FERNS; THEIR DISTRIBUTION AND ANTIQUITY

'It has been shown that certain forms persist with very little change, from the oldest to the newest fossiliferous formations; and thus show that progressive development is a contingent, and not a necessary, result of the nature of living matter.' Huxley.

The Ferns as a whole represent a section of the vegetable kingdom which traces its ancestry as far into the past as any group of plants. Impressions of leaves on the shales of the Coal-measures and on rocks of the earlier Devonian period are hardly distinguishable in form and in the venation and shape of the leaflets from the finely divided fronds of modern ferns. Until a few years ago these Palaeozoic fossils were generally regarded as true ferns, and it was believed that ferns played a conspicuous part in the vegetation of the earliest periods, of which we have any botanical knowledge. Conclusions based on external form must frequently be revised in the light of more trustworthy evidence. It was shown in the later part of the nineteenth century by the late Professor Williamson of Manchester, whose researches into the plants of the Coal age shed a flood of light on the ancestry and inter-relationship of many existing plants, that some of the fern-like leaves which have long been familiar to those who search among the shales of the refuse heaps of collieries, were borne on stems differing in anatomical features from those of any known fern. The investigation of the structure of the leaves and their supporting stems led to the recognition of certain extinct genera of Palaeozoic plants of exceptional interest, to which the term generalised type is aptly applied. Associated with anatomical and other characters such as we now regard as the attributes of ferns, these plants exhibit other features not met with in modern ferns but characteristic of a group of seed-bearing plants known as the Cycads. Recent research has revealed the existence of several such generalised types which, by their combination of characters now met with in distinct sub-divisions of the plant-kingdom, clearly indicate the derivation of Ferns, and Cycads as we know them to-day, from a common stock. It was in the first instance by means of anatomical evidence—obtained by the microscopical examination of sections of petrified fragments of stems and leaves—that the generalised nature of these Palaeozoic plants was recognised. Nothing was known as to the reproductive organs. Ferns as now represented in the floras of the world are essentially seed-less plants. As the author of Hudibras wrote:

The reproductive organs or spores borne on the fronds of a fern produce, on germination, a thin green structure, known as the prothallus, less than an inch in length: this bears the sexual organs, and as the result of the union of the male and female cells, the embryo fern-plant begins its existence as a parasite on the inconspicuous prothallus, until after unfolding its first green leaf and thrusting a slender root into the ground, it starts its career as an independent organism[2]. In this life-cycle the seed plays no part.

It is noteworthy that the absence of any indication of spore-capsules and spores, in the case of some of the supposed fern leaves from the Coal-measures, caused some suspicion in the mind of an Austrian Palaeobotanist as to the right of such specimens to be classed among the ferns. This opinion, based in the first place on negative evidence and but little regarded by other authors, has in recent years been proved correct. In 1904 a paper was read before the Royal Society by Professor Oliver and Dr Scott(43) in which evidence was brought forward pointing to the conclusion that one of these generalised plants bore true seeds. Subsequently Dr Kidston published an account of some specimens of another of these Palaeozoic plants in which was actually shown an organic connexion between undoubted seeds and pieces of a fern-like frond(44). Without entering into further details, these and similar discoveries may be summarised as follows:—Many of the supposed Fern-fronds of Palaeozoic age, particularly those characteristic of the Coal-measures, are the leaves of plants which in their anatomical characters combined features now shared by true Ferns and by the Cycads. The reproductive organs of these Palaeozoic genera differed widely from those of existing ferns; the male organs, while not unlike the spore-capsules and spores of certain ferns, recall the male organs of living Conifers and Cycads, and the female organs were represented by seeds of a highly complex form. These seed-bearing plants have been called Pteridosperms, a name which expresses the combination of fern-like features with one of the distinguishing attributes of the higher plants, namely the possession of seeds. The ancestors of Pteridosperms are as yet unknown; it is, however, reasonable to assume that there existed in some pre-Carboniferous epoch a group of simple plants from which both Ferns and Pteridosperms were derived. In the forests of the Coal age true Ferns probably occupied a subordinate position in relation to the Pteridosperms.

The question of the relationship between different families of recent ferns and the older known fossil members of the group is beyond the scope of this book. Evidence has been discovered in recent years which warrants the statement that, although none of those Carboniferous ferns were generically identical with existing forms, they very clearly foreshadowed some of those structural features which characterise more than one family of present-day Ferns. The records of the older Mesozoic formations afford abundant evidence of the existence of certain types of Ferns showing a very close resemblance to recent species.

An enquiry into the geographical distribution of living Ferns reveals facts of special interest in connexion with the relative antiquity of different genera and families. The wide distribution of the Bracken fern has already been referred to: it is abundant in Tasmania; its vigour in the island is well illustrated by Mr Geoffrey Smith's statement that constant attention is necessary to keep it from invading newly opened country(45). On Mount Ophir in the Malay Peninsula the cosmopolitan bracken occurs in association with the two genera Matonia and Dipteris, ferns which are among the most striking examples of links with a remote past and have a restricted geographical range. With Osmunda regalis, the Royal Fern, the Bracken is conspicuous in the marsh vegetation of the Bermudas; it flourishes on the Atlas Mountains, in the Canary Islands, in Abyssinia, on Mt Kenia, in British East Africa, in the Himalayas, and is in fact generally distributed in the tropics in both the north and south temperate zones.

The Royal Fern (Fig. 9) is another British species with a wide distribution; it occurs in Northern Asia and in North America; it is common in the Siberian forests and lives in several tropical countries, extending to Southern India and Cape Colony, and in South America it is represented by a closely allied species. Though at the present day Osmunda regalis is one of the rare English Ferns, its occurrence in the submerged forest-beds round our coasts and in pre-Glacial beds points to its former abundance in the British area generally. The Royal Fern is a member of a family now represented by two genera, Osmunda and Todea.

With the exception of Todea barbara, with its large spreading fronds and a short root-covered stem, which occurs in Australia and Cape Colony, all the species of this genus are filmy ferns with semitransparent fronds adapted to a moisture-laden atmosphere. The maximum development of the genus is in New Zealand.

Todea barbara affords an instance of discontinuous distribution; it was no doubt once widely spread in circumpolar regions and now survives only in South Africa and in Australia.

There are satisfactory reasons for regarding the Bracken Fern, with its world-wide range in present-day floras, as a comparatively modern species now in full vigour. Its anatomical and other features are consistent with the view that it is a late product of evolution, and as yet no indication has been given by the records of the rocks of an ancient lineage. The Osmunda family, on the other hand, is undoubtedly an extremely old branch of the fern group. A comparison of the Royal Fern with the Bracken shows that their stems are constructed on very different plans, and we have good reasons for speaking of the structural peculiarities of the former as those of a more primitive type. Moreover, the discontinuous geographical range of some members of the Osmunda family is in itself an indication of antiquity. There is another point which may have a bearing in this question of antiquity, namely the fact that the spores of Osmunda are green and do not possess the powers of indurance inherent in the spores of the majority of ferns which are not green. It has recently been contended by Professor Campbell of Stanford University that the delicate green spores of the Liverworts, plants closely allied to the Mosses, constitute an argument in favour of the antiquity of these plants(46). Certain Liverworts are cosmopolitan in their range, e.g. the genera Riccia and Marchantia.

If certain genera are widely distributed, notwithstanding the fact that their reproductive cells, by which dispersal is effected, are ill-adapted to withstand unfavourable conditions or to endure prolonged desiccation, it would seem reasonable to conclude that their emigration has been accomplished slowly and with difficulty. Ferns such as Osmunda, with green and short-lived spores, would thus be handicapped in competition with other genera provided with more efficient means of dispersal and better equipped for the vicissitudes of travel.

The inferences as to antiquity deduced from a study of the existing species of Osmunda and Todea receive striking confirmation from the testimony of fossils. Some of the oldest known Palaeozoic ferns, though differing too widely from the existing Osmundas and Todeas to be included in the same family, afford distinct glimmerings of Osmundaceous characters, which at a later period became individualised in the direct ancestors of the modern forms. Our knowledge of the past history of the Osmunda family has recently been considerably extended and placed on a firmer basis by the researches of Dr Kidston and Professor Gwynne-Vaughan. These authors have recognised in some exceptionally well-preserved fern-stems from Permian rocks in Russia, anatomical features which point unmistakably to close relationship with the recent members of the family(47)(48).

Passing higher up the geological series, fertile fern fronds with spore-capsules and spores practically identical with those of Osmunda have been found in the Jurassic plant-beds of Yorkshire and in rocks of approximately the same age in many parts of the world. From Jurassic strata in New Zealand a petrified fern-stem has been described (Osmundites Dunlopi), almost identical in structure with the surviving species. Cretaceous and Tertiary examples of similar ferns might be quoted; but enough has been said to establish the claim of the Royal Fern and other members of the Osmunda-family to an ancestry which possibly extends even farther back than that of any other existing family of Ferns.

A brief reference may be made to another fern now represented by several species widely disseminated in tropical and sub-tropical countries. The genus Gleichenia occurs abundantly in the warmer regions of both the Old and New World. The fronds may usually be recognised by their habit of growth (Fig. 10); in several species the main axis is repeatedly forked and a small bud between the divergent branches of the forks forms a characteristic feature. The leaflets are either long and narrow like the teeth of a comb or short and bluntly rounded. Moreover the anatomy of the creeping stem affords a ready means of identification. We have satisfactory evidence of the occurrence of Gleichenia in European floras during both the Jurassic and Cretaceous periods. Numerous fragments of plants were obtained some years ago, not far from Brussels, from the Wealden strata in which the famous skeletons of Iguanodon were discovered. Visitors to the Natural History Museum in Brussels are no doubt familiar with the skeletons of this enormous herbivorous animal: in the same gallery are exhibited the remains of the fossil plants from the Iguanodon beds. Some of these fragments are pieces of fern fronds identical in form with those of existing Gleichenias. The microscopical examination of some exceptionally well preserved fragments of Wealden stems discovered by Prof. Bommer of Brussels enabled him to recognise the Gleichenia type of structure and thus to confirm the inconclusive evidence furnished by fragmentary leaves. The most interesting records in regard to the former occurrence of Gleichenia in Northern Europe we owe to the late Oswald Heer of Zurich, who has described many examples of Gleichenia fronds from rocks of Lower Cretaceous age in Disco Island on the west coast of Greenland in latitude 70° N. The same type of fern is recorded also from upper Jurassic beds in the north-east of Scotland, in the Wealden rocks of Sussex, as well as from other European localities. It is clear that the Gleichenia-family, no longer represented in north temperate floras, was in the Jurassic period, and especially in the early days of the Cretaceous period, widely spread in Europe, extending well within the Arctic circle. It may be that the original home of Gleichenia was in the far North at a time when climatic conditions were very different from those which now prevail. Gleichenia, like many other northern plants, retreated to more southern regions where, in the warmer countries of the world, many species still flourish widely separated in space and time from the place of their birth.

The ferns so far mentioned have a more or less extended distribution at the present day. In the case of Pteridium aquilinum, the cosmopolitan Bracken Fern, wide range would seem to be correlated with comparatively recent origin; on the other hand, the facts of palaeobotany show that the wide distribution of Osmunda, a type of fern which differs in many important respects from members of the family (Polypodiaceae) to which the Bracken belongs, is not inconsistent with an exceptionally ancient family-history. There are, however, certain genera of ferns which afford remarkable examples of restricted geographical distribution associated with great antiquity. The island of Juan Fernandez, 420 miles off the coast of Chili, the home for four years of Alexander Selkirk (to whose adventures we owe Defoe's creation of Robinson Crusoe), is interesting also from a botanical point of view. The vegetation of this oceanic island, 20 square miles in area with basaltic cliffs rising to a height of 3000 ft. above the sea, includes more than 40 species of ferns, eight of which occur nowhere else. One of these endemic ferns is Thyrsopteris elegans, the only representative of the genus; it is readily distinguished by its large and graceful fertile fronds, examples of which may occasionally be seen on a plant of this species in the Royal Gardens at Kew: the sporangia are produced in circular cups which replace the ordinary leaflets on the lower branches of the frond and hang from the short axis like miniature clusters of grapes. It is noteworthy that among the fragmentary remains of the fern vegetation of the Jurassic flora in England and in other parts of Europe specimens occur with fertile segments practically identical with those of the Juan Fernandez species. Students of fossil plants are occasionally led away by the temptation to identify imperfect specimens with rare existing species to which they exhibit a superficial resemblance, and this is well illustrated by the frequent use of the generic name Thyrsopteris for Jurassic and Lower Cretaceous ferns which are too imperfect to be determined with any degree of certainty We have, however, satisfactory grounds for the assertion that the Juan Fernandez fern affords a striking confirmation of the truth of Darwin's dictum that 'Rarity, as geology tells us, is the precursor to extinction.' In this remote oceanic island, for reasons which we cannot explain, there lingers an isolated type which belongs to another age.

The following passage, which forms a fitting introduction to an account of two other genera of ancient ferns, is taken from a description of an ascent of Mount Ophir in the Malay Peninsula by Dr A. R. Wallace in his well-known book on the Malay Archipelago:—'After passing a little tangled jungle and swampy thickets, we emerged into a fine lofty forest.... We ascended steadily up a moderate slope for several miles, having a deep ravine on the left. We then had a level plateau or shoulder to cross, after which the ascent was steeper and the forest denser till we came out upon the Padang-Batu, or stone-field.... We found it to be a steep slope of even rock, extending along the mountain side farther than we could see. Parts of it were quite bare, but where it was cracked and fissured there grew a most luxuriant vegetation, among which the pitcher plants were the most remarkable.... A few coniferae of the genus Dacrydium here first appeared, and in the thickets, just above the rocky surface, we walked through groves of those splendid ferns, Dipteris Horsfieldii and Matonia pectinata, which bear large spreading fronds on slender stems, 6 or 8 feet high'(49).

The two genera Matonia and Dipteris afford exceptionally striking examples of survivals from the past. Matonia is represented by two species, Matonia pectinata (Fig. 11), which grows abundantly on the upper slopes of Padang-Batu in dense thickets on the rock faces where, as Mr Tansley states, its associates are a species of Gleichenia, Dipteris, and a little Pteridium aquilinum (Bracken Fern). Matonia pectinata occurs also on Bornean mountains at an altitude of over 3000 ft. and descends to the coast on some of the Malay islands. The other species of the genus, Matonia sarmentosa, has so far been found in one locality only, Niak, Sarawak, where it was discovered by Mr Charles Hose. Matonia pectinata has a creeping stem covered with a thick felt of brown hairs bearing tall fan-shaped fronds divided into numerous comb-like branches thickly set with narrow linear leaflets on which circular clusters of spore-capsules are sparsely scattered. In some respects Matonia is unlike other ferns; the fronds constitute a striking feature, and the anatomy of the stem is still more distinctive. In the form, development, and arrangement of the sporangia (spore-capsules)—organs which from the constancy of their characters have long been recognised as the most useful basis for classification—Matonia exhibits distinctive features.

In order to emphasise the isolated position of the genus it has recently been placed in a separate family, the Matonineae, of which it is the sole living representative. The restricted geographical range of Matonia, considered in connexion with the clearly marked peculiarities in structure and form, leads us to expect other evidence in support of the natural inference that the genus is a survivor of a once more vigorous and widely spread family. If Matonia were a recently evolved type which has not spread far from its original home, we should expect it to conform more closely than it does to other ferns in the Malay region. Even assuming for the sake of argument that variation may occur per saltum, and new forms may be produced differing in more than the finer shades of small variation from their parents, the peculiar features of Matonia are too pronounced and its individual characteristics too obvious to warrant the assumption of recent production. It is, however, from the testimony of the rocks that we obtain confirmation of the opinion that these Malayan species are plants on the verge of extinction. In shales of Jurassic age exposed on the Yorkshire coast at Gristhorpe Bay and in iron-stained rocks of the same age between Whitby and Scarborough, well preserved leaves have been found agreeing in the shape of the frond, as also in the form of the leaflets and of the groups of sporangia which they bare, with those of Matonia pectinata.

The exposure by a stroke of the hammer, on the fractured surface of a rock picked up on the beach at Hayburn Wyke (a few miles south of Whitby), of a piece of fern frond which is unmistakably closely allied to the species described by Wallace on Mount Ophir, establishes a link between the Jurassic and the present era and presents a fascinating problem in geographical distribution. These fossil Matonias are known to students of ancient plants as species of the genus Matonidium, a name adopted by a German botanist for specimens apparently identical with those from the Yorkshire coast discovered in slightly younger rocks (Wealden) in North Germany. The same type has been found also in sediments of Wealden age on the Sussex coast. Other leaf-impressions agreeing closely with those of Matonidium have been obtained from the Yorkshire Jurassic rocks and these are assigned to another genus Laccopteris, an extinct member of the family Matonineae. It is not merely in the habit of the fronds and in the shape and venation of the leaflets that these fossil ferns resemble the existing species, but the more important features exhibited by the spore-capsules supply additional evidence. It has already been pointed out that the stems of Matonia are characterised by a type of structure unknown in an identical form in any other recent fern.

A few years ago Prof. Bommer discovered fragments of leaves and stems in Wealden beds a few miles from Brussels sufficiently well preserved to reveal the details of internal organisation. Some of these fossils were found to possess structural features identical with those of the Malayan species of Matonia. A full account of the fossil representatives of the Matonia family would be out of place in a general essay on Links with the Past, but brief reference may be made to some of the data which throw light on the geological history of the family. In strata classed by geologists as Rhaetic, a phase of earth-history between the Triassic and Jurassic eras (see p. 42), species of Laccopteris and allied forms have been described from several other countries; from Jurassic and Wealden strata examples of both Laccopteris and Matonia have been found in Germany, Portugal, Belgium, Austria, and elsewhere. From rocks of Cretaceous age, higher in the series than the Wealden strata, well preserved impressions of a Matonidium have been discovered in Moravia. The Matonineae were widely distributed in Europe during the Rhaetic and Jurassic periods, but, so far as we know, the family did not survive in the northern hemisphere beyond the limits of the Cretaceous period. It is noteworthy that, in spite of the preservation of the remains of Jurassic and Cretaceous floras in many extra-European regions, notably in India, South Africa, Australia, China, and Tonkin, no specimens have been found which can with confidence be assigned to the Matonineae. A single fossil has, however, been described from Queensland which may be a piece of a Laccopteris frond.

There is some evidence that ferns very similar to Matonia existed in North America during the Mesozoic period. It would be in the highest degree rash to assume that the Matonineae played no part in the Jurassic vegetation of India, South Africa, and other southern lands, but there can be little doubt that the family was especially characteristic of European floras during a portion of the Mesozoic era. It would seem that subsequent to the Wealden period the ancestors of Matonia dwindled in numbers and their geographical range became much more restricted.

The records of Tertiary rocks have hitherto added nothing to our knowledge of the distribution of the family subsequent to the Cretaceous period. All we can say is that the existing species of Matonia are the last survivors of a family which in the Jurassic period overspread a wide area in Europe and probably extended to the other side of the Atlantic. Exposed to unfavourable climatic conditions and possibly affected by the revolution in the plant world consequent on the appearance of the Flowering Plants, the Matonineae gradually retreated beyond the equator until the two surviving species found a last retreat in the Malayan region.

The fern spoken of by Dr Wallace as Dipteris Horsfieldii (perhaps better known as Dipteris conjugata (Fig. 12)), which grows with Matonia pectinata on Mount Ophir and in the Malay region generally, is one of seven species of a genus characterised by a somewhat wider geographical range than Matonia. Dipteris conjugata extends to the Philippines, Samoa, Fiji, New Caledonia, New Guinea and Central China; its fronds, like those of Matonia, are borne on long slender stalks attached to a creeping stem; they have a broad lamina divided by a deep median sinus into two symmetrical halves and each half is cut up into segments with a saw-like edge. Several stout ribs spread through the lamina from the apex of the long stalk like the open fingers of a hand; from these ribs smaller veins are given off at a wide angle, and these in turn give rise to a reticulum of finer veins forming a skeletal system like that in the leaves of an oak and many other flowering plants.

Numerous groups of spore-capsules are borne on the lower surface of the broad lobed frond. The leaves of other species of Dipteris have the same type of structure, but in some the segmentation of the lamina is carried further and the leaf consists of numerous long and narrow segments with one or two main ribs. Dipteris is represented in the flora of Assam, and it is interesting to find that a species recently discovered in Borneo is more closely connected with the Assam type than with those of the Malay region. Until a few years ago the genus Dipteris was included in the large family Polypodiaceae of which nearly all our British ferns are members, but the discovery of certain distinguishing features in the structure of the sporangia showed that these Eastern and Southern species form a fairly well-defined group worthy of family rank.

In the Rhaetic plant-beds of Northern and Central Europe, of North America, Tonkin, and elsewhere, numerous fossil leaves have been discovered which in shape, venation, and in the manner of occurrence of the sporangia bear a close resemblance to species of Dipteris. Ferns of this type were abundant in the Jurassic floras of the northern hemisphere, and it is interesting to find impressions of Dipteris-like leaves both in the Jurassic rocks of the Yorkshire coast as well as in slightly newer beds of the same geological period on the north-east coast of Sutherland.

It is impossible to say with confidence how nearly these Rhaetic and Jurassic ferns were related to the existing species, as our knowledge of them is less complete than in the case of the fossil representatives of the Matonineae, but there can be no reasonable doubt that in Dipteris as in Matonia we have a connecting link between the present and a remote past.