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].

Fig. 231.

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.

RECENT FERNS

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].

Fig. 233. Polypodium Billardieri Br. (¼ nat. size.) Middle Island, New Zealand. From specimens in the Cambridge Herbarium.

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.

Fig. 235. Hemitelia capensis R. Brown. Nat. size. a, Pinna of normal frond.
[From a specimen in the British Museum. M.S.]

Fig. 236a. Pteris aquilina.
Part of leaf from greenhouse. (¼ nat. size.) After Boodle.

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.

Fig. 236b. Pteris aquilina.
Leaf from the same plant grown out of doors. (¼ nat. size.)
After Boodle.

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.

Fig. 237.

  1. Matonia pectinata (petiole).
  2. M. pectinata (stem).
  3. Gleichenia dicarpa (stem): p, petiole; pp, protophloem; position of protoxylem indicated by black dots.
  4. Matonidium.
  5. 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).

Fig. 238. Stele of Trichomanes scandens: px, protoxylem; s, endodermis.
From Tansley, after Boodle.

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.

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.