I. Leptosporangiate Filicales.

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

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.]

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
Schizaeaceae
Gleicheniaceae
Matonineae		 Simplices (Bower). The sporangia are relatively large and all the sporangia in a sorus have a simultaneous origin: the annulus is oblique.
Loxsomaceae
Hymenophyllaceae
Cyatheaceae
Dennstaedtiinae		 Gradatae (Bower). Sporangia arise in basipetal succession on a more or less elongated receptacle (portion of the leaf lamina which projects as a cushion or column on which the sporangia are borne); annulus oblique; indusium, if present, in the form of a cup or flap of tissue arising from the base of the sorus.
Polypodiaceae
Parkeriaceae		 Mixtae (Bower). This division includes the Polypodiaceae, by far the largest family of ferns. The sporangia are characterised by their 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.
Dipteridinae The Dipteridinae include species with the characters 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.

Fig. 221.

  1. Osmunda cinnamomea (after Faull).
  2. Todea barbara, p, phloem; s, sclerenchyma.
  3. Osmunda regalis (after Luerssen).
  4. 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.

Fig. 222. Schizaea elegans. (Slightly reduced.) A few of the segments terminate in narrow fertile lobes.

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.

Fig. 224.

(A, B, after Prantl; C, G, H, I, after Bower.)

Fig. 225. Gleichenia dicarpa. (⅓ nat. size.)

Fig. 226.

(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].

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.

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.

Fig. 229.

(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.

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.