FILICALES.
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].”
[Sidenote: PTERIDOSPERMS]
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.
[Illustration: FIG. 220. Young fronds of (A) _Angiopteris evecta_ and
(B) _Cycas revoluta_. (Reduced.)]
(_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.]
(_a_) _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 ╭ _Simplices_ (Bower). The sporangia are relatively
Schizaeaceae ┤ large and all the sporangia in a sorus have a
Gleicheniaceae │ simultaneous origin: the annulus is oblique.
Matonineae ╰
╭ _Gradatae_ (Bower). Sporangia arise in basipetal
Loxsomaceae │ succession on a more or less elongated receptacle
Hymenophyllaceae │ (portion of the leaf lamina which projects as a
Cyatheaceae ┤ cushion or column on which the sporangia are
Dennstaedtiinae │ borne); annulus oblique; indusium, if present,
│ in the form of a cup or flap of tissue arising
╰ from the base of the sorus.
╭ _Mixtae_ (Bower). This division includes the
│ Polypodiaceae, by far the largest family of ferns.
Polypodiaceae │ The sporangia are characterised by their
Parkeriaceae ┤ relatively small size, the presence of a slender
│ stalk, the absence of regular orientation or
│ sequence in development, and by the presence of a
╰ vertical annulus.
╭ The Dipteridinae include species with the characters
Dipteridinae ┤ of the _Mixtae_, and one species in which the
╰ sporangia develope simultaneously (_Simplices_).
=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.
[Illustration: FIG. 221.
A. _Osmunda cinnamomea_ (after Faull).
B. _Todea barbara_, p, phloem; s, sclerenchyma.
C. _Osmunda regalis_ (after Luerssen).
D. _Todea barbara_ (½ nat. size).]
=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.
[Illustration: FIG. 222. _Schizaea elegans._ (Slightly reduced.) A few
of the segments terminate in narrow fertile lobes.]
[Illustration: FIG. 223. _Aneimia rotundifolia._ (From the Royal
Gardens, Kew. ⅓ nat. size.)]
=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.
[Illustration: Fig. 224.
A. _Aneimia flexuosa._
B. _A. phyllitidis._
C. _Hymenophyllum dilatatum._
D, E, F, G. _Matonia pectinata_; _i_, indusium.
H. _Thyrsopteris elegans._
I. _Gleichenia circinata._
(A, B, after Prantl; C, G, H, I, after Bower.)]
[Illustration: FIG. 225. _Gleichenia dicarpa._ (⅓ nat. size.)]
[Illustration: FIG. 226.
A, B. _Gleichenites Rostafinskii_, Raciborski.
C. _Gleichenia dicarpa._ (Nat. size.)
D, E. _Gleichenia dichotoma._ (Reduced.)
(A, B, after Raciborski; C, after Hooker; D, E, after Goebel.)]
_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].
[Illustration: FIG. 227. _Matonia pectinata._ (⅕ nat. size.) M.S.]
=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.
[Illustration: FIG. 228. _Matonia pectinata._ From a photograph by Mr
Tansley of a group of plants in a wood on Gunong Tundok, Mount
Ophir.]
=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_.
[Illustration: FIG. 229.
A. _Thyrsopteris elegans._ B. _Cyathea spinulosa._
C. _Davallia concinna._ D. _Dicksonia coniifolia._
E. _Alsophila excelsa._ F, G. _Dicksonia culcita._
(A, after Diels and Kunze; B, D, F, G, after Hooker; E, after
Bower.)]
=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.
[Illustration: FIG. 230. _Dicksonia Bertercana_ Hook. Fertile and
sterile pinnae. (Nat. size. British Museum Herbarium.)]
=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].
[Illustration: FIG. 231.
A, A′. _Dipteris quinquefurcata_ (type-specimen in the Kew
Herbarium).
B, C, E, G. _D. conjugata._ (C, ⅛ nat. size.)
D. _Polypodium quercifolium._
F. _Dipteris Wallichii._
(D, after Luerssen.)]
=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.
[Sidenote: RECENT FERNS]
[Illustration: FIG. 232. _Davallia aculeata._ (⅖ nat. size.)]
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].
[Illustration: FIG. 233. _Polypodium Billardieri_ Br. (¼ nat. size.)
Middle Island, New Zealand. From specimens in the Cambridge
Herbarium.]
[Illustration: FIG. 234. _Polypodium quercifolium._ (Much reduced: M,
Mantle-leaves.)]
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.
[Illustration: FIG. 235. _Hemitelia capensis_ R. Brown. Nat. size.
_a_, Pinna of normal frond. [From a specimen in the British Museum.
M.S.]]
[Illustration: FIG. 236_a_. _Pteris aquilina._
Part of leaf from greenhouse. (¼ nat. size.) After Boodle.]
[Sidenote: PTERIS]
The resemblance between some of the filmy Hymenophyllaceae and
thalloid Liverworts[718] is worthy of mention as one of the many
possible pitfalls to be avoided by the palaeobotanical student. The
long linear fronds of such genera as _Vittaria_ and _Monogramme_
might well be identified in a fossil state as the leaves of a
grass-like Monocotyledon, or compared with the foliage of _Isoetes_
or _Pilularia_. The resemblance of some fern leaves with reticulate
venation to those of Dicotyledons has led astray experienced
palaeobotanists; it is not only the anastomosing venation in the leaves
of several ferns that simulates dicotyledonous foliage, but the
compound leaves of many dicotyledons, e.g. _Paullinia thalictrifolia_
(Sapindaceae) and species of Umbelliferae, may easily be mistaken for
fronds of ferns.
[Illustration: FIG. 236 _b_. _Pteris aquilina._
Leaf from the same plant grown out of doors. (¼ nat. size.)
After Boodle.]
[Sidenote: RECENT FERNS]
The dichotomously lobed lamina of some Schizaeas, e.g. _S. dichotoma_
and _S. elegans_ (fig. 222), bears a close resemblance to the leaves of
_Baiera_ or _Ginkgo_[719]. The original description by Kunze[720] of
the South African Cycad _Stangeria paradoxa_ as a Polypodiaceous fern
illustrates the difficulty, or indeed impossibility, of distinguishing
between a sterile simply pinnate fern frond and the foliage of some
Cycads. The deeply divided segments of _Cycas Micholitzii_[721]
simulate the dichotomously branched pinnae of _Lygodium dichotomum_,
and the leaves of _Aneimia rotundifolia_ (fig. 223) and other species
are almost identical in form with the Jurassic species _Otozamites
Beani_, a member of the Cycadophyta.
There are certain facts in regard to the geographical distribution of
ferns to which attention should be directed. Mr Baker in his paper on
fern distribution writes: “With the precision of an hygrometer, an
increase in the fern-vegetation marks the wooded humid regions[722].”
If in a collection of fossil plants we find a preponderance of ferns
we are tempted to assume the existence of such conditions as are
favourable to the luxuriant development of ferns at the present day.
On the other hand, we must bear in mind the wonderful plasticity of
many recent species and the fact that xerophilous ferns are by no means
unknown in present-day floras.
Ferns are admirably adapted to rapid dispersal over comparatively wide
areas. Bower[723] estimates that in one season a Male Fern may produce
about 5,000,000 spores: with this enormous spore-output are coupled a
thoroughly efficient mechanism for scattering the germs and an unusual
facility for wind-dispersal. When Treub[724] visited the devastated
and sterilised wreck of the Island of Krakatau in 1886, three years
after the volcanic outburst, he found that twelve ferns had already
established themselves; the spores had probably been carried by the
wind at least 25 to 30 miles. It is not surprising, therefore, to find
that many ferns have an almost world-wide distribution; and, it may
be added, in view of their efficient means of dispersal, wide range
by no means implies great antiquity. Prof. Campbell[725] has recently
called attention to the significance of the wide distribution of
Hepaticae in its bearing on their antiquity; the spores are incapable
of retaining vitality for more than a short period, and it is argued
that a world-wide distribution can have been acquired only after an
enormous lapse of time. If we apply this reasoning to the Osmundaceae
among ferns, it may be legitimate to assume that their short-lived
green spores render them much less efficient colonisers than the
great majority of ferns; if this is granted, the wide distribution of
Osmundaceous ferns in the Mesozoic era carries their history back to a
still more remote past, a conclusion which receives support from the
records of the rocks.
The Bracken fern which we regard as characteristically British is a
cosmopolitan type; it was found by Treub among the pioneers of the New
Flora of Krakatau; in British Central Africa, it greets one at every
turn “like a messenger from the homeland[726]”; it grows on the Swiss
Alps, on the mountains of Abyssinia, in Tasmania, and on the slopes
of the Himalayas. The two genera _Matonia_ (fig. 228) and _Dipteris_,
which grow side by side on Mount Ophir in the Malay Peninsula, are
examples of restricted geographical range and carry us back to the
Jurassic period when closely allied types flourished abundantly in
northern latitudes. Similarly _Thyrsopteris elegans_, confined to Juan
Fernandez, exhibits a remarkable likeness to Jurassic species from
England and the Arctic regions.
The proportion of ferns to flowering plants in recent floras is a
question of some interest from a palaeobotanical point of view; but we
must bear in mind the fact that the evolution of angiosperms, effected
at a late stage in the history of the earth, seriously disturbed the
balance of power among competitors for earth and air. The abundance
of ferns in a particular region is, however, an unsafe guide to
geographical or climatic conditions. Many ferns are essentially social
plants; the wide stretches of moorland carpeted with _Pteris aquilina_
afford an example of the monopolisation of the soil by a single
species. In Sikkim Sir Joseph Hooker speaks of extensive groves of tree
ferns, and in the wet regions of the Amazon, Bates[727] describes the
whole forest glade as forming a “vast fernery.” In a valley in Tahiti
_Alsophila tahitiensis_ is said to form “a sort of forest almost to
the exclusion of other ferns[728].” In the abundance of _Glossopteris_
(figs. 334, etc.) fronds spread over wide areas of Permo-Carboniferous
rocks in S. Africa, Australia, and India, we have a striking instance
of a similar social habit in an extinct fern or at least fern-like
plant.
_Acrostichum aureum_, with pinnate fronds several feet long, is an
example of a recent fern covering immense tracts, but this species[729]
is more especially interesting as a member of the Filicineae
characteristic of brackish marshes and the banks of tropical rivers
in company with Mangrove plants and the “Stemless Palm” _Nipa_. This
species exhibits the anatomical characters of a water-plant and
affords an interesting parallel with some Palaeozoic ferns (species of
_Psaronius_) which probably grew under similar conditions.
=The Anatomy of Ferns.=
The text-book accounts of fern-anatomy convey a very inadequate idea
of the architectural characters displayed by the vascular systems
of recent genera. When we are concerned with the study of extinct
plants it is essential to be familiar not only with the commoner
recent types, but particularly with exceptional or aberrant types. The
vascular system of many ferns consists of strands of xylem composed
of scalariform tracheae associated with a larger or smaller amount of
parenchyma, surrounded either wholly or in part (that is concentric
or bicollateral) by phloem: beyond this is a pericycle, one layer
or frequently several layers in breadth, limited externally by an
endodermis, which can usually be readily recognised. The vascular
strands are embedded in the ground-tissue of the stem consisting of
thin-walled parenchyma and, in most ferns, a considerable quantity of
hard and lignified mechanical tissue. The narrow protoxylem elements
are usually characterised by a spiral form of thickening, but in
slow-growing stems the first-formed elements are frequently of the
scalariform type.
A study of the anatomy of recent ferns both in the adult state and
in successive stages of development from the embryo has on the whole
revealed “a striking parallelism[730]” between vascular and sporangial
characters in leptosporangiate ferns. For a masterly treatment of our
knowledge of fern anatomy from a phylogenetic point of view reference
should be made to Mr Tansley’s recently published lectures: within the
limits of this volume all that is possible is a brief outline of the
main types of vascular structure illustrated by recent genera.
[Illustration: FIG. 237.
A. _Matonia pectinata_ (petiole).
B. _M. pectinata_ (stem).
C. _Gleichenia dicarpa_ (stem): _p_, petiole; _pp_, protophloem;
position of protoxylem indicated by black dots.
D. _Matonidium._
E. _Trichomanes reniforme_: _pp_, protophloem. (C, E, after Boodle;
D, after Bommer.)]
To Prof. Jeffrey[731] we owe the term protostele which he applied to
a type of stele consisting of a central core of xylem surrounded by
phloem, pericycle, and endodermis. While admitting that steles of this
type may sometimes be the result of the modification of less simple
forms, we may confidently regard the protostele as representing the
most primitive form of vascular system. The genus _Lygodium_ affords
an example of a protostelic fern; a solid column of xylem tracheae
and parenchyma is completely encircled by a cylinder of phloem
succeeded by a multi-layered pericycle and an endodermis of a single
layer of cells. In this genus the stele is characterised by marginal
groups of protoxylem; it is exarch. An almost identical type is
represented by species of _Gleichenia_, but here the stele is mesarch,
the protoxylem being slightly internal (fig. 237, C). _Trichomanes
scandens_ (fig. 238) has an exarch protostele like that of _Lygodium_;
but, as Boodle[732] has suggested, the protostelic form in this case
is probably the result of modification of a collateral form of stele
such as occurs in _Trichomanes reniforme_ (fig. 237, E). A second type
of stele has been described in species of _Lindsaya_[733] in which
the xylem includes a small group of phloem near the dorsal surface.
This _Lindsaya_ type is often passed through in the development of
“seedling” ferns and may be regarded as a stage in a series leading
to another well-marked type, the solenostele. The solenostele[734],
a hollow cylinder of xylem lined within and without by phloem,
pericycle, and endodermis, occurs in several genera belonging to
different families, e.g. _Dipteris_, species of _Pteris_, species of
_Lindsaya_, _Polypodium_, _Jamesonia_, _Loxsoma_, _Gleichenia_ and
other genera. In a smaller number of ferns the stele consists of what
may be called a medullated protostele similar to the common form of
stele in _Lepidodendron_: this type is found in species of _Schizaea_
and in _Platyzoma_ (fig. 239). It is important to notice that in
the solenostele and as a rule in the medullated protostele when a
leaf-trace passes out from the rhizome stele the vascular cylinder is
interrupted by the formation of a foliar gap (_Platyzoma_[735], fig.
239, is an exception). This fact has been emphasized by Jeffrey[736]
who draws a distinction between the Lycopodiaceous type of stele, which
is not broken by the exit of leaf-traces, and the fern stele in which
foliar gaps are produced: the former he speaks of as the cladosiphonic
type (_Lycopsida_) and the latter as the phyllosiphonic (_Pteropsida_).
[Illustration: FIG. 238. Stele of _Trichomanes scandens_: _px_,
protoxylem; _s_, endodermis. From Tansley, after Boodle.]
[Illustration: FIG. 239. _Platyzoma microphylla_. _l.t._, leaf-trace;
_i.e._, internal endodermis. (After Tansley; modified from
Boodle.)]
The transition to a hollow cylinder of xylem from a protostele
may be described as the result of the replacement of some of the
axial conducting tracheae by parenchyma or other non-vascular
tissue consequent on an increase in diameter of the whole stele
and the concentration of the true conducting elements towards the
periphery[737].
The occurrence of the internal cylinder of phloem, pericycle, and
endodermis in a solenostele is rendered intelligible by a study of
fern seedlings and by a comparative examination of transitional types
connecting protosteles and solenosteles through medullated protosteles
and steles of the _Lindsaya_ type. A further stage in stelar evolution
is illustrated by what is termed the dictyostele, the arrangement of
vascular tissue characteristic of _Nephrodium Filix-mas_, _Cyathea_
(fig. 240), _Polypodium vulgare_ and many other common ferns.
[Illustration: FIG. 240. _Cyathea Imrayana._ (From Tansley after de
Bary.) (Sclerenchyma represented by black bands.)]
If a solenostele is interrupted by leaf-gaps at intervals sufficiently
close to cause overlapping, a transverse section at any part of the
stele will show apparently separate curved bands of concentrically
arranged xylem and phloem, which on dissection are seen to represent
parts of a continuous lattice-work or a cylinder with the wall pierced
by large meshes. The manner of evolution of the dictyostele has been
ably dealt with by Gwynne-Vaughan[738] and other authors. In a few
ferns, e.g. _Matonia pectinata_[739], a transverse section of the
stem (fig. 237, B) reveals the presence of two or in some cases three
concentric solenosteles with a solid protostele in the centre: this
_polycylic_ type may be regarded as the expression of the fact that in
response to the need for an adequate water-supply to the large fronds,
ferns have increased the conducting channels by a method other than by
the mere increase of the diameter of a single stele. Fig. 237, A, shows
the vascular tissue of a petiole of _Matonia_ in transverse section.
The two genera of Osmundaceae, _Todea_ and _Osmunda_, are peculiar
among recent ferns in having a vascular cylinder composed of separate
strands of xylem varying considerably in shape and size, from
=U=-shaped strands with the concavity facing the centre of the stem
and with the protoxylem in the hollow of the =U=, to oval or more
or less circular strands with a mesarch protoxylem or without any
protoxylem elements (fig. 221, A, B). These different forms are the
expression of the change in contour or in structure which the parts
of the lattice-work undergo at different levels in the stem[740].
Beyond this ring of xylem bundles is a continuous sheath of phloem of
characteristic structure. A transverse section of a stem of _Osmunda
regalis_ may show 15 or more xylem strands; in _O. Claytoniana_ there
may be as many as 40. In _Todea barbara_ (fig. 221, B) the leaf-gaps
are shorter, and in consequence of the less amount of overlapping the
xylem cylinder becomes an almost continuous tube. The recent researches
of Kidston and Gwynne-Vaughan[741] have resulted in the discovery of
fossil Osmundaceous stems with a complete xylem ring, the stele being
of the medullated protostele type; in another extinct member of the
family the stele consists of a solid xylem core. The Osmundaceous
type of stele is complicated in _O. cinnamomea_ (fig. 221, A) by the
occurrence of local internal phloem and by an internal endodermis,
a feature which leads Jeffrey to what I believe to be an incorrect
conclusion that the vascular arrangement found in _Osmunda regalis_ has
been evolved by reduction from a stele in which the xylem was enclosed
within and without by phloem. New facts recently brought to light
enable us to derive the ordinary Osmundaceous type from the protostele
and solenostele. It is worthy of remark that the Osmundaceae occupy a
somewhat isolated position among recent ferns; their anatomy represents
a special type, their sporangia differ in several respects from those
of other leptosporangiate ferns and in some features _Osmunda_ and
_Todea_ agree with the Eusporangiate ferns. The possession of such
distinguishing characters as these suggests antiquity; and the facts
of palaeobotany, as also the present geographical range of the family,
confirm the correctness of this deduction.
Before leaving the stelar structure of leptosporangiate fern stems,
a word must be added in regard to a type of structure met with in
the Hymenophyllaceae. In this family _Trichomanes reniforme_ (fig.
237, E) may be regarded, as Boodle suggests, as the central type: the
stele consists of a ring of metaxylem tracheae, the dorsal portion
having the form of a flat arch and the ventral half that of a straight
band. This flattened ring of xylem encloses parenchymatous tissue
containing scattered tracheae some of which are protoxylem elements. In
_Trichomanes radicans_ the rhizome is stouter than in _T. reniforme_
and the stele consists of a greater number of tracheae. The stele is
cylindrical like that shown in fig. 238, but the centre is occupied by
two groups of protoxylem and associated parenchyma. In _Hymenophyllum
tunbrigense_ the stele is of the subcollateral type; the ventral plate
of the xylem ring has disappeared leaving a single strand of xylem with
endarch protoxylem and completely surrounded by phloem. _Trichomanes
muscoides_ possesses a still simpler stele consisting of a slender
xylem strand with phloem on one side only. Reference has already been
made to the occurrence in this family of the protostelic type. The
Hymenophyllaceae afford a striking illustration of the modification in
different directions of stelar structure connected with differences in
habit, and of the correlation of demand and supply as shown in the
varying amount of conducting tissue in the steles of different species.
The leaf-trace in a great number of ferns is characterised by its
=C=-shaped form[742] as seen in transverse section: this in some
genera, e.g. _Matonia_ (fig. 237, A), is complicated by the spiral
infolding of the free edges of the =C=; in other ferns (e.g. some
Cyatheaceae) (fig. 278, C) the sides of the =C= are incurved, while in
some species the xylem is broken up into a large number of separate
strands.
An elaborate treatment of the leaf-traces of ferns was published a few
years ago by MM. Bertrand and Cornaille[743] in which the authors show
how the various systems of vascular tissue in the fronds of ferns maybe
derived from a common type. As Prof. Chodat[744] justly remarks this
important work has not received the attention it deserves, the neglect
being attributed to the strange notation which is adopted[745].
The roots of ferns are characterised by a uniformity of plan in marked
contrast to the wide range of structure met with in the stem and to
a less extent in the leaves. The xylem may consist of a plate of
scalariform tracheae with a protoxylem group at each end, or the stele
may include six or more alternating strands of xylem and phloem.
II. =Marattiales= (Eusporangiate isosporous Filicales).
The Marattiaceae, the single family of ferns included in the
Marattiales, comprise the genera _Angiopteris_, _Archangiopteris_,
_Marattia_, _Danaea_, and _Kaulfussia_, which are for the most
part tropical in distribution. These genera are characterised by
eusporangiate sori or synangia, the presence of stipules at the base of
the petioles, and by the complex arrangement of the vascular tissue. In
view of the fact that many fossil ferns show a close resemblance to the
recent Marattiaceae, the surviving genera are briefly described. The
prothallus is green and relatively large.
_Angiopteris._ This genus occurs in Polynesia, tropical Asia, and
Madagascar; it is characterised by a short and thick fleshy stem
bearing large bipinnate leaves which occasionally show a forking of
the rachis[746], a feature reminiscent of some Palaeozoic fern-like
fronds. One of the large plants of _Angiopteris evecta_ in the Royal
Gardens, Kew, bears leaves 12 feet in length with a stalk 6 inches
in diameter at the base. The sessile or shortly stalked and rather
leathery linear or broadly lanceolate pinnules have a prominent
midrib and dichotomously branched lateral veins. The surface of an
old stem is covered with the thick stumps of petioles enclosed by
pairs of fleshy stipules (fig. 241, A) and bears numerous fleshy
roots, which hang free in the air or penetrate the soil. The young
fronds (fig. 220, A) exhibit very clearly the characteristic circinate
vernation. The proximal part of each primary pinna is characterised by
a pulvinus-like swelling. The sporangia, in short linear elliptical
sori near the edge of the pinnules, consist of free sporangia (fig.
242, A–D) provided with a peculiar type of “annulus”[747], in the form
of a narrow band of thicker-walled cells, which extends as a broad
strip on either side of the apex. An examination of sections through
the sporangia of _Angiopteris_ in different planes[748] illustrates
the difficulty of determining the precise nature of the annulus in a
petrified sporangium which is seen only in one or two planes. Many of
the sporangia from the English Coal-Measures, compared by authors with
those of Leptosporangiate ferns, are in all probability referable to
the Marattiaceous type.
[Illustration: FIG. 241.
A. _Angiopteris evecta._ (Considerably reduced.)
B. _Marattia fraxinea._ Stipule. M.S.]
The vascular system[749] of the stem constitutes a highly complex
dictyostelic or polycylic type which may consist of as many as nine
concentric series of strands of xylem surrounded by phloem, with
large sieve-tubes and a pericycle which abuts on the parenchymatous
ground-tissue without any definite endodermal layer. A peculiarity in
the vascular strands is that the first-formed elements of the phloem
lie close to the edge of the xylem, the metaphloem being therefore
centrifugal in its development. The ground-tissue is devoid of
mechanical tissue and is penetrated by roots, a few of which arise
from the outer vascular strands while others force their way to the
surface from the more internal dictyosteles. Leaf-traces, consisting
of several strands, are given off from the outermost cylinder and a
segment of the second dictyostele moves out to fill the gap formed in
the outermost network, while the gap in the second cylinder receives
compensating strands from the third. A few layers below the surface of
the petiole there is a ring of thick-walled elements (_s_, fig. 243),
and in both petiole and stem numerous mucilage ducts and tannin-sacs
occur in the ground-tissue. It has been shown by Farmer and Hill[750]
that in some of the vascular strands in an _Angiopteris_ stem a few
secondary tracheae are added to the primary xylem by the activity of
the adjacent parenchyma. The vascular bundles in the petiole form more
or less regular concentric series; they have no endodermis and are
characterised also by the large size of the sieve-tubes (_st_, fig.
243).
[Illustration: FIG. 242. A–D. _Angiopteris evecta_.
A. Apex of sporangium showing “annulus.”
B. Sori.
C. Sporangium.
D. Section of sporangium, showing the two lateral bands of
thick-walled cells.
E. _Danaea_: _a_, roof of synangium, with pores; _b_, sporangial
cavities; _v_, vascular bundle; _i_, indusium.
(D, after Zeiller.)]
The roots of Marattiaceous ferns (fig. 244) are characterised by the
larger number of xylem and phloem groups; the stele is polyarch and not
diarch, tetrarch or hexarch as in most Leptosporangiate ferns.
[Illustration: FIG. 243. _Angiopteris evecta_. Section of petiole
(considerably reduced) and of a single vascular bundle
(magnified): _px_, protoxylem; _st_, sieve-tubes.]
[Illustration: FIG. 244. _Angiopteris evecta_. Transverse section of
root, with part of the stele magnified: _s_, sieve-tubes; _p_,
phloem; _px_, protoxylem.]
_Archangiopteris_. This monotypic genus, discovered by Mr Henry in
South Eastern Yunnan, was described by Christ and Giesenhagen in
1899[751]. The comparatively slender rhizome has a fairly simple
vascular system[752]. The simply-pinnate leaves bear pinnules like
those of _Danaea_, but the sori agree with those of _Angiopteris_
except in their greater length and in the larger number of sporangia.
_Marattia_. This genus, which extends “all round the world within
the tropics[753],” includes some species which closely resemble
_Angiopteris_, while others are characterised by more finely divided
leaves with smaller ultimate segments. The fleshy stipules occasionally
have an irregularly pinnatifid form (fig. 241, B). The sporangia are
represented by oval synangia[754] (fig. 245, A; the black patches
at the ends of the lateral veins) composed of two valves, which on
ripening come apart and expose two rows of pores formed by the apical
dehiscence of the sporangial compartments (fig. 245, A′, B). In
_Marattia Kaulfussii_ the sori are attached to the lamina by a short
stalk (fig. 245, B, B′) and the leaf bears a close resemblance to
those of the Umbelliferous genera _Anthriscus_ and _Chaerophyllum_.
The vascular system is constructed on the same plan as that of
_Angiopteris_ but is of simpler form.
[Illustration: FIG. 245.
A. _Marattia fraxinea_. A′. A single synangium showing the two
valves and pores of the sporangial compartments.
B, B′. _M. Kaulfussii_.
C. _Kaulfussia_ (synangium showing pores of sporangial
compartments).
D, E. _Marattiopsis Münsteri_.
(C, after Hooker; D, E, after Schimper.)]
_Danaea._ Danaea, represented by about 14 species confined to tropical
America, is characterised by simple or simply pinnate leaves with
linear segments bearing elongated sori extending from the midrib
almost to the margin of the lamina. Each sorus consists of numerous
sporangia in two parallel rows united into an oblong mass partially
overarched by an indusium (fig. 242, E, _i_) which grows up from the
leaf between the sori. In the portion of a fertile segment shown in
fig. 242, E, the apical pores are seen at _a_; and at _b_, where the
roof of the synangium has been removed, the spore-bearing compartments
are exposed. The vascular system[755] agrees in general plan with that
characteristic of the family.
_Kaulfussia._ The form of the leaf (Vol. I. p. 97, fig. 22) closely
resembles that of the Horse Chestnut; the stem is a creeping
dorsiventral rhizome with a vascular system in the form of a “much
perforated solenostele[756].” The synangia are circular, with a median
depression; each sporangial compartment opens by an apical pore on the
sloping sides of the synangial cup (fig. 245, C)[757].
Copeland has recently described a Marattiaceous leaf which he makes
the type of a new genus, _Macroglossum alidae_. The sori are nearer
the margin than in _Angiopteris_ and are said to consist of a greater
number of sporangia. The photograph[758] of a single pinna which
accompanies the brief description hardly affords satisfactory evidence
in support of the creation of a new genus. The structure of a petiole
which I have had an opportunity of examining, through the kindness of
Mr Hewitt of Sarawak, shows no distinctive features.
III. =Ophioglossales.= (Isosporous and Eusporangiate.)
The three genera, _Ophioglossum_, _Botrychium_, and _Helminthostachys_,
are characterised by the division of the leaves into a sterile
and a fertile lobe. The fertile lobe in _Ophioglossum_ bears two
rows of spherical sporangia sunk in its tissue; in _Botrychium_
and _Helminthostachys_ the spores are contained in large sporangia
with a stout wall[759]. The prothallus is subterranean and without
chlorophyll. In the British species of _Ophioglossum_, _O. vulgatum_
(the adder’s tongue fern), an almost cosmopolitan species, the sterile
part of the frond is of oval form and has reticulate venation. In _O.
pendulum_ and _O._ _palmatum_ the lamina is deeply lobed. In the genus
_Botrychium_, represented in Britain by _B. Lunaria_, both sterile
and fertile branches of the frond are pinnately divided, while in
_Helminthostachys_ the sporangia are borne on sporangiophores given off
from the margin of the fertile branch of a frond similar in habit to a
leaf of _Helleborus_.
[Illustration: FIG. 246. _Ophioglossum vulgatum._ Transverse section of
petiole and single bundle: _p_, phloem; _px_, endarch protoxylem.]
[Illustration: FIG. 247. _Botrychium virginianum_: _e_, endodermis;
_c_, cambium; _x_, xylem. A, diagrammatic section of stem; B,
portion of the stele and endodermis enlarged.
(A, after Campbell; B, after Jeffrey.)]
The stem of _Ophioglossum_ is characterised by a dictyostele of
collateral bundles with endarch protoxylem: the vascular system of the
leaf-stalk is also composed of several separate strands (fig. 246).
In _Botrychium_ the stele is a cylinder of xylem surrounded externally
by phloem. This genus affords the only instance among ferns of a
plant in which the addition of secondary tracheae occurs on a scale
large enough to produce a well-defined cylinder of secondary xylem
traversed by radial rows of medullary-ray cells[760] (fig. 247). The
unsatisfactory nature of the evidence in regard to the past history of
the Ophioglossales renders superfluous a fuller treatment of the recent
species.