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.
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 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.
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).
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.
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.
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.
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).
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.
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.
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.
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.
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.