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:
'Who would believe what strange bugbears
Mankind creates itself, of fears?
That spring like fern, that insect weed,
Equivocally, without seed.'
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.
[2] The life-history of a Fern is clearly described by Prof. Bower in a
recent volume in this series.
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.
[Illustration: Fig. 9. _Osmunda regalis_ Linn. Fertile frond, (2/5 nat.
size.)]
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.
[Illustration: Fig. 10. _Gleichenia dicarpa_ Br. (1/2 nat. size.)]
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).
[Illustration: Fig. 11. _Matonia pectinata._ A group of plants in a
wood on Gunong Tundok, Mount Ophir. (Photograph by Mr A. G. Tansley.)]
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.
[Illustration: Fig. 12. _Dipteris conjugata_ Rein. and, in the middle
of the upper part of the photograph, a frond of _Matonia pectinata_ R.
Brown. Mount Ophir. (Photograph by Mr A. G. Tansley.)]
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.