PTERIDOPHYTA (Gr. πτέρις, fern, and φυτόν plant), or as they are frequently called, the Vascular Cryptogams, the third of the large subdivisions of the vegetable kingdom. The Ferns form the great majority of existing Pteridophytes; the importance and interest of the other groups, of which the Club-mosses and Horsetails are the most familiar examples, depend largely on the fact that they are the surviving representatives of large families of plants which flourished in earlier geological periods. (See Palaeobotany.)
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| Fig. 1.—Diagrammatic sketches of prothalli of— |
a, Equisetum. b, Lycopodium cernuum. c, L. phlegmaria. d, L. clavatum. e, Selaginella. f, Botrychium virginianum. g, Helminthostachys. h, A Fern. i, Salvinia. |
The relation which exists between the two alternating stages or generations, which together constitute the complete life-cycle of all plants higher than the Thallophyta, is perhaps the most Life history. natural characteristic of the Pteridophyta. From the germinated spore of a fern plant, which must not be confused with the “seed” of seed-bearing plants, a small, flat, green organism is developed; this is the prothallus (gametophyte, sexual generation; fig. 7). As the result of fertilization of an ovum produced by this, the fern plant (sporophyte, asexual generation) originates; from it spores are ultimately set free, with the germination of which the life-history again commences. The point common to all Pteridophyta is that from the first the gametophyte is an independent organism, while the sporophyte, though in the first stages of its development it obtains nutriment from the prothallus, becomes physiologically independent when its root develops. This independence of the two generations for the greater part of their lives distinguishes this group on the one hand from the Bryophyta (in which the sporophyte is throughout its life attached to the gametophyte), and on the other hand from the Gymnosperms and Angiosperms (in which the more or less reduced gametophyte remains enclosed within the tissues of the sporophyte). The gametophyte, which is usually dorsiventral, though in some cases radially symmetrical (fig. 1, b), is a small thallus attached to the soil by rhizoids. In structure it is equally simple, being composed of parenchymatous tissue without any clearly marked conducting system. Usually it grows exposed to the light and contains chlorophyll, but subterranean saprophytic prothalli also occur in the Lycopodiaceae and Ophioglossaccae (fig. 1, c, d, f, g). In the heterosporous forms the gametophyte is more or less reduced (fig. 1, e, i). The reproductive organs ultimately produced on the same or on different individuals are of two kinds, the antheridia and archegonia; the origin of both is from single superficial cells of the prothallus. The antheridium (fig. 8) at maturity consists of a layer of cells forming the wall which encloses a group of small cells; from each of the latter a single motile spermatozoid originates. The archegonium (fig. 9) consists of a more or less projecting neck and the venter, which is usually enclosed by the tissue of the prothallus. A central series of cells can be distinguished in it, the lowest of which is the ovum; above this come the ventral canal cell and one or more canal cells. When the archegonium has opened by the separation of the terminal cells of the neck, the disintegration of the canal cells leaves a tubular passage, at the base of which is the ovum (fig. 9, b). Down this canal the spermatozoid, which in the Ferns has been shown to be attracted by reason of its positive irritability to malic acid, passes and fuses with the ovum. After fertilization the latter surrounds itself with a cell-wall and develops into the sporophyte. The early segmentation of the embryo differs in the several groups, but usually the first leaf or leaves, the apex of the stem and the first root are differentiated early, while a special absorbent organ (the foot) maintains for some time the physiological connexion between the sporophyte and the prothallus. The sporophyte is always highly organized both as regards form and structure. Root, stem and leaf can be distinguished even in the simplest forms, and the plant is traversed by a well-developed vascular system. The reproductive organs of the sporophyte are the sporangia, within which the spores are produced; the sporangia are often borne on or in relation to leaves, which may be more or less distinct from the foliage leaves in form and structure (cf. fig. 2). The cells of the wall of the sporangium are usually so constructed as to determine the dehiscence of the sporangium and the liberation of its spores. The spores produced in each sporangium vary from very many to a single one in the case of some heterosporous forms. These latter bear spores of two kinds, microspores and megaspores, in separate sporangia. From the microspore an extremely reduced male prothallus and from the megaspore the female prothallus, develops (cf. fig. 1, e). The spores of the homosporous Vascular Cryptogams are usually of small size; the prothalli produced from them usually bear both antheridia and archegonia, though under special conditions an imperfect sexual differentiation may result. The complete life-history, with its regular alternation of gametophyte and sporophyte, is now known in all except a few rare genera of recent Pteridophyta, and will be described in connexion with the several groups. A cytological difference of great importance between the two generations can only be mentioned in passing. The nuclei of the cells of the sexual generation possess a definite number of chromosomes and this number is also characteristic of the sexual cells. On fertilization the number is doubled and all the cells of the spore-bearing generation have the double number. On the formation of the spores a reduction to the number characteristic of the gametophyte takes place.
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Fig. 2.—Diagrammatic sketches of spore-producing members of— |
a, Equisetum. b, Lycopodium. c, Psilotum. d, Ophioglossum. e, Kaulfussia. f, Angiopteris. g, Nephrodium. h, Salvinia. |
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(All except d represent vertical sections of sporangiophore or sorus.) |
The systematic arrangement of the Vascular Cryptogarns for the purposes of identification and description necessarily Classification. remains unchanged, while the comparative morphology is being more fully worked out. But modifications in the order of placing the natural groups are of importance in expressing the results of such investigations. Such a scheme may be laced here in a tabular form before entering on the consideration of the life-history, natural history, morphology, and classification of the several groups:—
| Pteridophyta. | |||||
| I. | Equisetales | Equisetaceae. | |||
| Calamariaceae. | |||||
| II. | Sphenophyllales | Sphenophyllaceae. | |||
| Cheirostrobaceae. | |||||
| III. | Psilotales | Psilotaceae. | |||
| IV. | Lycopodiales | Lycopodiaceae. | |||
| Selaginellaceae. | |||||
| Lepidodendraceae. | |||||
| Isoetaceae. | |||||
| V. | Ophioglossales | Ophioglossaceae. | |||
| VI. | Filicales | Filicaceae | Marattiaceae. | ||
| Osmundaceae. | |||||
| Schizaeaceae. | |||||
| Gleicheniaceae. | |||||
| Matoniaceae. | |||||
| Loxsomaceae. | |||||
| Hymenophyllaceae. | |||||
| Cyatheaceae. | |||||
| Polypodiaceae. | |||||
| Hydropterideae | Salviniaceae. | ||||
| Marsiliaceae. | |||||
These main subdivisions are of unequal size and importance. The Sphenophyllales are only known in a fossil state, while the Equisetales, Lycopodiales and Filicales include both living and extinct representatives. The small groups of recent plants forming the Psilotales and Ophioglossales are given independence in this scheme of classification owing to their exact affinities with the other phyla being at present doubtful.
I. Equisetales.—The plants of the single living genus Equisetum, which vary in height from a few inches to 40 ft., have subterranean rhizomes, from which the erect shoots arise. The habit of the plant depends on the degree of branching rather than upon the foliage. The internodes are elongated and hollow. The leaves are borne in whorls, those of each whorl cohering, except at their extreme tips, to form a sheath. The leaves of successive whorls alternate with one another, and this applies also to the branches which arise in the axil of the leaf sheath. In most species many of these buds, which alternate with the leaves, remain dormant, but in others the aerial shoots are copiously and repeatedly branched. In some species branches of the rhizome with tuberous internodes are formed, which serve as a means of vegetative reproduction. The roots which arise from the base of the lateral buds remain undeveloped on the aerial stem. The vascular bundles equal in number the leaf-teeth from which they enter the stem and form a single ring. Each bundle runs downwards through one internode and then divides into two branches which insert themselves on the alternating bundles entering at this node. The young stems, and the older stems of certain species, are clearly monostelic; but in other species an inner and outer endodermis may be present, or an endodermal layer surrounds each bundle. The vascular bundles themselves are collateral, the xylem consisting of the protoxylem, towards the centre of the stem, and two groups of xylem, between which the phloem is situated; the protoxylem elements soon break down, giving rise to the carinal canal. Only the median or carinal strand of xylem is common to stem and leaf; the lateral cauline strands possibly represent the remains of a centripetally developed mass of primary xylem. There is no secondary thickening except at the node in E. maximum, where some short tracheides, arranged in radial rows, arise from a cambium. The stems, the surface of which exhibits a number of ridges with intervening furrows, perform the greater part of the work of assimilation. The chlorophyll-containing tissue reaches the surface at the sides and base of the furrows, where stomata of peculiar form occur in the epidermis, while subepidermal strands of sclerenchyma occupy the ridges. In the cortical tissue beneath each furrow a wide intercellular space is present running the length of the inter node, and called the vallecular canal. The central cylinder of the root, in which there are several xylem and phloem strands, has around it a two-layered endodermis, the inner layer of which appears to take the place of a pericycle. The sporangia are borne upon lateral outgrowths of the axis (the sporangiophores), which arise in whorls and are associated in definite strobili or cones (fig. 3, C); at the base of the cone an outgrowth of the axis like a rudimentary leaf sheath (the annulus) is present, Each sporangiophore (fig. 3 D) consists of a stalk expanding into a peltate disk of hexagonal outline; from the inner surface of the latter six to nine large sporangia hang parallel with the stalk. The single vascular bundle supplies a branch to the base of each sporangium. The latter arises from a number of superficial cells, the cells destined to form the spores being derived from a single one of these. A tapetal layer is derived from the cells surrounding the sporogenous group, and the arrest of a number of the spore-mother-cells further contributes to the nourishment of the remainder, each of which gives rise to four spores. The outermost layer of the cell-wall of the ripe spore splits along spiral lines, giving rise to the elaters; these two long strips of wall, attached by their middle points to the spore, tend to straighten out in dry, and close round the spore in damp air. They thus assist in the opening of the sporangium, which takes place by a slit on its inner face. Further, several spores will be likely to germinate together owing to their elaters becoming entangled; a fact of some importance, since the antheridia and archegonia, though occurring sometimes on the same prothallus, are more often borne on separate individuals. The prothalli contain abundant chlorophyll, and are dorsiventral. Those that bear the antheridia are the smaller, and are either filamentous, or flattened, and irregularly lobed. The antheridia are deeply sunk in the tissue; the spermatozoids consist of a spiral of two or three coils, the numerous cilia being attached to the pointed anterior end. The female prothalli, which are sometimes branched, consist of a thick cushion bearing thin, erect lobes, at the base of which the archegonia are situated. The necks of the latter are short, the central series of cells consisting of ovum, ventral canal cell and one or two canal cells. The half of the embryo directed towards the archegonial neck gives rise to the apex of the stem and a sheath of three leaves, the other half to the small foot and the primary root. The first shoots are of limited growth, being replaced by lateral branches, which gradually acquire the number of leaf-teeth characteristic of the species.
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(C, D, E from Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.) |
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Fig. 3.—Equisetum maximum. |
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A, Longitudinal section of the rhizome, including a node and portions of the adjoining internodes; k, septum between the two internodal cavities, hh; gg, vascular bundles; l, vallecular canal; s, leaf-sheath. B, Transverse section of the rhizome; g, vascular bundle; l, vallecular canal. C, Fertile shoot showing two leaf-sheaths and the terminal strobilus. D, E, Sporophylls bearing sporangia, which in E have opened. |
Fossil species, some of which attained a great size, are known, to which the name Equisetites is given, since they appear to be closely allied to the existing forms. Two other extinct genera, Phyllotheca and Schizoneura, may be mentioned here. Abnormal specimens of Equisetum in which the strobilus is interrupted by whorls of leaves are of interest for comparison with the fructiiication of Phyllotheca. The most important and best known of the extinct Equisetales are, however, the Calamites (see Palaeobotany: Palaeozoic). In the primary structure of the stem the Calamites present many points of resemblance to Equisetum, but secondary thickening went on in both stem and root. These plants, which appear to have grown in swampy soil, thus attained the dimensions of considerable trees. The leaves, which were of simple form (except in Archaeocalamites, where they forked), were inserted in whorls at the nodes; they were either free from one another or cohered by their bases into a sheath. The branches alternated in position with the leaves, and sprang from just above the insertion of the latter. Some of the branches terminated in cones, which present a general similarity to those of Equisetum. This similarity is closest in Archaeocalamites, an ancient type found in Upper Devonian rocks; in this the strobilus consists of peltate sporangiophores inserted in whorls on the axis. In the other Calamarian strobili known the whorls of sporangiophores are separated by whorls of bracts. In some the sporangiophores stood midway between the sterile whorls, while in others they approached the whorl above or below. There is a close resemblance between these sporangiophores and those of Equisetum, but as a rule only four sporangia were borne on each. Some Calamites were heterosporous, sporangia with microspores and megaspores being found in the same cone.
Our knowledge of the extinct Equisetales, full as it is with respect to certain types, does not suffice for a strictly phylogenetic classification of the group. The usual subdivision is into Equisetaceae including Equisetum and Equisetites (with which Phyllotheca and Schizoneura may be provisionally associated), and Calamariaceae, including Calamites and Archaeocalamites.
II. Sphenophyllales.—The two very distinct genera Sphenophyllum and Cheirostrobus, included in this group, are known only from the Palaeozoic rocks. Though the high specialization of this ancient group of plants renders the determination of their natural affinities difficult, indications are afforded by anatomy and the morphology of the strobilus.
In general appearance the species of Sphenophyllum (the remains of Cheirostrobus known do not allow of any idea of its habit being formed) present some resemblances to the Equisetales. The long, sparingly branched stem bore at the somewhat swollen nodes whorls of six to eighteen wedge-shaped or linear leaves, which did not alternate in successive whorls. Both the broader and narrower leaves may be more or less deeply divided, and both forms may occur on the same shoot. From the relation of the thickness of the stem to its length it may be inferred that the shoots of Sphenophyllum derived support from adjoining plants. Without entering into detail regarding the anatomy, it may be stated that secondary thickening took place in both genera. The single stele in the stem consisted of the phloem surrounding a solid central strand of xylem, the groups of protoxylem being situated at the projecting angles. In Sphenophyllum, in which the transverse section of the xylem is triangular, there were three or six protoxylem groups; in Cheirostrobus they were more numerous. The anatomy of the stem is thus very unlike that characteristic of the Equisetales, and presents essential points of resemblance to the Lycopodiales and especially to the Psilotales. The general morphology of the cones, on the other hand, suggests some affinity with the Equisetales. The cone of Sphenophyllum consisted of an axis bearing at the nodes whorls of bracts, united below into a sheath. The overlapping bracts afforded protection to the sporangia, which were borne on sporangiophores springing from the upper surface of the coherent bracts near their origin from the axis; two sporangiophores usually arose from each bract, and sometimes adhered to its upper surface for some distance. Each bent round at the upper end, and bore one or two sporangia on the side turned towards the axis. The mature sporangium had a wall of a single layer of cells, which were larger towards the base, where they continued into the epidermis of the sporangiophore. In Sphenophyllum fertile both the ventral lobes of the sporophyll (corresponding to the sporangiophores in other species) and the dorsal lobes, which in other species are sterile, were developed as peltate sporangiophores. In other species of Sphenophyllum, which are known only as impressions, single sporangia, or groups of four, appear to have been inserted directly on the upper surface of the bracts. In Cheirostrobus a similar relation of sporangiophores to bracts existed, but here each bract was divided into three segments. From each segment, near its base, a stalked peltate sporangiophore arose; this bore four sporangia, which hung parallel to the stalk. That these three sterile segments, with their sporangiophores, are together comparable to one of the bracts of Sphenophyllum, with its sporangiophores, is shown by the vascular supply in each case being derived from a single leaf-trace. So far as is at present known, the Sphenophyllales were homosporous. The differences between the two genera described above are sufficiently marked to justify the division of the Sphenophyllales into the two orders Sphenophyllaceae and Cheirostrobaceae. A consideration of the characters of both shows that the Psilotales are the nearest living representatives of the Sphenophyllales, while resemblances suggesting actual relationship exist between this group and the Equisetales and Lycopodiales. It has been suggested that the Sphenophyllales may have sprung from a very old stock which existed prior to the divergence of the latter groups. So long, however, as our knowledge of these phyla is confined, as at present, to specialized forms, the nature of the relationship between them must remain to some extent hypothetical.
III. Psilotales.—The two genera Psilotum and Tmesipteris, which are provisionally isolated in this group, have usually been classed with the Lycopodiales. Recent work both on their anatomy and on the morphology and structure of their spore-producing organs has however tended to show that their peculiarities can be best understood in the light of our knowledge of the Sphenophyllales. Some authorities place them in this group and there is much to be said in support of the close relationship implied. The Psilotaceae, however, differ from the Sphenophyllales in a number of definite features, such as the arrangement of the leaves singly and not in whorls, and the mode of branching. These differences and our comparatively imperfect knowledge of the Sphenophyllaceous plants which most closely resemble the Psilotaceae appear to justify the provisional isolation of the latter as a distinct group, showing affinities with both the Sphenophyllales and Lycopodiales. In both Psilotum and Tmesipteris the functions of the root-system, which is completely absent, are performed by leafless rhizomes bearing absorbent hairs and inhabited by an endophytic fungus. Psilotum lives epiphytically or in soil rich in humus, while Tmesipteris is epiphytic (and, it has been suggested, partially parasitic) upon stems of tree ferns: the former has small scale-like leaves; those of the latter are of considerable size. The stem is monostelic, the protoxylem groups being towards the periphery of the xylem, the development of which is thus centripetal; the centre of the stele is occupied by sclerenchymatous tissue. The leaves, which bear the sporangia, are dichotomous, and do not form definite cones, but alternate in irregular zones with the foliage leaves. The sporophylls may exceptionally undergo further dichotomies and bear more numerous synangia. The sporangia of the Psilotaceae are associated in synangia, which occupy the same position relatively to the sporophyll, as the single sporangium of Lycopodium or the group of sporangia in Spenophyllum majus. The careful study of the development of the synangium of Tmesipteris, which consists of two loculi, and of Psilotum, which consists of three, has shown that their structure can be explained as originating by the septation of a single sporangium resembling that of Lycopodium. Other views of the nature of the Psilotaceous synangium are, however, possible, and indeed the existence of both simple and complicated sporangiophores in the Sphenophyllaceae leaves the question open as to whether the synangium in existing Psilotaceae is a relatively simple type of sporangiophore which has persisted unaltered or is the result of reduction from a more elaborate structure. There is some reason to believe that the prothallus of Psilotum resmbles some Lycopodium prothalli, but conclusive evidence is wanting; that of Tmesipteris is unknown.
IV. Lycopodiales.—The living representatives of this group are of small size compared with the related plants which lived in Palaeozoic times. A large proportion of the living species are tropical, though others have a wide distribution. As general characteristics of the Lycopodiales, the simple form of the leaves, which are generally of small size, and the situation of the sporangia on the upper surface of the sporophylls, which are often associated in cones, close to their insertion on the axis, may be mentioned; there are both homosporous and heterosporous forms, the prothalli exhibiting corresponding differences. A number of species of Lycopodium are epiphytic and those of Isoëtes live submerged in water. Vegetative reproduction is effected in various ways: by the separation of the branches of a creeping stem in some Lycopodia, the persistence through the winter of the apex of the shoot in L. inundatum, and by the formation of leafy bulbils on the aerial stem of L. Selago and others. A highly specialized means of vegetative reproduction is seen in the tubers of Phylloglossum and the embryos of some Lycopods. The modifications shown by the gametophyte of Lycopodium will be described below. All such special relations of the plant to its environment, which might be expected in the few forms of a large group which has persisted beyond the others, are less marked in the genus Selaginella. It would appear as if the latter was more suited to the conditions of the existing flora, and many of the specific forms within it may rather be regarded as recently evolved than as simply persistent.
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| (From Strasburger's Lehrbuch der Botanik.) |
| Fig. 4.—Lycopodium clavatum. |
| A, Old prothallus. |
| B, Prothallus bearing young sporophyte. |
| G, Portion of a mature plant showing the creeping habit, the adventitious roots and the specialized erect branches bearing the strobili or cones. |
| H, Sporophyll bearing the single sporangium on its upper surface. |
| J, Spore, highly magnified. |
Lycopodiaceae.—This order contains the two genera Phylloglossum and Lycopodium; the former has a single species, confined to Australia, Tasmania and New Zealand, while nearly one hundred species of Lycopodium are known. Erect and creeping terrestrial plants and pendulous epiphytes occur in the latter genus. The simple leaves, which are of small size and do not possess a ligule, are arranged spirally around the branched stem in the majority of the species. The roots of the erect forms often grow downwards in the cortex of the stem to reach the soil. The anatomy of Lycopodium presents considerable variety in detail, but the stem is always monostelic and the development of the xylem centripetal, the protoxylems being situated at the periphery of the stele; pericycle and endodermis surround the stele, and the wide cortex may be more or less sclerenchymatous. The central cylinder of the root often shows a striking resemblance to that of the stem. The Lycopodiaceae are homosporous. The spores are formed in sporangia of considerable size, situated on the upper surface and near the base of the sporophylls. The latter may differ from the foliage leaves and be arranged in definite cones, or the two may be similar and occupy alternate zones of a shoot with continued growth; sometimes rudiments of sporangia are found at the bases of the leaves (fig. 4). In the development of the sporangium the sporogenous tissue is derived from a number of superficial cells by divisions parallel to the surface. The tapetum is derived from the layer of cells surrounding the sporogenous group. Short trabeculae of sterile tissue have been found to project into the cavity of the sporangium of some species. The spores, when liberated by the dehiscence of the sporangium, give rise to the prothallus, which is now, owing mainly to the investigations of Treub and Bruchmann, known in a number of tropical and temperate species. In habit and mode of life of the prothallus these present striking differences, which may be correlated with the situations inhabited by the sporophyte, and are perhaps to be regarded as adaptations which have enabled the species to survive. Thus in L. cernuum and others the prothallus is green and grows on the surface of the soil (fig. 1, b); in the species living on the moors it is subterranean and saprophytic, though sometimes capable of developing chlorophyll when exposed to light (fig. 1, d); while in L. Phlegmaria and other epiphytic forms the prothallus consists of fine branches growing saprophytically in rotting wood (fig. 1, c). A comparison of these various types would appear to indicate that the primitive form of prothallus in the genus was radially symmetrical and contained chlorophyll. The prothalli of L. cernuum come nearest to this; in them the meristem forms a zone slightly below the summit, which may bear a number of green lobes. The different forms of the prothallus found in L. Selago give an idea of how the more extremely modified types could be derived from such a prothallus as that of L. cernuum. All the saprophytic prothalli contain an endophytic fungus in definite layers of their tissue. The antheridia and archegonia are produced above the meristematic zone, and are more or less sunk in the tissues of the prothallus. The most important difference in the sexual organs concerns the length of the archegonial neck; this is shortest and has only a single canal cell in L. cernuum, while in L. complanatum it is longer than in any other Vascular Cryptogam, and contains a number of canal cells. The spermatozoids are biciliate. The embryo in L. cernuum and other forms with superficial green prothalli is attached to the prothallus by a small foot, and develops at first as a tuberous body (the protocorm) bearing rhizoids; this forms a number of simple leaves, and upon it the apex of the shoot arises later. In the saprophytic forms the protocorm is absent, and in some of them the foot is of large size (fig. 4, B). When new individuals of species which possess a protocorm arise vegetatively from the leaves or roots of young plants, the protocorm appears in the young sporophyte. This fact leads to the consideration of Phylloglossum, which resembles the embryo of Lycopodium cernuum in so many respects that it has been spoken of as a permanently embryonic form of Lycopod: it is in some respects the simplest existing Pteridophyte. Its prothallus resembles that of L. cernuum, but wants the crown of assimilating lobes. The plant is reproduced by tubers, which resemble the protocorm in bearing first a number of protophylls and later the upright shoot with its single terminal strobilus. The sporangia agree with those of Lycopodium in structure and position.
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| (From Strasburger's Lehrbuch der Botanik.) |
| Fig. 5.—Selaginella. |
| A, S. helvetica (nat. size). |
| B, S. denticulata, young plant attached to the megaspore (enlarged). |
Selaginellaceae.—The single genus of
this order (Selaginella) contains between
three and four hundred species. There
is considerable diversity among them as
regards external form, the majority having
dorsiventral aerial shoots with dimorphic
leaves (fig. 5, A), while in others the shoots
are radially symmetrical and the leaves
alike. The stem contains one, two or several
steles; in one species the stele is tubular.
The phloëm completely surrounds the
xylem, which usually develops from two
protroxylem groups. In the aerial stem
of the British species (S. spinosa) the
radial stele has a number of protoxylem
groups arranged round the periphery,
much as in Lepidodendron. The cells of
the endodermis are developed as trabeculae,
which traverse the continuous
air-space surrounding each stele. The
simple, uni-nerved leaves have a ligule near the base; the base
of the ligule is somewhat sharply marked off from the other
tissues of the leaf. In some species a depression of the leaf-surface
encloses the ligule, regarding the function of which little is known.
The roots, the stele of which is monarch, may arise directly from
the stem, or are borne on rhizophores, which spring from the
shoot at the point of branching, and root on reaching the soil.
In structure they resemble the roots, but their morphological nature
is uncertain. The sporophylls are arranged radially in the cones,
which are terminal on the branches. A single sporangium is borne
on the axis just above the insertion of each sporophyll. Selaginella
is heterosporous, the megasporangia being often found towards the
base of the cone. The development of the micro- and megasporangia
is the same up to the stage of isolation of the
spore mother-cells. The sporogenous tissue, which is referable
to several archesporial cells, is surrounded by a tapetum, mostly
derived from the sporogenous group. In the microsporangium
all the mother-cells undergo the tetrad division, giving rise to the
numerous microspores. In the megasporangium, on the other hand,
the four megaspores, which arise from a single mother-cell, are
nourished at the expense of the other sporogenous cells and of the
tapetum. On germination the micro spores give rise to a reduced
prothallus, consisting of the small cell first cut off and a wall of cells
enclosing two to four central ones; from these latter the biciliate
spermatozoids originate. The megaspore becomes filled with the
female prothallus, the formation of cell-walls commencing at the
pointed end of the spore, where from the first the nuclei are more
numerous, and later extending to the base. The surface of the
prothallus, which is exposed when the thick wall of the spore is
ruptured, may produce a few rhizoids; upon it the archegonia, consisting
of a short neck and the central series of ovum, ventral canal
cell and canal cell, arise (fig. 1, e). After fertilization the embryo
forms a short suspensor; the apex of the stem, with a leaf on each side
of it, is first distinguishable; at the base of this is the foot; while
the root arises on the farther side of the latter. Thus the position
of the root in Selaginella is different from what obtains in the other
Vascular Cryptogams. A point of interest in this heterosporous
genus is that the formation of the prothallus may commence before
the megaspore is liberated from the sporangium.
Lepidodendraceae.—This order includes only extinct forms, the best known of which are the plants placed in the genera Lepidodendron and Sigillaria. These plants, a fuller description of which must be sought in the article Palaeobotany: Palaeozoic, underwent secondary increase in thickness and attained the size of large trees; the aerial stem was more or less branched dichotomously. The leaves, which were of simple form and provided with a ligule, were, as the leaf-scars on the stem show, variously arranged. In Sigillaria the latter form vertical rows, while in Lepidodendron the arrangement is a complicated spiral. The stem had a single stele, the primary xylem of which was polyarch and centripetally developed. The upright stems were attached to the soil by a number of dichotomously branched members (Stigmaria), which, whatever their morphological nature may be, appear to have performed the function of roots: they bore numerous cylindrical appendages, which penetrated the soil on all sides. The cones, which in some instances at least were heterosporous, presented a general resemblance to those of Lycopodium and Selaginella, a single sporangium being situated on the upper surface of each sporophyll. The cavities of the large sporangia were sometimes traversed by trabeculae of sterile tissue resembling those found in Isoëtes. In some of the heterosporous forms (Lepidocarpon, Miadesmia) the sporangia were sometimes surrounded by an integument; and since only a single megaspore attained maturity, the structure of the megasporangium suggests a comparison with an ovule.
Isoëtaceae.—The single genus (Isoëtes) contains about fifty, mostly aquatic, species, though a few are amphibious or terrestrial. The plants present considerable uniformity in general habit, consisting of a short, unbranched stem, bearing the closely-crowded awl-shaped leaves, which in the larger species attain the length of a foot. Each leaf bears a ligule resembling that of Selaginella in structure and position. The stem is monostelic, the centre of the stele being occupied by a mass of short tracheides; but little can be said as to the primary structure of the central cylinder, which appears to be reduced. A meristematic zone forms a short distance outside the xylem, from which secondary tissue is developed both internally and externally; that to the inside contains both xylem and phloëm elements. By the unequal development of the secondary cortex the stem becomes two- or three-lobed; the roots, which branch dichotomously, spring from the furrows between the lobes. The leaves have a single main bundle, and in the mesophyll are four longitudinal series of large intercellular spaces separated by transverse diaphragms. The sporangia, which are situated singly on the adaxial surface of the leaves, between their insertion on the stem and the ligule, arise from a considerable number of epidermal cells. The cells composing the young sporangium are at first similar, but ultimately become differentiated into sterile trabeculae, which may stretch from the inner to the outer wall, and the mother-cells of the spores. The latter are more numerous in the microsporangium than in the megasporangium. The tapetal layer is partly formed from the sporangial wall and partly as a layer covering the trabeculae. The spores, which are set free by the rotting of the sporangial wall, germinate much as in the case of Selaginella, though the similarity may be a case of independent resemblance. Important points of difference are found in the multiciliate spermatozoids, and in the embryo, which has no suspensor.
The several orders of Lycopodiales described above, while presenting a number of features in common, are distinctly isolated from one another. A natural classification of such specialized plants can only be obtained when the extinct forms are more fully known. What is known at present, while it does not indicate the phylogeny of the Lycopodiales, at least shows that such living orders as Lycopodiaceae and Selaginellaceae cannot be regarded as forming a linear series. The difficulty is increased when it is borne in mind that the small surviving forms probably have a long geological history, and may have coexisted with the Lepidodendraceae. For these reasons no attempt has been made to arrange the orders in larger divisions, since such a division as that of the ligulate and eligulate forms, while convenient for practical purposes, may not express the phylogeny of the group. The Psilotaceae, formerly included in the Lycopodiales, have been described separately owing to-their resemblance to the Sphenophyllales. It remains to be mentioned that the Isoëtaceae have been regarded as more nearly allied to the Filicales than to the former, near which they are here placed.
V. Ophioglossales.—The peculiarities of this small order of Pteridophyta render their systematic position a matter of doubt, especially in the absence of evidence as to their geological history, and justify their separation for the present from the other main natural groups. In the three genera, Ophioglossum, Botrychium and Helminthostachys, there is an underground rhizome, from which one leaf or a few leaves with sheathing bases are produced annually; the roots arise in more or less definite relation to the insertion of the leaves. The latter are simple, or irregularly lobed in Ophioglossum, more or less compoundly pinnate in Botrychium and palmately pinnate in Helminthostachys. The fertile branch or branches are situated on the adaxial surface of the leaves, and may be simple, as in Ophioglossum (fig. 2, d), or more or less compound, the degree of branching in the sterile and fertile segments exhibiting a general parallelism. The stem is monostelic, the arrangement of the xylem and phloem being collateral. The endodermis and pericycle surround the whole stele in Botrychium and Helminthostachys; in Ophioglossum each bundle has a separate sheath. Well-marked secondary thickening occurs in Botrychium. In the roots of Ophioglossum and Botrychium and in the first formed roots of Helminthostachys an endophytic fungus is present, forming a mycorhiza—the stele in the larger roots has the usual radial arrangement of xylem and phloem; monarch roots occur in Ophioglossum. The morphology of the fertile spike is a disputed question, upon the answer to which the systematic position of the Ophioglossaceae largely rests. The spike is most simple in Ophioglossum, where it bears on each side a row of large sporangia, which hardly project from the surface, the vascular bundles occupying a central position. In the young spike, which arises when the leaf is still very small, a band of tissue derived from superficial cells is distinguishable along either side; this sporangiogenic band gives rise to the sporogenous groups, the sterile septa between them, and the outer walls of the sporangia. The spike of Helminthostachys corresponds to that of Ophioglossum, but in it the sporangia are borne on two lateral rows of branched sporangiophores. The sporangia themselves resemble those of Botrychium, which project from the ultimate subdivisions of the branched spike; each is developed from a number of cells, the sporogenous tissue arising from a single cell. Two diverse views of the morphology of the fertile spike in these plants have been entertained. The older view was that it was a fertile segment of the leaf; and though its ventral position presents a difficulty, this must be regarded as a possible explanation; the occasional occurrence of sporangia on the lamina in Botrychium has been regarded as supporting it. On the other hand, the spike has been explained as due to the elaboration of a single sporangium occupying a similar position with regard to the leaf as in the Lycopodiales, and evidence of considerable weight has been brought forward in support of this interpretation. The important bearing of this question on the relationship of the Ophioglossaceae to the phyla of the Filicales and Lycopodiales will be obvious.
The position of the fertile spike in relation to the leaf corresponds to that of the synangium or sporangiophores in the Psilotales and Sphenophyllales. The Ophioglossaceae are homosporous, and the prothalli, which are known in species of all three genera, are subterranean and saprophytic (fig. 1, f, g). The prothallus of O. pedunculosum, as observed by Mettenius, subsequently reached the surface and produced green lobes; those of the other species known are wholly saprophytic, and contain an endophytic fungus. Those of Ophioglossum are cylindrical, while the dorsiventral prothallus of Botrychium bears the sexual organs on the upper surface. They present a general, but probably homoplastic, resemblance to the saprophytic prothalli of certain Lycopodia. Important points of difference exist, however, in the apical position of the meristem of the Ophioglossaceous prothalli, in the presence of a basal cell to the archegonium, and in the multiciliate spermatozoids. In these respects, in the megaphyllous habit and in certain anatomical features, the Ophioglossaceae approach the Filicales. Some species of Botrychium have recently been found to have embryos provided with a suspensor. The position of the Ophioglossaceae can at present only be regarded as an open question, in considering which the possible antiquity of the group must be borne in mind.
|
| (From Strasburger's Lehrbuch der Botanik.) |
| Fig. 6.—Scolopendrium vulgare. |
VI. Filicales.—This group of Pteridophyta differs from the others in being well represented in our present flora by forms, many of which can be regarded not as archaic types which have persisted to the present day, but as having been evolved in comparatively recent periods. The Ferns exhibit a wide range in size from the minute epiphytic Hymenophyllaceae, with leaves barely a centimetre in length, to gigantic tree-ferns 80 ft. or more in height. A general characteristic of their habit is the large size of the leaves, which are often highly compound, relatively to the stern. Some ferns have a longer or shorter erect stem often clothed by the persistent bases of the leaves; in others the stem creeps on the surface of the substratum or is subterranean. Its surface is clothed with filamentous or scaly hairs (paleae), which protect the growing point; and adventitious roots spring from it. The position of the branches varies in the group; they are only exceptionally axillary (Hymenophyllaceae, Botryopterideae). The anatomy of the stele in the stem exhibits on the whole a progression from a solid protostele through a tubular solenostele to one or more circles of separate steles derived by the breaking up of the solenostele. The leaf-traces usually interrupt the continuity of the stele of the axis on their departure. The sporangia are borne in groups (sori) on the under surface of the leaves; sometimes the fertile leaves differ more or less from the purely vegetative ones. The form of the sorus and the structure of the sporangium are of great systematic importance. The sorus is frequently protected by an outgrowth from the surface or margin of the leaf called the indusium. Heterospory is only known in the Hydropterideae. The prothallus developed from the spore is green and in most cases dorsiventral, bearing the archegonia and antheridia on the under surface.
Some of the more striking adaptive modifications in the gametophyte and sporophyte, and certain effects of altered external conditions which have been ascertained experimentally, may be briefly mentioned. The dorsiventrality of the prothallus has been shown to depend mainly on the illumination, the filamentous form being retained in feeble light; a similar result is obtained when the prothalli are cultivated in water. These facts may have a bearing on the filamentous prothalli of some Hymenophyllaceae. The reproduction of the prothallus by gemmae in species of Trichomanes, Vittaria and Monogramme is another interesting adaptation; the prothallus of Gymnogramme leptophylla is perennial, the sporophyte being annually borne on it. The phenomena of apogamy and apospory which have now been observed in a number of Ferns, may be mentioned here. In the former the prothallus produces one or more fern-plants vegetatively, the projection which develops into the sporophyte in many cases occupying the position of an archegonium. In some apogamous Ferns sporangia may occur on the prothallus and the vegetative organs of the sporophyte may also occur singly. In apospory the converse phenomenon is seen, the gametophyte springing vegetatively from the sporangium, receptacle of the sorus, or leaf-margin of the fern-plant. In a number of cases, though not in all, apospory appears to be correlated with a failure of the sporangia to develop.
|
| (From Strasburger's Lehrbuch der Botanik.) |
| Fig. 7.—Nephrodium filix-mas. |
| A, Prothallus viewed from the lower surface; ar, archegonia; an, antheridia; rh, rhizoids (much enlarged). |
| B, Prothallus bearing a young fern plant; b, first leaf; w, primary root. |
|
| (From Strasburger's Lehrbuch der Botanik.) |
| Fig. 8.—Polypodium vulgare. |
| A, Mature antheridium. |
| B, Empty antheridium; p, prothallial cell; 1, 2, cells of antheridial walls; 3, cap cell. |
| C, D, Spermatozoids. |
The adaptations in the vegetative organs of the sporophyte are similar to those in the Flowering Plants. Thus there are a few Ferns which climb, others are water plants, while many, especially those which live as epiphytes, are more or less xerophytic. Some of the epiphytic forms (Polypodium quercifolium, Platycerium) have strongly dimorphic leaves, the sterile leaves serving in some cases to catch falling débris, and thus to provide the plant with soil. Lastly, the symbiotic relation between the plant and ants is found in Ferns, the rhizome of Polypodium carnosum containing cavities inhabited by these insects. The existence of these myrmecophilous Ferns suggests a possible explanation of the nectaries on the leaves of some other species, such as the Common Bracken.
The main existing groups of the Filicaceae may now be briefly described, with special reference to the characters of gametophyte and sporophyte, which have been found of value in determining affinities.
Marattiaceae.—These are ferns of considerable size, the large leaves of which are borne on a short, erect, swollen stem (Angiopteris, Marattia), or arise from a more or less horizontal rhizome (Danaea, Kaulfussia). The leaves, at the base of which are two large stipule-like outgrowths, have a thick leaf-stalk, and are simple or simply pinnate in Danaea, pinnate in Archangiopteris, bi- to tri-pinnate in Marattia and Angiopteris, and digitately lobed in Kaulfussia. The stem, from the ground tissue of which sclerenchyma is absent, has a complicated system of steles arranged in concentric circles; the thick roots, the central cylinders of which have several alternating groups of xylem and phloem, arise in relation to these. The pinnae, except in a few filmy forms, are thick; in Kaulfussia large pores derived from stomata occur in the epidermis. The sori are borne on the under surface of the pinnae, usually in a single row on either side of the midrib, but in Kaulfussia dotted over the expanded lamina. The large sporangia, each of which originates from a number of superficial cells, are here incompletely separated from one another and arranged in a single circle forming a synangium. The association is closest in Danaea, where the individual sporangia of the elongated sorus, which is sunk in a depression of the leaf, open by pores; in Marattia and Kaulfussia (fig. 2, e) they dehisce by slits on the inner face; while in Angiopteris (fig. 2, f) they are almost free from one another. The spores produce a green prothallus of large size, the sexual organs of which hardly project from the surface. The cotyledon and stem grow up vertically through the prothallus, the root turning downwards into the soil.
|
| (From Strasburger's Lehrbuch der Botanik.) |
| Fig. 9.—Polypodium vulgare. |
| A, Unopened archegonium; o, ovum; k″, ventral canal cell; k′ neck-canal-cell. |
| B, Mature opened archegonium. |
Osmundaceae.—The two genera of this group, Osmunda and Todea, have thick erect stems, covered with the closely crowded leaf bases. The stem is monostelic, the vascular tissues being separated into curved groups comparable with collateral vascular bundles, which surround the pith. The somewhat thick roots are diarch. The leaves are large and pinnate; their, lamina is usually thick, though filmy species of Todea occur. The leaf-base shows indications of stipular outgrowths. In Todea the sori, each of which consists of a single circle of bulky sporangia, are borne on the under surface of the pinnae. In Osmunda the region of the leaf which bears the sporangia has its lamina little developed; the leaf thus bears sterile and fertile pinnae, or, as in O. cinnamomea, sterile and fertile leaves may be present. The sporangia originate from single cells, though surrounding cells may contribute to the formation of the stalk. The latter is thick and short, and the wall of the sporangium, which opens by a median slit, has a group of thick-walled cells at the summit, forming the annulus. The prothalli are similar to those of the other Filicaceae, but more massive; the same may be said of the archegonia and antheridia, which, however, project more than in the preceding group.
Schizaeaceae.—The anatomy of the stem differs in the four recent genera of this order, and presents a series possibly illustrating the origin of a number of concentric steles from a solid stele, the intermediate step being represented by those forms in which the central cylinder is tubular. The sporangia are borne singly or in sori of two or three on the margin or under surface of leaves, the fertile pinnae of which differ more or less from the sterile segments. The sporangium is of considerable size, and dehisces by a median slit, the annulus being a more or less definitely limited horizontal ring of cells near the apex. The prothallus and sexual organs may resemble those of the Polypodiaceae; in Aneimia and Mohria the prothallus, though flattened, is not bilaterally symmetrical, the growing point being on one side; a filamentous type of prothallus is known in Schizaea.
Gleicheniaceae.—These forms have a horizontal rhizome, from which simply pinnate leaves arise in Platyzoma, while Gleichenia bears compound pinnate leaves with continued apical growth. The rhizome usually has a solid central cylinder in Gleichenia, while that of Platyzoma is tubular. The sporangia arise simultaneously in the sorus, which is borne on the under surface of the ordinary pinna; in those species with large sporangia the latter form a single circle, in others sporangia may also arise from the central part of the receptacle. The annulus is horizontal and the dehiscence median. The prothalli, while resembling those of the Polypodiaceae, have points of similarity with those of the preceding groups.
Matoniaceae.—This contains the single genus Matonia, two species of which are known from the eastern tropics. They are of special interest, since they have been shown to be the surviving forms of a group species which have been identified from Jurassic and Cretaceous rocks. The living species have a long rhizome, from the upper surface of which the large leaves arise; these are branched in a pedate manner, each branch being pinnate. The structure of the rhizome is complicated, a transverse section showing that the centre may be occupied by a solid stele, outside of which are two tubular steles. The sori are borne on the under surface of the pinnae, each consisting of a single series of large sporangia covered by a coriaceous indusium, which is attached to the central part of the receptacle. The sporangium, which corresponds on the whole to that of the Gleicheniaceae, has a somewhat oblique annulus; the dehiscence also is not truly median. The gametophyte is unknown.
Loxsomaceae.—The single genus Loxsoma has a tubular stele in its rhizome, which bears leaves resembling those of some Davallias. The elongated receptacle of the marginal sori is surrounded by a basal cup-shaped indusium. The sporangia, which arise in basipetal succession on the receptacle, dehisce by a median slit, though the annulus is somewhat oblique; they have resemblances to the Gleicheniaceae. When mature, the sporangia are raised above the margin of the indusium by the elongation of the receptacle, thus facilitating the dispersion of the spores. The gametophyte is unknown.
Hymenophyllaceae.—This group, which contains the two genera Hymenophyllum and Trichomanes, is characterized by the prevalent “filmy” texture of the leaves. Many of the species inhabit situations in which the air is constantly moist, especially in the tropics; some are terrestrial; others, some of which are very minute, are epiphytic on tree-stems. A single solid central cylinder is found in the rhizome. The sori, which are marginal, have a long receptacle, bearing the sporangia in basipetal succession, and are surrounded by a cup-shaped indusium. The sporangia present a considerable range in size, the largest being found in species of Hymenophyllum, the smallest in Trichomanes. Each has an almost horizontal annulus resembling that of Gleichenia, but the dehiscence is lateral. The gametophyte in Hymenophyllum is flat and variously lobed; that of Trichomanes may be similar, but in other species is filamentous. The archegonia and antheridia present points of similarity to those of the Gleicheniaceae.
Cyatheaceae.—This order includes the majority of existing tree-ferns, as well as some of smaller size. The stem has a ring of flattened steles. The sorus has a somewhat elongated receptacle, on which the sporangia arise basipetally; the indusium may be cup-shaped, bivalve or wanting. The dehiscence of the sporangium is almost transverse, as in the Polypodiaceae, but the annulus is slightly oblique. The prothalli correspond to those of the next group.
Polypodiaceae.—This group, which contains the remaining ferns, includes a number of distinct lines of descent and will doubtless require subdivision as our knowledge of the morphology of the genera classed in it becomes extended. Space will not allow of an account of the progress already made in this direction. The stem in the more primitive forms has a tubular stele (solenostele); for the most part two to many steles, arranged in a ring (dictyostele). In a number of genera, which there is reason to regard as relatively primitive, the sporangia show the same regular basipetal succession as in some of the preceding groups; in the great majority, however, the succession is not regular, but those of various ages are intermixed in the sorus (fig. 2, g). The sporangia dehisce by a transverse slit, the annulus being truly vertical or, in some of the genera in which they are regularly arranged, very slightly oblique. The structure of the prothallus and sexual organs will be evident from figs. 7, 8 and 9; some of the more interesting modifications have been referred to above.
Our knowledge of the extinct Filicales cannot be readily summarized, since it is in a transition state, owing to the recent evidence which has shown that many of the fern-like plants of the Palaeozoic period belonged to a group of seed-bearing plants derived from a filicineous ancestry. There is, however, abundant evidence that the Ferns were represented in the most ancient floras known, though they were not such a dominant group as has hitherto been supposed. The best known of these ancient Ferns belong to the Botryopterideae; the characters of this group point to its having been the starting-point of several series of existing Ferns (see Palaeobotany: Palaeozoic).
A consideration of the Filicaceae as arranged above will show that the several sub-orders may in general terms be said to form a series between those in which the sorus consists of a single circle of bulky sporangia and those Polypodiaceae in which the numerous small sporangia appear to be grouped without order in the sorus. When the survey is extended to the extinct Ferns of which the fructification is known, many of those from the more ancient rocks are found to group themselves with the existing sub-orders with large sporangia, such as the Marattiaceae, Gleicheniaceae and Schizaeaceae; the Polypodiaceae, on the other hand, do not appear until much later. The extinct forms cannot be dealt with in detail here; but it may be pointed out that their order of appearance affords a certain amount of direct evidence that the existing Ferns with a single circle of large sporangia in the sorus are relatively primitive. The series which can be constructed from a study of the sorus is in general supported by the anatomy of the sporophyte, and by the structure and sexual organs of the gametophyte. A more detailed investigation of all the characters of the Ferns will be needed before the course of evolution thus broadly indicated can be traced, but the results obtained afford a deeper insight into the general method of progression and the selective factors in the process. On the ground mainly of an examination of the sorus and sporangium, Bower has shown that the Filicaceae may be divided into three groups—the Simplices, Gradatae and Mixtae—in which the sporangia arise simultaneously, in basipetal succession, or irregularly in the sorus respectively. The first includes the Marattiaceae, Osmundaceae, Schizaeaceae, Gleicheniaceae and Matoniaceae; the second the Loxsomaceae, Hymenophyllaceae, Cyatheaceae and the Dennstaedtineae (a group including species placed in the Synopsis Filicum in Dicksonia and Davallia); while the remaining Polypodiaceae constitute the Mixtae. The change from the one type of sorus to the other may have taken place in several different lines of descent, some of which have been traced. A consideration of the biology of the sorus gives an insight into the advantages obtained by the one type over the preceding, as regards protection, spore production and the dispersal of the spores, and thus indicates the way in which natural selection may have acted. The differences in the form and mode of dehiscence of the sporangia (those of the Simplices having median dehiscence and a horizontal annulus, those of the Gradatae a more or less oblique position of the annulus and of the plane of dehiscence, while in the Mixtae the annulus is vertical and the dehiscence transverse) stand in relation to the position of the sporangia in the sorus relatively to one another. The application of the important criteria which Bower has thus pointed out to the construction of a strictly phylogenetic classification of the Filicaceae cannot be made until the anatomy, the sexual generation and the palaeobotanical evidence have been further examined from this point of view. Though on this account and because the subdivisions Simplices, Gradatae and Mixtae do not correspond to definite phylogenetic groups, they have not been used in classifying the Ferns above; they are of great importance as an advance towards a natural classification.
Hydropterideae.—Two very distinct orders of heterosporous Filicales, the Salviniaceae and the Marsiliaceae, are included in this group. The difficulty of determining their exact relationship to the other orders of Ferns is increased by the more or less completely aquatic habit of the plants and the modifications and reductions in structure associated with this. The absence of an annulus from their indehiscent sporangia makes it impossible to compare them with the other Ferns in respect of this important character. It has been suggested with considerable probability that the Marsiliaceae are allied to the Schizaeaceae, while the Salviniaceae may possibly be related to the Hymenophyllaceae or to some other family of the Gradatae. Space will only permit of a brief general account of the more obvious features of the several genera, the structure and life-history of which are known in great detail. Unlike as they are in many respects, the two orders agree in being heterosporous. The microspores on germination produce a small, greatly reduced male prothallus bearing one or two antheridia which give rise to a number of spirally coiled, multiciliate spermatozoids. The single large megaspore contained in each megasporangium produces a small prothallus, which bears one or a few archegonia; these are exposed on the surface of the prothallus at the summit of the germinated megaspore (fig. 1, i ).
1. The Salviniaceae include the two genera Salvinia (fig. 10) and Azolla. The small dorsiventral plants are in both cases floating aquatics. Azolla has roots depending from the lower surface of the stem into the water, while these organs are completely wanting in Salvinia, their place being taken functionally by highly divided leaves borne on the ventral surface of the stem. Nostoc colonies are constantly present in a special cavity of the dorsal lobe of the leaf in Azolla. The sporangia in both genera are associated in sori enclosed by indusia springing from the base of the receptacle. In Salvinia (fig. 2, h) the sori are borne towards the base of the submerged leaves, in Azolla on the reduced ventral lobe of the leaf. They consist either of microsporangia or megasporangia, which are arranged in basipetal succession on the receptacle. In the megasorus of Azolla there is only the one terminal, functional sporangium. The microspores are united by means of hardened protoplasm into one or more masses, while the solitary megaspores have a more or less complicated episporium.
|
| (Reduced. After Bischoff from Strasburger's Lehrbuch der Botanik.) |
| Fig. 10.—Salvinia natans. |
| A, From the side.B, From above. |
2. The Marsiliaceae also include two genera, Marsilia and Pilularia, the latter of which is found in Britain. The plants grow as a rule in marshy places, though some species of Marsilia are xerophytic. The creeping stem produces roots from the ventral surface and leaves from the dorsal surface; the leaves when young are circinately coiled. The leaves are simple and linear in Pilularia, but in Marsilia bear a pinnate four-lobed lamina. The highly specialized sporocarps are borne on the basal portions of the leaves, as a rule singly, but in some species of Marsilia in numbers. The development of the sporocarp shows that it corresponds to a pinna, although when mature it may appear to occupy a ventral position in relation to the vegetative portion of the leaf. It has a complicated structure in both genera; in Pilularia its shape is nearly spherical, while in Marsilia it is elongated and bean-shaped. The sori are developed in depressions and are thus protected within the resistant outer wall of the sporocarp. There are usually four sori in Pilularia, while in Marsilia they form two longitudinal rows. Each sorus includes both microsporangia, with numerous spores, and megasporangia, each of which contains a single megaspore with a complicated wall. Enclosed within the sporocarp they can endure a period of drought, but on the return of moist conditions are extruded from the sporocarp by the swelling of a special mucilaginous tissue and the spores become free. The development of the prothalli is in general similar to that of the Salviniaceae, though the resemblance may be homoplastic. The stem in the less reduced forms is solenostelic with sclerenchymatous ground tissue occupying the centre of the stele.
In the absence of direct evidence from Palaeobotany, and bearing in mind the modifications associated with adaptation to an aquatic life in other plants, the recognition of any more definite affinity for these heteros porous ferns than that indicated above appears to be inadvisable. Further evidence is necessary before they can be removed from such a position of convenience as is assigned to them here and placed in proper relation to the series of the Filicaceae.
The several phyla of Pteridophyta having now been briefly described, their relationship to one another remains for Phylogeny. consideration. The available evidence does not suffice to solve this question, although certain indications exist. In the earliest land vegetations of which we have any sufficient record specialized forms of Equisetales, Lycopodiales, Sphenophyllales and Filicales existed, so that we are reduced to hypotheses founded on the careful comparison of the recent and extinct members of these groups. In this connexion it may be pointed out that the fuller study of the extinct forms has as yet been of most use in emphasizing the difficulty of the questions at issue. It has thus led to a condition of uncertainty as regards the relationship of the great groups of Vascular Cryptogams, in which, however, lies the hope of an ultimate approach to a satisfactory solution. The study of the Sphenophyllales, however, as has been pointed out above, appears to indicate that the Equisetales and Lycopodiales may be traced back to a common ancestry. As to the relationship of the Filicales to the other phyla, evidence from extinct plants appears to be wanting. If, as has been suggested by Bower, the strobiloid types are relatively primitive, the large-leaved Pteridophyta must be supposed to have arisen early from such forms. The question cannot be discussed fully here, but enough has been said above to show that in the light of our present knowledge the main phyla of the Vascular Cryptogams cannot be placed in any serial relationship to one another.
It may even be regarded as an open question whether some of them may not have arisen independently and represent parallel lines of evolution from Bryophytic or Algal forms. This leads us to consider the question whether any indications exist as to the manner in which the Pteridophyta arose. It will be evident that no direct record of this evolution can be expected, and recourse must be had to hypotheses founded on the indirect evidence available. There appears to be no reason to doubt that the sexual generation is homologous with the thallus of a Liverwort, or of such an Alga as Coleochaete. It is with regard to the origin of the spore-bearing generation of the Pteridophyta that differences of opinion exist. This, though at first dependent on the prothallus, soon becomes independent. It may be regarded as derived from a wholly dependent sporogonium not unlike that of some of the simpler Bryophyta; the latter are assumed to have arisen from primitive Algal forms, in which, as the first step in the interpolation of the second generation in the life cycle, the, fertilized ovum gave rise to a group of swarm spores, each of which developed into a new sexual plant. On this view the origin of the sporophyte is looked for in the gradual development of sterile tissue in the generation arising from the fertilized ovum, and a consequent postponement of spore-formation. Certain green Algae (e.g. Oedogonium, Coleochaete), the Bryophyta, and the simpler Pteridophyta, such as Phylloglossum, have been regarded as illustrating the method of progression, though there is no reason to regard the existing forms as constituting an actual series. For a discussion of this view, which regards the alternation of generations in Pteridophytes as antithetic and the two generations as not homologous with one another, reference may be made to the works of Celakovsky and Bower. Although the antithetic theory is supported by many facts regarding the life-history and structure of the group of plants under consideration, it is quite possible that a stage in which the sporophyte was wholly dependent on the gametophyte may never have been passed through in their evolution. The spore-bearing generation may throughout its phylogenetic history have been independent at one part of its life, and have been derived by modification of individuals homologous with those of the sexual generation, and not by the progressive sterilization of a structure the whole of which was originally devoted to asexual reproduction. A number of facts regarding the Algae, and also those relating to such deviations from the normal life cycle as apogamy or apospory, may be regarded as lending support to this view, which, in contrast to the theory of antithetic alternation, has been called that of homologous alternation. Without entering further into the discussion of these alternative theories, for which the literature of the subject must be consulted, it may be pointed out that on the latter view the strobiloid forms of Pteridophyta would not necessarily be regarded as primitive relatively to the large-leaved forms, and also that the early stages of the origin of the sporophyte in the two cases may have proceeded on different lines.
Another question of great interest, which can only be touched upon here and may fitly close the consideration of this division of the Vegetable Kingdom, concerns the evidence as to the derivation of higher groups from the Pteridophyta. The most important positive evidence on this point indicates that the most ancient Gymnosperms were derived from the Filicales rather than from any other phylum of the Vascular Cryptogams. Extinct forms are known intermediate between the Ferns and the Cycads, and a number of these have been shown to bear seeds and must be classed as Pteridospermae. These forms will, however, be found discussed in the articles treating of extinct plants and the Gymnosperms, but their recognition will serve to emphasize, in conclusion, the important position the Pteridophyta hold with regard to the existing flora.
Cultivation.—Numerous species of ferns, both temperate and tropical, are cultivated as valued ornamental plants. Species of the other groups are occasionally grown for scientific purposes in the larger botanic gardens, but their cultivation, which often presents special difficulties, need not be referred to here. While a number of ferns can be multiplied vegetatively, by buds formed on the leaves and in other ways, the regular mode of propagation is by sowing the spores shed from the ripe sporangia. The spores should be thinly sprinkled on the surface of the soil in well-drained pots, which should stand in saucers filled with water and be covered with glass plates. After the prothalli have attained some size and bear sexual organs the pots should be occasionally sunk in water so as to flood the prothalli for a few minutes and facilitate fertilization. The young plants developed on the prothalli should be carefully pricked out into other pans and later transferred to 3-in. pots. When the pots are fairly filled with roots the plants may be shifted into larger ones.
The best time for a general repotting of ferns is in spring, just before growth commences. Those with creeping rhizomes can be propagated by dividing these into well-rooted portions, and, if a number of crowns is formed, they can be divided at that season. In most cases this can be performed with little risk, but the Gleichenias, for example, must only be cut into large portions, as small divisions of the rhizomes are almost certain to die; in such cases, however, the points of the rhizomes can be led over and layered into small pots, several in succession, and allowed to remain unsevered from the parent plant until they become well rooted. In potting the well-established plants, and all those of considerable size, the soil should be used in a rough turfy state, not sifted but broken, and one-sixth of broken crooks or charcoal and as much sand as will insure free percolation should be mixed with it.
The stove ferns require a day temperature of 65° to 75°, but do not thrive in an excessively high or close dry atmosphere. They require only such shade as will shut out the direct rays of the sun, and, though abundant moisture must be supplied, the atmosphere should not be loaded with it. The water used should always be at or near the temperature of the house in which the plants are growing. Some ferns, as the different kinds of Gymnogrammae and Cheilanthes, prefer a drier atmosphere than others, and the former do not well bear a lower winter temperature than about 60° by night. Most other stove ferns, if dormant, will bear a temperature as low as 55° by night and 60° by day from November to February. About the end of the latter month the whole collection should be turned out of the pots and redrained or repotted into larger pots as required. This should take place before growth has commenced. Towards the end of March the night temperature may be raised to 60°, and the day temperature to 70° or 75°, the plants being shaded in bright weather. Such ferns as Gymnogrammas, which have their surface covered with golden or silver powder, and certain species of scaly surfaced Cheilanthes and Nothochlaena, as they cannot bear to have their fronds wetted, should never be syringed; but most other ferns may have a moderate sprinkling occasionally (not necessarily daily) and as the season advances sufficient air and light must be admitted.
Authorities.—Scott, Structural Botany: Flowerless Plants (London, 1896), Studies in Fossil Botany (Edinburgh, 1900);* Campbell, Mosses and Ferns (London, 1895);* Engler and Prantl, Die naturlichen Pflanzenfamilien (Theil i. Abth. 4.; Leipzig, 1898-1902); Bower, The Origin of a Land Flora (London, 1908); Goebel, Organography of Plants (Oxford, 1905); Hooker and Baker, Synopsis Filicum (London, 1874); Baker, Fern Allies (London, 1887); Christ, Die Fankräuter der Erde (Jena, 1897); Seward, Fossil Botany, vol. i. (Cambridge, 1898). In those works marked with an asterisk copious references to the recent literature of the subject will be found.