushion, while in the latter the surface of the stem (as in Bothrodendron) is perfectly smooth. Here also the distinction has proved not to hold good, S. Brardi, for FIG. 14. - Sigillaria Brardi. Part of surexample, showing both face of stem, showing five leaf-scars. conditions on the same (X 11.) stem. All these names, vb, Print of vascular bundle. however, are still in use pa, Parichnos. as descriptive terms.
lg, Ligule. Generally, the Eusigillariae are characteristic of the older Carboniferous strata, the Subsigillariae of the Upper Coal Measures and Permian. The leafscars throughout the genus show essentially the same prints as in Lepidodendron, differing only in details, and here also a Ligule was present (fig. 14).
The anatomy of Sigillaria is not so well known as that of Lepidodendron, for specimens showing structure are comparatively rare, a fact which may be correlated with the infrequency of branching in the genus. The structure of a Clathrarian Sigillaria (S. Menardi), from the Permian of Autun, was accurately described by Brongniart as long ago as 1839, and a similar species, S. spinulosa (= S. Brardi) was investigated by Renault in 1875, but it was long before we had any trustworthy data for the anatomy of the ribbed forms. This gap in our knowledge has now been filled up, owing to Bertrand's investigation of a specimen referred by him to S. elongata, followed by the detailed researches of Kidston and Arber on Sigillaria elegans, scutellata and mamillaris. The structure of the ribbed Sigillariae, as at present known, essentially resembles that of a medullate Lepidodendron, though the ring of primary wood is narrower. Its outer margin is crenulated, the leaf-traces being given off from the middle of each bay. Secondary wood was formed in abundance, precisely as in most species of Lepidodendron. In the Subsigillarian species S. Menardi the primary wood is broken up into distinct bundles, while in S. spinulosa their separation is sometimes incomplete. The secondary cortex or periderm attained a great development, and in some cases shows considerable differentiation. On the whole, the anatomy of Sigillaria is closely related to that of the preceding group, and in fact a continuous series can be traced from the anatomically simplest species of Lepidodendron to the most modified Sigillariae. The leaves of Sigillaria are in some cases almost identical in structure with those of Lepidodendron, but in certain species (S. scutellata and S. mamillaris) there is evidence that they were of the Sigillariopsis type, the leaf being traversed by two parallel vascular strands, derived from the bifurcation of the leaf-trace.
The nature of the fructification of Sigillaria was first satisfactorily determined in 1884 by Zeiller, who found the characteristic Sigillarian leaf-scars on the peduncles of certain large strobili (Sigillariostrobus). The cones, of which several species have been described, bear a strong general resemblance to Lepidostrobus, differing somewhat in the form of the sporophylls and some other details. The megaspores (reaching 2 mm. or more in diameter) were found lying loose on the sporophylls by Zeiller; the sporangia containing them were first observed by Kidston, in a species from the Coal Measures of Yorkshire. That the cones were heterosporous there can be no doubt, though little is known as yet of the microsporangia. The discovery of Sigillariostrobus, which was the fructification of Subsigillariae as well as of the ribbed species, has finally determined the question of the affinities of the genus, once keenly discussed; Sigillaria is now clearly proved to have been a genus of heterosporous Lycopods, with the closest affinities to Lepidodendron. Stigmaria. - On present evidence there is no satisfactory distinction to be drawn between the subterranean organs of Sigillaria and those of Lepidodendron and its immediate allies, though some progress in the identification of special forms of Stigmaria has recently been made. These organs, to which the name Stigmaria was given by Brongniart, have been found in connexion with the upright stems both of Sigillaria and Lepidodendron. In the Coal Measures they commonly occur in the underclay beneath the coal-seams. Complete specimens of the stumps show that from the base of the aerial stem four Stigmarian branches were given off, which took a horizontal or obliquely descending course, forking at least twice. These main Stigmarian axes may be 2 to 3 ft. in diameter at the base, and 30 or 40 ft. in length. Their surface is studded with the characteristic scars of their appendages or rootlets, which radiated in all directions into the mud. Petrified specimens of the main Stigmaria are frequent, and those of its rootlets extraordinarily abundant. The two parts are very different in structure: in the main axis, as shown in the common Coal Measure form Stigmaria ficoides, the centre was occupied by the pith, which was surrounded by a zone of wood, centrifugally developed throughout. In other species, however, the centripetal primary xylem is represented. Phloem, surrounding the wood, is recognizable in good specimens; in the cortex the main feature is the great development of periderm. The rootlets, which branched by dichotomy, contain a slender monarch stele exactly like that in the roots of Isoetes and some Selaginellae at the present day; they possessed, however, a complex absorptive apparatus, consisting of lateral strands of xylem, connecting the stele with tracheal plates in the outer cortex. The morphology of Stigmaria has been much discussed; possibly the main axes, which do not agree perfectly either with rhizomes or roots, may best be regarded as comparable with the rhizophores of Selaginellae; they have also been compared with the embryonic stem, or protocorm, of certain species of Lycopodium; the homologies of the appendages with the roots of recent Lycopods appear manifest. It has been maintained by some palaeobotanists that the aerial stems of Sigillaria arose as buds on a creeping rhizome, but the evidence for this conclusion is as yet unconvincing.
Lycopoditeae
Under this name are included the fossil Lycopods of herbaceous habit, which occur occasionally, from the Devonian onwards. One such plant, Miadesmia, has already been referred to, as one of the seed-bearing Lycopods. In some Lycopoditeae the leaves were all of one kind, while others were heterophyllous, like most species of Selaginella. The genus Selaginellites, Zeiller, is now used to include those forms in which the fructification has proved to be heterosporous. In Selaginellites Suissei there was a definite strobilus bearing both microand megasporangia; in each of the latter from 16 to 24 megaspores were contained; in Selaginellites primaevus, however, the number of megaspores was only 4, and the resemblance to a recent Selaginella was thus complete. Selaginellites elongatus, another heterosporous species, is remarkable for having no differentiated strobilus, a condition not known in the recent genus. The antiquity of the Selaginella type indicates that this group had no direct connexion with the Lepidodendreae, but sprang from a distinct and equally ancient herbaceous stock. There is, however, some evidence that Isoetes, which in several respects agrees more nearly with the Lepidodendreae, may actually represent their last degenerate survivors (see Pleuromeia, in § II., MEsozoic). No homosporous Lycopoditeae have as yet been recognized.
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IV. Filicales. - Of all Vascular Cryptogams the Ferns have best maintained their position down to the present day. Until recently it has been supposed that the class was well represented in the Palaeozoic period, and, indeed, that it was relatively, and perhaps absolutely far richer in species even than in the recent flora. Within the last few years, however, the position has completely changed, and the majority of the supposed Palaeozoic (After Weiss. Scott, Studies.) Ferns are now commonly regarded as more probably seed-bearing plants, a conclusion for which, in certain cases, there is already convincing evidence. The great majority of specimens of fossil fern-like plants are preserved in the form of carbonaceous impressions of fronds, often of remarkable perfection and beauty. The characters shown by such specimens, however, when, as is usually the case, they are in the barren state, are notoriously unstable, or of small taxonomic value, among recent plants. Hence palaeobotanists have found it necessary to adopt a purely :artificial system of classification, based on form and venation of the frond, in the absence of adequate data for a more natural ,grouping. The well-known form-genera Pecopteris, Sphenopteris, Odontopteris, &c., are of this provisional nature. The majority of these fronds have now fallen under suspicion and .can no longer be accepted as those of Ferns; the indications often point to their having belonged to fern-like Spermophyta, as will be shown below.
It has thus become very difficult to decide what Palaeozoic plants should still be referred to the Filices. The fructifications by themselves are not necessarily decisive, for in certain cases the supposed sporangia of Marattiaceous Ferns have turned out to be in reality the microsporangia or pollen-sacs of seed-bearing plants (Pteridosperms). It is, however, probable that a considerable group of true Ferns, allied to Marattiaceae, existed in Palaeozoic times, side by side with simpler forms. In one respect the fronds of many Palaeozoic Ferns and Pteridosperms were peculiar, namely, in the presence on their rachis, and at the base of their pinnae, of anomalous leaflets, often totally different in form and venation from the ordinary pinnules. These curious appendages (Aphlebiae), at first regarded as parasitic growths, have been compared with the feathery outgrowths which occur on the rachis in the Cyatheaceous genus Hemitelia, and with the 'anomalous pinnules found in certain species of Gleichenia, at the points of bifurcation of the frond.
Marattiaceae
A considerable number of the Palaeozoic fern-like plants show indications - more or less decisive - of Marattiaceous affinities; some account of this group will first be given. The reference of these ferns to the family Marattiaceae, so restricted in the recent flora, rests, of course, primarily on evidence drawn from the fructifications. Typically Marattiaceous sori, consisting of exannulate sporangia united to form synangia, are frequent, and are almost always found on fronds with the character of Pecopteris, large, repeatedly pinnate leaves, resembling those of Cyatheaceae or some species of Nephrodium. In certain cases the anatomical structure of these leaves is known, and found to agree generally with that of recent coriaceous fern-fronds. The petiole was usually traversed by a single vascular bundle, hippocrepiform in section - a marked point of difference from the more complex petioles of recent Marattiaceae. There is evidence that in many cases these Pecopteroid fronds belonged to arborescent plants, the stems on which they were borne reaching a height of as much as 60 ft. These stems, known as Megaphytum when the leaves were in two rows, and as Caulopteris in the case of polystichous arrangement, are frequent, especially in the Permian of the Continent; when petrified, so that their internal structure is preserved, the name Psaronius is employed. The structure is often a complex one, the central region containing an elaborate system of numerous anastomosing steles, accompanied by sclerenchyma; the cortex is permeated or coated by a multitude of adventitious roots, forming a thick envelope to the stem. The whole structure bears a general resemblance to that of recent Marattiaceae, though differing in detail. We will now describe some of the fructifications, which are grouped under generic names of their own; these genera, as having a more natural basis, tend to supersede the artificial groups founded on vegetative characters. The genus Asterotheca includes a number of Ferns, chiefly of Coal Measure age, with fronds of the Pecopteris type. The sori, or synangia, ranged in two series on the under-side of the fertile pinnules, are circular, each consisting of 3 to 6 sporangia, attached to a central receptacle and partly united to each other (fig. 15, A); the sporangia separated when mature, dehiscing by a ventral slit. Stur's genus Hawlea (fig. 15, H), characterized by the separation of the sporangia, may only represent an advanced stage of an Asterotheca. In Ptychocarpus the fusion of the sporangia to form the synangium was much more complete; Scolecopteris resembles A sterotheca, but each synangium is stalked. In all these genera there is an obvious similarity to the synangia of Kaulfussia, while in some respects Marattia or Danaea is approached. In another Pecopteroid genus, Sturiella, the synangia resemble those of Asterotheca, but each sporangium is provided with a band of enlarged cells of the nature of an annulus (fig. 15, D). As a similar differentiation, though less marked, appears in the recent genus Angiopteris, the presumption is in favour of the Marattiaceous affinities of Sturiella, which also shows some relation to the genus Corynepteris (see below, Botryopterideae). In the genus Danaeites, from the Coal Measures of the Saar, the synangia are much like those of the recent Danaea, each sporangium opening by an apical pore. In the Grand' Eurya of Stur the sporangia appear to have been free from each other, as in Angiopteris. On the whole there is thus good evidence for the frequency of Marattiaceae in the Palaeozoic period, though the possibility that the fructifications may really represent the microsporangia of fern-like spermophytes must always be borne in mind. In a certain number of genera the reference to Marattiaceae is much more doubtful. In Dactylotheca, for example (fig. 15, C), a Pecopteroid (After various authors. Scott, Studies.) FIG. 15. - Group of Palaeozoic fructifications of Ferns or Pteridosperms.
A, Asterotheca. t, Pinnule bearing 8 synangia. 2, Synangium in side view. 3, In section, magnified.
B, Renaultia. i, Fertile pinnule, nat. size. 2, Sporangium, enlarged.
C, Dactylotheca, as in B.
D, Sturiella. Section of pinnule and synangium. a, Vascular bundle; c, hairs; b, d, annulus, magnified.
E, Oligocarpia. Sorus in surface-view, magnified.
F, Crossotheca. Fertile pinnule, bearing several tufts of microsporangia, magnified.
G, Senftenbergia. Group of annulate sporangia, magnified.
H, Hawlea. Synangium after dehiscence, magnified.
J, Urnatopteris. I, Part of fertile pinna, nat. size. 2, Sporangia, showing apical pores, magnified.
Of the above, A, D, E, G and H, probably belong to true Ferns; F is the male fructification of a Pteridosperm (Lyginodendron); the rest are of doubtful nature.
genus, ranging throughout the Carboniferous, the elongated sporangia individually resemble those of Marattiaceae, but they are completely isolated, the characteristic grouping in sori being absent; the same remark applies to the Sphenopteroid Renaultia of Zeiller (fig. 15, B); the foliage of Sphenopteris, one of the most extensive of Palaeozoic frond-genera, with many different types of fructification, resembled that of various species of Asplenium or Davallia. In many fern-like plants of this period the fronds were dimorphic, the fertile leaves or pinnae having a form quite different from that of the vegetative portions. This was the case in Urnatopteris (Kidston), with Sphenopteroid sterile foliage; the sporangia, borne on the filiform pinnules of the fertile rachis, appear to have dehisced by an apical pore (fig. 15, J). The magnificent Devonian Fern Archaeopteris hibernica, with a somewhat Adiantiform habit, bore special fertile pinnae; the fructification is still imperfectly understood, but the presence of stipules, observed by Kidston, has been adduced in support of Marattiaceous affinities. In all these cases there is reason to suspect that the plants may have been Pteridosperms, rather than Ferns.
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Other Families
The Marattiaceae are the only recent family of Ferns which can be supposed to have existed in anything like its present form in Palaeozoic times. Of other recent orders the indications are meagre and dubious, and there can be no doubt that a large proportion of Ferns from the older rocks (in so far as they were Ferns at all) belonged to families quite distinct from any which we recognize in the flora of our own day. Little or nothing is known of Palaeozoic Ophioglossaceae. Certain fructifications have been referred to Gleicheniaceae (Oligocarpia, fig. 15, E), Schizaeaceae (Senftenbergia, fig. 15, G), Hymenophyllaceae and Osmundaceae, and on good grounds, so far as the external characters of the sporangia are concerned; our knowledge of most of the Ferns in question is, however, far too incomplete to justify us in asserting that they actually belonged to the families indicated. In the case of the Osmundaceae there is good evidence, from anatomical characters, for tracing the family back to the Palaeozoic; their oldest members show a distinct relationship to the Botryopterideae, described in the next paragraph. Numerous more or less isolated fern-sporangia occur in the petrified material of the Carboniferous formation; the presence of an annulus is a frequent character among these specimens, while synangic sori are rare; it is thus certain that families remote from the Marattiaceae were abundantly represented during this period.
Botryopterideae
The family Botryopterideae, first discovered by Renault, stands out with striking clearness among the Palaeozoic Ferns, and differs widely from any group now in existence. The Botry- opterideae are chiefly known from petrified specimens; in the genus Botryopteris and certain species of Zygopteris we have a fairly complete g knowledge of all parts of the plant. The type-genus Botryopteris, represented in the Permo-Carboniferous of France and in both the Lower and Upper Carboniferous of Great Britain, had a rhizome, with a very simple monostelic structure, bearing spirally arranged compound leaves, with lobed pinnules, probably of a somewhat fleshy texture. In the French species, B. forensis, the plant was covered with characteristic jointed hairs, which have served to identify the various organs on which they occur. The sporangia were large pyriform sacs, shortly stalked, and borne in tufts on the branches of the fertile rachis, which developed no lamina. Each sporangium had, on one side only, a longitudinal or slightly oblique annulus, several cells in width; the numerous spores were all of the same size; certain differences among them, which have been interpreted as indicating heterospory, have now proved to depend merely on the state of preservation. The genus Zygopteris, of which numerous Carboniferous and Permian species are known, likewise had a monostelic stem, but the structure of its vascular cylinder was somewhat complex, resembling that of the most highly differentiated Hymenophyllaceae, with which some species of Zygopteris also agreed in the presence of axillary shoots. There is evidence that the stem in some species was a climbing one; the pinnate leaves, arranged on the stem in a two-fifths spiral, were dimorphic, the sterile fronds resembling some forms of (From a drawing by Mrs D. H. Scott. Scott, Studies.) FIG. 17. - Stauropteris oldhamia. Three sporangia borne on branchlets of the rachis. In A the stomium (st) or place of dehiscence is shown. B is cut tangentially. In C, p is the palisade tissue of the rachis. (X about 35.) Sphenopteris. The petioles have a somewhat complex structure, the bundle often having, in transverse section, the form of an H; it has been proposed to subdivide the genus on the details of the petiolar structure. It is characteristic of Zygopteris and its near allies that two rows of pinnae were borne on each side of the rachis, at least in the fertile fronds. On the fertile rachis the sporangia were borne in tufts, much as in the preceding genus; they were still larger, reaching 2.5 mm. in length, and had a multiseriate annulus, extending, however, to both sides of the sporangium (see fig. 16, A and B). In Stauropteris, a genus showing some affinity with Zygopteris, the branched rachis of the fertile frond terminates in fine branchlets, each bearing a single, spherical sporangium, without any differentiated annulus (fig. 17). The spores in the sporangia have been found in a germinating condition; the stages of germination correspond closely with those observed in recent homosporous ferns (fig. 18). This fact strongly confirms the conclusion, drawn from morphological and anatomical characters, that the Botryopterideae were true Ferns. The genus Corynepteris of Baily is interesting from the fact that its sporangia, while individually similar to those of Zygopteris, were grouped in sori or synangia, resembling those of an Asterotheca. The family Botryopterideae appears to have included a number of other genera, though in most cases the evidence from vegetative structure is alone available. The genus Diplolabis of Renault, shows much in common with Zygopteris as regards anatomical structure, but resembles Corynepteris in possessing a synangic fructification. The genus Asterochlaena of Corda with a deeplylobed stele, goes back to the Devonian. The family as a whole is of great interest, as presenting points of contact with various recent orders, especially Hymenophyllaceae, Osmundaceae and Ophioglossaceae; the group appears to have been a synthetic one, belonging to a primitive stock (the Primofilices of Arber) from which the later Fern families may have sprung.
A number of genera of Palaeozoic " fern-fronds " have been described, of the fructification of which nothing is known. This is the case, for example, with Diplotmema, a genus only differing from Sphenopteris in the dichotomy of the primary pinnae, and with Mariopteris, which bears a similar relation to Pecopteris. The same holds good of the Pecopteroid Ferns included under Callipteris and Callipteridium. In such cases, as will be explained below, there is a strong presumption that the fronds were not those of Ferns, but of seed-bearing plants of the new class Pteridospermeae.
On the present evidence it appears that the class Filicales was well represented in the Palaeozoic flora, though by no means so dominant as was formerly supposed. The simpler Ferns (Primofilices) of the period are for the most part referred to the remarkable family Botryopterideae, a group very distinct from A - C D (From a drawing by Mr L. A. Boodle. Scott, Studies.) FIG. 18. - Stauropteris oldhamia. Four germinating spores from the interior of a sporangium. All four are putting out rhizoids. In C, lying horizontally, an additional cell has been cut off between rhizoid and spore. (X 335.) any of the more modern families, though showing analogies with them in various directions. On the other hand there was the far more complex Marattiaceous type, strikingly similar in both vegetative and reproductive characters to the recent members of the family. Although doubts have lately been cast on the authenticity of Palaeozoic Marattiaceae owing to the difficulty in distinguishing between their fructifications and the pollenbearing organs of Pteridosperms, the anatomical evidence (stem of Psaronius) strongly confirms the opinion that a considerable group of these Ferns existed.
Spermophyta
The Pteridospermeae, for which Potonie's name Cycadofilices is still sometimes used, include all the fern-like plants which, on the evidence available, appear to A (After Renault.) FIG. 16. - Zygopteris pinnata. A, Group of sporangia, in surface view.
B, Single sporangium, in transverse section, showing annulus on both sides, magnified.
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have been reproduced by means of seeds. The cases in which such evidence is decisive are but few, namely, Lyginodendron oldhamium, Neuropteris heterophylla, Pecopteris Pluckeneti, Aneimites fertilis and Aneimites tenuifolius. In the first-named plant the structure, both of the vegetative and reproductive organs, is known, and the evidence, from comparison and association, is sufficiently strong. In the other cases there is direct proof of continuity between seed and plant, but only the external characters are known. Ina great number of forms, amounting to a majority of the Palaeozoic plants of fern-like habit, the indirect evidence is in favour of their having possessed seeds. We will begin with the Lyginodendreae, a group in which the anatomical characters indicated a systematic position between Ferns and Cycads, long before the reproductive organs were discovered.
Lyginodendreae
Of the genus Heterangium, which still stands very near the true Ferns, several species are known, the oldest.
structure bears a general resemblance to Heterangium, the primary wood consists for the most part of short wide tracheides; probably, ?.?
r (After Williamson. Scott, Studies.) FIG. 19. - Heterangium Grievii. Restoration of Stem, shown partly in transverse and longitudinal section, partly in surface view.
x, Primary wood. hy, Hypoderma.
x 2 , Secondary wood. l.t, l.t, Leaf-traces.
p.c, Phloem and pericycle. r, Adventitious root. Several c, Cortex. leaf-bases are shown.
being H. Grievii, of Williamson, from the Lower Carboniferous of Scotland. This plant had a long, somewhat slender, ridged stem, the ridges corresponding to the decurrent bases of the spirally arranged leaves (fig. 19). The specimens on which the genus was founded are petrified, showing structure rather than habit, but conclusive evidence has now been obtained that the foliage of H. Grievii was of the type of Sphenopteris (Diplotmema) elegans (fig. 20), and was thus in appearance altogether that of a Fern, with somewhat the habit of an Asplenium. The stem has a single stele, resembling in general primary structure that of one of the simpler species of Gleichenia; there is no pith, the wood extending to the centre of the stele. The leaf-traces, where they traverse the cortex, have the structure of the foliar bundles in Cycads, for they are of the collateral type, and their xylem is mesarch, the spiral elements lying in the interior of the ligneous strand. The leaf-traces can be distinguished as distinct strands at the periphery of the stele, as shown in fig. 21. Most of the specimens had formed a zone of secondary wood and phloem resembling the corresponding tissues in a recent Cycad; the similarity extended to minute histological details, as is shown especially in H. tiliaeoides, a Coal Measures species, where the preservation is remarkably perfect. The cortex was strongly constructed mechanically; in addition to the strands of fibres at the periphery, horizontal plates of stone-cells were present in the inner cortex, giving both stem and petiole a transversely striated appearance, which has served to identify the different parts of the plant, even in the carbonized condition (cf. figs. 19 and 20). The single vascular bundle which traversed the petiole and its branches was concentric, the leaves resembling those of Ferns in structure as well as in habit. Heterangium shows, on the whole, a decided preponderance of Filicinean vegetative characters, though in the leaf-traces and the secondary tissues the Cycads are approached. The organs of reproduction are not yet known, though there is a probability that an associated seed allied to Lagenostoma (see below) belonged to Heterangium. In the Coal Measure genus Megaloxylon, of Seward, which in (After Stur. Scott, Studies.) FIG. 20. - Sphenopteris elegans (foliage of Heterangium Grievii). Part of frond. (3 nat. size.) as the secondary tissues increased, it had become superfluous for conducting purposes, and was adapted rather for water-storage. In the genus Lyginodendron, of which L. oldhamium, from the (Scott, Studies.) FIG. 21. Heterangium Grievii. Part of the stele of the stem in transverse section, showing a primary xylem-strand and adjacent tissues. (X 135.) px, Protoxylem of strand. c.p, Conjunctive tissue.
x, Centripetal. Secondary wood.
x', Centrifugal primary wood. cb, Cambium.
mx, Part of the internal wood. ph 2 , Phloem.
Coal Measures, is now the best-known of all Palaeozoic plants, the central wood has disappeared altogether and is replaced by pith; the primary wood is only represented in the leaf-trace strands, which form a ring of distinct collateral bundles around the pith; (From a model after Oliver.) FIG. 23. - Lagenostoma Lomaxii (the seed of Lyginodendron). Restoration of a seed, enclosed in the lobed cupule, which bears numerous glands. (X about 15.) thus the " medullate-monostelic " structure characteristic of the higher plants was already attained. The individual bundles, however, have the same structure as in Heterangiurn, and agree (From a photograph. Scott, Studies.) FIG. 24. - Capitate gland on the cupule of Lagenostoma Lomaxii. (X 70.) closely with the foliar bundles of Cycads. The secondary tissues, which are highly developed, are also of a Cycadean character (fig. 22, Plate). The vegetative organs of the plant are very completely known; the foliage has proved to be that of a Sphenopteris, identical with the species long known under the name of S. Honinghausi. Apart from the important advance shown in the anatomy of the stem, Lyginodendron agrees structurally with Heterangium. There is reason to believe that Lyginodendron oldhamium was a climbing plant comparable in some respects to such recent Ferns as Davallia aculeata. The roots were at first like those of Marattiaceae but grew in thickness like the roots of Gymnosperms.
The first definite evidence of the mode of reproduction of Lyginodendron oldhamium was due to F. W. Oliver, who in 1903 identified the seed, Lagenostoma Lomaxii, by means of the glands on its cupule, which agree exactly with those on the associated leaves and stems of the plant (cf. figs. 24 and 25). No similar glands are known on any other Palaeozoic plant. Lagenostoma Lomaxii is a small barrel-shaped seed (5.5 by 4.2 5 mm. when mature) enclosed in a husk or cupule, which completely enveloped it when young, but was ultimately open (figs. 23 and 26 and fig. 27 from another species). The seed was stalked, and there is an exact agreement in structure between the vascular strands of the stalk and cupule of the seed, and those of the rachis and leaflets of Lyginodendron, thus confirming the evidence from the glands. The seed itself is of a Cycadean type, and radially symmetrical. The single integument is united to the nucellus, except at the top, and is traversed by about nine vascular strands. In the apex of the nucellus, as in most Palaeozoic seeds and in recent Cycads, a pollen-chamber, for the reception of the pollen-grains or microspores, is excavated (fig. 26). In Lagenostoma the pollen-chamber has a peculiar From a photograph. Scott, Studies.) FIG. 25. - Capitate Gland on the Petiole of Lyginodendron oldhamium. (X 70.) structure, a solid column of tissue rising up in the middle, leaving only a narrow annular crevice, in which pollen-grains are found. The neck of the flask-shaped pollen-chamber projected a little from the micropyle and no doubt received the pollen directly. The seed, which need not be described in further detail, was a highly organized structure, showing little trace of the cryptogamic megasporangium from which we must suppose it to have been derived. From the structure of the seed-bearing stalk, and from the analogy of the similar form Lagenostoma Sinclairi (fig. 27) it appears that the seed was borne on a leaf, or part of a leaf, reduced to a branched rachis.
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The male organs of Lyginodendron were discovered by Kidston, a year or two after the seeds were identified. They are of the type known as Crossotheca, formerly regarded as a Marattiaceous fructification. The genus is characterized by the arrangement of the sporangia, which hang down from the lower surface of the little oval fertile leaflets, the whole resembling an epaulet with its fringe (fig. 15, F; fig. 28). In the case of Lyginodendron the Crossotheca occurs in connexion with the vegetative parts of the frond. Each fertile pinnule bore six, or rarely seven fusiform microsporangia, described as bilocular: not improbably each may represent a synangium. The microspores are tetrahedral. This is the first case in which the pollen-bearing organs of a Ptoridosperm have been identified with certainty It will be seen that, while the seeds of Lyginodendron were of an advanced Cycadean type, the microsporangiate organs were more like those of a Fern, the reproductive organs thus showing the same combination of characters which appears in the vegetative ct pc. A, Micropylar region.
B, Body of seed.
C, Chalazal region.
B D, Stalk.
c, Cupule, surrounding seed.
vb, Vascular bundles of stalk, cupule and integument.
cp, " Canopy," or waterreservoir, at top of integument.
pc, Cavity of pollenchamber.
cc, Central column.
apc, Aperture of pollenchamber.
(After Oliver. Scott, Studies.) FIG. 26. - Lagenostoma Lomaxii. Diagram of seed in median longitudinal section.
structure. The family Calamopityeae, allied anatomically to Lyginodendreae, is of Devonian and Lower Carboniferous age.
Cycadoxyleae
A few Coal Measure and Permian stems (Cycadoxylon and Ptychoxylon) resemble Lyginodendron in the general character of their tissues, but show a marked reduction of the primary wood, together with an extensive development of anomalous wood and bast around the pith, a peculiarity which appears as an individual variation in some specimens of Lyginodendron oldhamium. It is probable that these stems belonged to plants with the fructification and foliage of Cycads, taking that group in the widest sense. It is only quite at the close of the Palaeozoic period that Cycads begin to appear. The Lyginodendreae type of structure, however, appears to have formed the transition not only to the Cycadales, but also to the extinct family Cordaiteae, the characteristic Palaeozoic Gymnosperms (see p. 107).
Medulloseae
In some respects the most remarkable family of the Cycad-fern alliance is that of the Medulloseae,seed-bearing plants often of great size, with a fern-like foliage, and a singularly corn- (After Arber. Scott, .Studies.) plex anatomical structure with FIG. 27. - Lagenostoma Sinclairs. out parallel amongrecent plants. Two seeds, enclosed in lobed cupules Some of the Medulloseae must and borne on branches of the rachis. have had a habit not unlike (X 5.) that of tree-ferns, with compound leaves of enormous dimensions, belonging to various frondgenera - especially, as has now been proved, to Alethopteris and Neuropteris; these are among the most abundant of the Carboniferous fronds commonly attributed to Ferns, and extend back to the Devonian. In habit some species of Alethopteris resembled the recent Angiopteris, while the Neuropteris foliage may be compared with that of an Osmunda. The Medullosa stems have been found chiefly in the Permo-Carboniferous of France and Germany, but a Coal Measures species (M. anglica) has been discovered in Lancashire. The great anatomical characteristic of the stem of the Medulloseae is its polystelic structure with secondary development of wood and bast around each stele. In M. anglica, the simplest species known, the steles are uniform, and usually only three in number; the structure of the stem is essentially that of a polystelic Heterangium. In the PermoCarboniferous species, such as M. stellata and M. Leuckarti, the arrangement is more complicated, the steles showing a differentiation into a central and a peripheral system; the secondary growth was extensive and unequal, usually attaining its maximum on the outer side of the peripheral steles. In certain cases the structure was further complicated by the appearance of extrafascicular zones exterior to the whole stelar system. The spirally arranged petioles (Myeloxylon) were of great size, and their decurrent bases clothed the surface of the stem; their (From structure is closely similar to that Studies.) of recent Cycadean petioles; in FIG. 28. - Crossotheca Honing- fact, the leaves generally, like hausi, the male fructification of those of Stangeria at the present Lyginodendron. Fertile leaflets, day, while fern like in habit bearing sporangia, and sterile, were Cycadean in structure. In leaflets on the rachis of the same the case of Medullosa anglica we leaf. (X 2.) have an almost complete knowledge of the vegetative organs - stem, leaf and root; Cycadean characters no doubt predominate, but the primary organization of the stem was that of a polystelic Fern. In the new genus Sutcliffia, also from the Coal Measures of Lancashire, the stem had a single, large central stele, from which smaller strands were given off, forming a kind of network, which gave rise to the numerous concentric leaf-traces which entered the (After Kidston. Scott, Studies.) FIG. 29. - Neuropteris heterophylla. Seed, attached to a branch of the rachis bearing two vegetative leaflets. (X 2.) petioles. This plant may be regarded as anatomically the most primitive of the Medulloseae.
| PALAEOZOIC] |
In one member of the Medulloseae, there is direct evidence of reproduction by seeds, for in Neuropteris heterophylla Kidston has demonstrated that large seeds, of the size of a hazel-nut, were borne on the frond (fig. 29). In this case the internal structure is not known, but another seed, Trigonocarpus Parkinsoni, associated with, and probably belonging to, the Alethopterid species, Medullosa anglica, occurs in the petrified condition and has been fully investigated. This is a large seed, with a very long micropyle; it has a beaked pollen-chamber, and a complex integument made up of hard and fleshy layers, closely resembling the seed of a modern Cycad; the nucellus, however, was free from the integument, each a sketch after Kidston. Scott, having its own vascular system. Various other seeds of the same type are known, and in a great number of instances Grand' Eury has found the fronds of Neuropterideae (Medulloseae) in close association with definite species of seeds, so there can be little doubt that the whole family was seed-bearing. Very little is known at present of the male organs. Some authors have been so much impressed by the similarity of this extinct family to the Cycads, that they have regarded them as being on the direct line of descent of the latter group; it is more probable, however, that they formed a short divergent phylum, distinct, though not remote, from the Cycadean stock.
Pecopterideae
It has now been established that the form-genus Pecopteris, once regarded as representing the typical Marattiaceous foliage, was in part made up of seed-bearing plants. In 1905 Grand' Eury discovered