Fossil plants, Vol. 2

This division of the Pteridophyta includes both the true ferns (Filicineae) and the less familiar water-ferns or Hydropterideae. The almost complete absence of satisfactory evidence in regard to the geological history of the latter renders this group of secondary importance from a palaeobotanical standpoint, but, on the other hand, we possess a wealth of material bearing on the past history and relative antiquity of the true ferns.

The study of extinct types has so far rendered no substantial help towards bridging the wide gap between the Filicales and the lower plants. As Mr Tansley[675] says in his admirable lectures on The Evolution of the Filicinean Vascular System, “The biggest gap in the plant kingdom at the present time is undoubtedly that which separates the Pteridophytes from the plants definitely below them in organisation, and directly we try to step behind the ferns we tumble into this abyss.” Resemblances long ago recognised between certain ferns and the cycads, a section of the Gymnosperms, were regarded by a few botanists as indications of blood-relationship, and the results of recent researches into the morphological characters of extinct Palaeozoic types are generally held to confirm these surmises. Prof. Chodat[676] of Geneva has recently challenged the validity of the arguments on which the affinity of cycads and ferns has been accepted by the great majority of botanists. Whether or not his criticisms stand the test of unbiassed examination, they must at least lead us to substitute a critical consideration of the facts for a mere repetition of conclusions which appeal to our imagination. Despite Prof. Chodat’s warning, we may still quote with confidence a phrase used in another connexion—ferns “are links in a chain and branches on the tree of life, with their roots in a past inconceivably remote[677].”

Transitional forms which are regarded as pointing to a common origin for ferns and cycads are known in abundance; other types have also been discovered which lead some authors to go so far as to derive the whole of the seed-bearing plants from an ancestry the descendants of which are represented by existing ferns. While hesitating to allow the ferns or fern-like plants the peculiar position of universal ancestors, we must admit that there is no group of plants with a history of greater importance from an evolutionary standpoint than that with which we are now concerned.

There are, however, some difficulties to face in attempting to decipher the history of the Filicineae as recorded in the earth’s crust. Few fossil plants are so familiar as the well-preserved carbonaceous impressions of compound leaves on the shales of our Coal-Measures, which were referred by older authors to recent genera and species of ferns and accepted by later writers as undoubted examples of Palaeozoic ferns. The common belief in the dominance of ferns in Palaeozoic floras is reflected in the novelist’s description of the Carboniferous period, “when the forms of plants were few and often of the fern kind[678].” We now know that very many of these Carboniferous leaves belonged to plants differing widely in morphological characters from the modern genera to which they exhibit so deceptive a resemblance. These pseudo-ferns, recently christened Pteridosperms or seed-bearing fern-like plants, are dealt with in a later chapter. The discovery of this extinct group has added enormously to our knowledge of plant-evolution and at the same time has rendered much more difficult the task of unravelling the past history of the true ferns. As soon as it was demonstrated that many familiar Palaeozoic “ferns” are not ferns, some authors went far towards concluding that however close might be the agreement between fossil and recent leaves suspicion of close relationship must be set aside. Like the earlier writers who described fossils as lusus naturae fashioned by devilish agency to deceive too credulous man, the discovery of seed-bearing plants with the foliage of ferns threatened to disturb the mental balance of palaeobotanists. The fact is, we cannot in some cases determine from leaf-form alone whether or not a fossil is a true fern; we may, as Professor Bower[679] suggests, regard all fern-like fossils as ferns until they are proved to be Pteridosperms, or in a spirit of scientific scepticism, we may at once admit that many Palaeozoic fern-like leaves must await further evidence before their true position can be determined. It is impossible, as Zeiller[680] says, in the present state of our knowledge to range fern-like Palaeozoic plants in two groups, one referred to Filicineae and the other to the Pteridosperms.

The following classification of the Filicales is based on that adopted by Prof. Engler in the latest edition of his Syllabus[681] and on the results of Bower’s[682] excellent work on the spore-bearing members of recent ferns.

The members of the Filicales are characterised by the same well-marked physiological division of labour in their vegetative parts as are the Lycopods; the plant is the asexual generation (sporophyte), while the sexual generation (gametophyte) is small and inconspicuous, either an independent green prothallus or a tissue more or less completely enclosed in the spore. The large size of the leaves, which in the young state are usually coiled like a crozier (fig. 220, A), is a striking characteristic of the ferns; they are megaphyllous in contrast to the microphylly of the Lycopods.

I. Leptosporangiate Filicales.

In these homosporous and heterosporous plants the sporangia are developed from single epidermal cells.

(a) Eufilicineae. The sporangia bear spores of one kind only; the wall of a sporangium consists of one layer of cells. In the great majority of cases the sporangia are characterised by the possession of a conspicuous row of thick-walled brown cells, the annulus[683], which serves as a mechanism for dehiscence and spore-dispersal. The fertile leaves, identical in form with the sterile, or more or less sharply contrasted, usually bear the sporangia on the under surface of the lamina in definite groups or sori, and not on the upper surface or grouped in strobili as in the Lycopodiales. The stem is dorsiventral or radial in structure, creeping or erect, frequently clothed with chaffy scales (ramenta) and less often with multicellular hairs. The sexual generation is represented by a small green prothallus which lives for a short period only and dies after nursing the fern-plant through its earliest stages.

(b) Hydropterideae. Heterosporous water-ferns differing considerably in habit from the true ferns. Each megasporangium contains a single megaspore and several microspores are produced in each microsporangium. The gametophyte is represented by tissue more or less enclosed in the spore. [Genera Salvinia, Azolla, Marsilia, Regnellidium, Pilularia. See Chapter XXVI.]

Eufilicineae.

The classification of the true ferns in common use is based almost exclusively on the structure of the sporangium, the form and position of the sori, and on the presence or absence of an indusium (the tissue which in some ferns partially or completely covers each sorus). In recent years there has been considerable activity in the investigation of fern anatomy with a view to elucidating the natural relationship between recent families or genera. The results of these researches are on the whole consistent with the scheme and grouping adopted in the Synopsis Filicum of Hooker and Baker and in general harmony with the main conclusions arrived at by Bower from an intensive study of the development of fern sporangia. The following classification is based on that of Bower who takes as a basis (i) the relative time of appearance of the sporangia in a single sorus, (ii) the structure of the sporangia and their orientation relative to the whole sorus, (iii) the productiveness of sporangia (spore-output).

Osmundaceae[684]. (Osmunda, Todea.)

Sporangia large and rather stouter than those of other Leptosporangiate ferns, borne in small groups (filmy species of Todea) in linear and frequently confluent sori (Todea barbara; fig. 221, D) or clustered round the axis of modified fertile pinnae with much reduced lamina (Osmunda). The annulus is represented by a group of thicker-walled cells a short distance below the apex (fig. 221, C). This family stands apart among the ferns; in some respects, e.g. in the more robust sporangia occasionally forming synangia, and in the presence of stipular wings, it forms a transitional series between the Leptosporangiate and Eusporangiate ferns. The only European species of Osmunda, O. regalis, is almost cosmopolitan in range; other species occur in North and South America, in the Far East, the Malay Peninsula, and in other regions, more especially in the temperate zones. Todea is represented by (i) the South African and Australian species, T. barbara, a fern with a stem, which may reach a height of several feet, thickly covered with adventitious roots and bearing large and somewhat leathery fronds; (ii) filmy species in New Zealand, New South Wales, New Caledonia, and elsewhere. A plant of the small tree-fern Todea Wilkesiana (Fiji, Samoa, and other islands) in the filmy-fern house at Kew, to which my attention was drawn by my friend Mr A. W. Hill, has a slender stem with the characteristic leaf-scars exposed; it presents a striking similarity to some of the fossil species of Osmundaceae described in a later chapter.

Schizaeaceae. (Schizaea, Aneimia, Lygodium, Mohria.)

Sporangia borne singly and not in groups (sori), readily recognised by the complete transverse apical annulus usually one layer of cells deep, but occasionally two layers in depth on the side opposite the line of dehiscence[685] (fig. 224, B). Schizaea (fig. 222) with the exception of one species in North America (S. pusilla) is characteristic of Northern India, the Malay region, Australia, New Caledonia, S. Africa, and elsewhere south of the Equator. Aneimia (figs. 223, 224, A, B), characterised by the fertile segments with reduced lamina, is chiefly American: the monotypic genus Mohria, resembling in habit the Polypodiaceous genus Cheilanthes, occurs in S. Africa and Madagascar, while species of Lygodium are widely spread tropical ferns, with one species in temperate North America. This family has disappeared from Europe.

Gleicheniaceae [Gleichenia, Platyzoma (= G. microphylla)].

Sporangia form circular naked sori composed of a variable number of sporangia, usually not more than ten and frequently fewer, characterised by an obliquely horizontal and almost complete annulus (fig. 224, I). In some species of Gleichenia (sect. Eugleichenia) the ultimate segments are very small and semicircular in form (fig. 226, C), in others (sect. Mertensia[686]) the segments are linear (fig. 226, D), and in many species the fronds are distinguished by the regular dichotomous branching (fig. 225), frequently showing an arrested rachis bud in the forks[687] protected by modified pinnules (fig. 226, D, E). In Platyzoma the leaves are simple, reaching a length of 20–30 cm., and bear small revolute oval segments.

Gleichenia is represented by several species in the tropics and extends to south temperate and Antarctic latitudes. The species G. dichotoma (= G. linearis) is one of the more successful tropical ferns, while G. moniliformis (by some authors recognised as a distinct genus, Stromatopteris) is peculiar to New Caledonia. The monotypic genus Platyzoma is a xerophilous Australian fern. The Gleicheniaceae are unrepresented in existing north temperate floras.

Matonineae. (Matonia.)

The genus Matonia, placed in the Cyatheaceae by Sir William Hooker and compared by other authors also with the Gleicheniaceae, is now included in a special family. The sori are circular and consist of 5–11 large sporangia (fig. 224, E, G) sessile on a central columnar receptacle which spreads out into an umbrella-like indusium (D, i) with its incurved margin tucked in below the ring of sporangia. The indusium is detached when the sporangia are ripe. The annulus is oblique and incomplete and often slightly sinuous; it agrees in the main with that of Gleichenia. The species Matonia pectinata is characterised by dichotomously branched fronds (figs. 227, 228) with long and slender petioles; the pinnae bear linear pinnules with forked lateral veins and occasional lateral anastomoses (fig. 224, F). The only other living representative is M. sarmentosa, discovered by Mr Charles Hose at Niah, Sarawak[688]: this species has long pendulous leaves apparently very different from those of M. pectinata, but the branching of the frond may be regarded as a modification of a primitive form of dichotomy[689]. A small bud occurs in the angle between the forked linear segments and the rachis, as in some species of Gleichenia[690]. Matonia is confined to the Malay region: M. pectinata grows in Western Borneo and in various localities in the Malay peninsula, while M. sarmentosa, has been found in one locality only; the latter species has recently been transferred to a new genus Phanerosorus, but in view of the practical identity in anatomical structure and the close agreement as regards the sori of the two species there would seem to be no justification for this change of name[691].

Loxsomaceae.

The New Zealand genus Loxsoma has marginal sori with a cup-like indusium surrounding an elongated receptacle bearing pear-shaped sporangia provided with a complete oblique annulus. The genus is chiefly interesting because of its isolated position; it agrees with Trichomanes (Hymenophyllaceae) in the structure of the sorus and with species of Dicksonia and Davallia in habit; it shows some resemblance also to Gleicheniaceae and Schizaeaceae[692]. A new type of fern described by Christ[693] from Costa Rica as Loxsomopsis costaricensis affords a striking instance of discontinuous distribution and emphasises the antiquity and generalised features of the family.

Hymenophyllaceae. (Hymenophyllum, Trichomanes.)

The sporangia, which are attached to a columnar receptacle or prolongation of a vein beyond the margin of the lamina, are characterised by an obliquely transverse annulus (fig. 224, C). A cup-like indusium surrounds the lower portion of the receptacle which is two-lipped in Trichomanes and entire in Hymenophyllum (fig. 270, C, D). These two filmy ferns have a wide distribution both in tropical and extra-tropical regions; they are represented in the British Isles by Hymenophyllum tunbrigense, H. Wilsoni, and Trichomanes radicans.

Cyatheaceae. (Cyathea, Hemitelia, Alsophila, Dicksonia, Thyrsopteris.)

The sporangia occur in indusiate or naked sori and have an obliquely vertical and incomplete annulus (fig. 229, E). In the great majority of cases the fronds are large and highly compound, but Cyathea sinuata Hook, a rare Ceylon species, bears simple narrow linear leaves. This family includes, with few exceptions, all the tree ferns[694]. The sori of Dicksonia are enclosed in a two-valved indusium (fig. 229, F. G); in the species represented in fig. 230 the fertile segments, which terminate in cup-like indusia, are characterised by the absence of a lamina and closely resemble those of Thyrsopteris (fig. 229, A). In Cyathea the indusium has the form of a cup which is at first closed and afterwards opens at the apex (fig. 229, B); in Hemitelia the indusium is much reduced and in Alsophila the sori are naked. Thyrsopteris is characterised by the reduced fertile pinnules bearing stalked sori in deep cups (fig. 229, A). The appearance of this fern “is very remarkable, for the cup-shaped sori hang down from the fronds in masses, looking just like masses of millet seed[695].” The sporangia are described by Bower[696] as large and of rather peculiar form. As seen in fig. 224, H, the annulus is continuous; it forms a twisted loop of cells which vary in shape and in the thickness of the walls. The Cyatheaceae are for the most part tropical ferns with a wide geographical range, usually in moist regions; they are, however, able to flourish under widely different temperature conditions. In Tasmania, as Diels[697] points out, tree ferns may occasionally be seen laden with snow, and on the west coast of New Zealand they overhang the edge of a glacier[698]. The monotypic genus Thyrsopteris is confined to Juan Fernandez. The Cyatheaceae no longer exist in Europe.

Dennstaedtiinae. (Microlepia, Dennstaedtia.)

This sub-tribe, instituted by Prantl, has been revived by Bower on the ground that the sori present features intermediate between those of Cyatheaceae and the Polypodiaceous genus Davallia. The sporangia have a slightly oblique annulus.

Polypodiaceae.

This section of the Leptosporangiate ferns, including several sub-tribes, comprises the great majority of recent genera. The sporangia form naked or indusiate sori and have a vertical incomplete annulus. In Plagiogyria[699] the oblique annulus and soral features suggest comparison with the Cyatheaceae. A more intimate acquaintance with Polypodiaceous ferns will undoubtedly demonstrate the existence of other generalised types[700].

From the point of view of the identification of fossil ferns it is important to bear in mind the very close resemblance presented by some Polypodiaceous species, e.g. species of Davallia (fig. 229, C), to Cyatheaceous ferns (cf. fig. 229, D).

Parkeriaceae. (Ceratopteris.)

The almost spherical and scattered sporangia are characterised by the peculiar form of the vertical annulus, which is composed of numerous cells differing in their greater breadth and smaller depth from those of a typical annulus. Exannulate sporangia have been described, while others occur showing different stages between a rudimentary and a complete ring. The single species of Ceratopteris, C. thalictroides, is an annual aquatic fern widely spread in tropical countries[701].

Dipteridinae. (Dipteris.)

The genus Dipteris, formerly included in the Polypodiaceae, has been assigned to a separate family partly on account of the slight obliquity of the vertical annulus (fig. 231, G) and on other grounds[702]. The four species Dipteris conjugata, D. Wallichii, D. Lobbiana (= D. bifurcata), and D. quinquefurcata (fig. 231) are characterised by a creeping rhizome bearing fronds reaching a length of 50 cm.; in D. conjugata and D. Wallichii the lamina is divided by a median sinus into two symmetrical halves, while in other species the leaf is dissected into narrow linear segments. The main dichotomously branched ribs are connected by lateral branches and these by tertiary veins, the delicate branches of which end freely within the square or polygonal areolae (fig. 231, A′, E). The naked sori are composed of numerous sporangia and filamentous hairs: while in some species the soral development conforms to that characteristic of the Mixtae, it has been shown that in one species, D. Lobbiana (= D. bifurcata[703]), the sporangia develope simultaneously as in the Simplices. Dipteris occurs in company with Matonia on Mt Ophir and elsewhere in the Malay peninsula; it extends to the Philippines, Samoa, New Caledonia, China, New Guinea, and the subtropical regions of Northern India.

The impossibility of drawing a hard and fast line between the divisions adopted in any system of classification is well illustrated by the ferns. In the main, the three-fold grouping suggested by Bower is probably consistent with the order of evolution of the true ferns. The Polypodiaceae, which are now the dominant group, are in all probability of comparatively recent origin, while the Gradatae and Simplices represent smaller subdivisions with representatives in remote geological epochs. The genera Loxsoma, Matonia and Dipteris afford examples of ferns exhibiting points of contact with more than one of Bower’s subdivisions: they are generalised types which, like many relics of the past, are now characterised by a restricted geographical range.

It is noteworthy that while certain vegetative features may in some cases be cited as family-characters, such features are not usually of much value from a taxonomic point of view. While the typical tree ferns are practically all members of the Cyatheaceae, a few members of other families, e.g. Todea barbara (Osmundaceae) and the monotypic Indian genus Brainea (Polypodiaceae), form erect stems several feet in height; but these differ in appearance from the Palm-like type of the Cyatheaceous tree ferns. On the other hand, the thin, almost transparent, leaf of Hymenophyllum tunbridgense and other filmy ferns is a character shared by several species of Todea, Asplenium resectum, and Danaea trichomanoides (Marattiaceae); the filmy habit is essentially a biological adaptation.

The form of frond represented by certain species of Gleichenia, characterised by a regular dichotomy of the axis and by the occurrence of arrested buds, is on the whole a trustworthy character, though Davallia aculeata (bearing spines on its rachis) (fig. 232) and Matonia sarmentosa have fronds with a similar mode of branching and also bear arrested radius-buds. A limited acquaintance with ferns as a whole often leads us to regard a certain form of leaf as characteristic of a particular species, but more extended enquiry usually exposes the fallacy of relying upon so capricious a feature. The form of leaf illustrated by Trichomanes reniforme is met with also in Gymnogramme reniformis and is fairly closely matched by the leaf of Scolopendrium nigripes. The fronds of Matonia pectinata (figs. 227, 228) bear a close resemblance to those of Gleichenia Cunninghami, Adiantum pedatum, and Cheiropteris palmatopedata[704].

The habit, leaf-form, and distribution of Ferns.

The full accounts of the structure and life-history of the common Male Fern, given by Scott in his Structural Botany and by Bower in the Origin of a Land Flora, render superfluous more than a brief reference to certain general considerations in so far as they may facilitate a study of fossil types.

In size Ferns have a wide range: at the one extreme we have the filmy fern Trichomanes Goebelianum[705], growing on tree stems in Venezuela, with leaves 2·5 to 3 mm. in diameter, and at the other the tree ferns with tall columnar stems reaching a height of 40 to 50 feet and terminating in a crown of fronds with a spread of several feet. A common form of stem is represented by the subterranean or creeping rhizome covered with ramental scales or hairs: the remains of old leaves may persist as ragged stumps, or, as in Oleandra, Polypodium vulgare and several other species, the leaf may be cut off by the formation of an absciss-layer[706] leaving a clean-cut peg projecting from the stem. As a rule the branches bear no relation to the leaves and are often given off from the lower part of a petiole, but in a few cases, e.g. in the Hymenophyllaceae, it is noteworthy that true axillary branching is the rule[707]. In the typical tree-fern the surface resembles that of a Cycadean trunk covered with persistent leaf-bases and a thick mass of roots. Among epiphytic ferns highly modified stems are occasionally met with, as in the Malayan species Polypodium (Lecanopteris) carnosum and P. sinuosum[708].

The leaves of ferns are among the most protean of all plant organs; as Darwin wrote, “the variability of ferns passes all bounds[709].” The highly compound tri- or quadripinnate leaves of such species as Pteris aquilina, Davallia and other genera stand for the central type of fern frond; others exhibit a well-marked dichotomy, e.g. Lygodium, Gleichenia, Matonia, etc., a habit in all probability associated with the older rather than with the more modern products of fern evolution. Before attempting to determine specifically fossil fern fronds, it is important to familiarise ourselves with the range of variability among existing species and more especially in leaves of the same plant. A striking example of heteromorphy is illustrated in fig. 233. Reinecke[710] has figured a plant of Asplenium multilineatum in which the segments of the compound fronds assume various forms. In Teratophyllum aculeatum var. inermis Mett., a tropical climbing fern believed by Karsten[711] to be identical with Acrostichum (Lomariopsis) sorbifolium,—an identification which Goebel[712] questions,—the fronds which stand free of the stem supporting the climber differ considerably from the translucent and much more delicate filmy leaves pressed against the supporting tree. From this fern alone Fée is said to have created 17 distinct species. In this, as in many other cases, differences in leaf-form are the expression of a physiological division of labour connected with an epiphytic existence. Some tropical species of Polypodium (sect. Drynaria), e.g. P. quercifolium (fig. 234 and fig. 231, D), produce two distinct types of leaf, the large green fronds, concerned with the assimilation of carbon and spore-production, being in sharp contrast to the small slightly lobed brown leaves which act as stiff brackets (fig. 234, M) for collecting humus from which the roots absorb raw material. Similarly in Platycerium the orbicular mantle-leaves differ widely from the long pendulous or erect fronds fashioned like the spreading antlers of an elk. In Hemitelia capensis, a South African Cyatheaceous species, the basal pinnae assume the form of finely divided leaves identified by earlier collectors as those of a parasitic Trichomanes (fig. 235). In a letter written by W. H. Harvey in 1837 accompanying the specimen shown in fig. 235, he says, “Apropos of Hemitelia, be it known abroad that supposed parasitical Trichomanes ... is not a parasite, but a part of the frond of Hemitelia.” The delicate reduced pinnae remain on the stem and form a cluster at the base of the fronds[713].

In many species the sporophylls are distinguished from the sterile fronds by segments with little or no chlorophyllous tissue, as in Onoclea struthiopteris[714] in which, each year, the plant produces a funnel-shaped group of sterile leaves followed later in the season by a cluster of sporophylls; or, as in many other genera, the fertile leaves are distinguished also by longer petioles and thus serve as more efficient agents of spore-dissemination. In Ceratopteris the narrow segments of the taller fertile leaves are in striking contrast to the broader pinnules of the submerged foliage leaves. Leaf-form is in many cases obviously the expression of environment; the xerophilous fern Jamesonia[715] from the treeless paramos of the Andes[716] is characterised by its minute leaflets with strong revolute margins and a thick felt of hairs on the lower surface; in others, xerophilous features take the form of a covering of overlapping scales (Ceterach), or a development of water-tissue as in the fleshy leaves of the Himalayan fern Drymoglossum carnosum. In the Bracken fern Boodle[717] has shown how the fronds may be classed as shade and sun leaves; the former are spreading and softer, while the latter are relatively smaller and of harder texture (fig. 236, a and b). Even in one leaf six feet high, growing through a dense bush of gorse and bramble, the lower part was found to have the features of a shade leaf, while the uppermost exposed pinnae were xerophilous.