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.

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