DIVISION III.
PTERIDOPHYTA (VASCULAR CRYPTOGAMS).
The alternation of generations is as distinct in this Division as in the Mosses, but the sexual generation consists of only a small thallus, the prothallium, which bears directly the sexual organs, antheridia and archegonia; and the asexual generation, which arises from the fertilisation of the oosphere, is no longer a single short-lived sporangium, but a highly developed, generally perennial, plant provided with stem, leaves and true roots (Ferns, Horsetails, etc.), the sporangia being borne on the leaves. In this latter generation the tissues are differentiated into epidermis, ground tissue and vascular tissue; in the last named the bundles are closed, and in the majority of cases concentric.
The sexual generation, gametophyte, or prothallium, is always a thallus, although not always green and leaf-like (Figs. 205, 215, 222, 229, 235, etc.) It is very small, even in cases where it attains the greatest development, and consists only of parenchymatous cells. The prothallium is nourished by hair-like roots (rhizoids) and has only a transitory existence, dying soon after the fertilisation of its oosphere.
The ANTHERIDIA exhibit great variations in structure which, however, must be considered as modifications of the fundamental type which is found in the Mosses. These modifications will be mentioned under the various families. The spermatozoids are always spirally-coiled, self-motile, protoplasmic bodies, with most frequently a large number of fine cilia on the anterior end (Figs. 206, 223, 234). They are formed principally from the nucleus of the mother-cell, and portions of the cytoplasm often remain for a time attached to their posterior end.
The ARCHEGONIA are more uniform throughout the entire Division, and more closely resemble those of the Mosses. They are, as in the previous Division, principally flask-shaped; but the central portion, which encloses the oosphere, is always embedded in the tissue of the prothallium, so that the neck, which is formed of 4 rows of cells, projects above the surface (Figs. 201 3, 222 h). The development of the archegonium in a Fern is seen in the accompanying figure (Fig. 201). The archegonium is developed from a surface cell, which divides into three cells by two walls in a direction parallel to the surface of the prothallium (Fig. 201). The most internal cell becomes the ventral portion of the archegonium. The external one (b) divides perpendicularly to the surface of the prothallium into four cells, which again divide parallel to the surface and form the neck (b, in 2 and 3). The intermediate cell projects upwards into the neck and divides into two, the lower one, after the separation of the ventral canal-cell, becoming the oosphere, and the upper one the neck-canal-cell (c, in 2 and 3).
Fig. 201.—Pteris serrulata. Development of archegonia.]
As in the Mosses, the divisional walls of the neck-canal-cells become mucilaginous, causing the rupture of the neck of the archegonium. Fertilisation takes place as in the Mosses, and the passage of the spermatozoids, along the neck, to the oosphere, has been observed. Water (rain or dew) is similarly necessary for the movements of the spermatozoids, and hence for fertilisation. The other classes of the Division chiefly deviate from the Ferns in having the archegonium sunk deeper into the prothallium, and the neck reduced in length (compare Fig. 201 with Figs. 216, 222, 235, 236).
According to the nature of the spores, the three classes of the Vascular Cryptogams are each divided into isosporous and heterosporous groups.
I. The isosporous Vascular Cryptogams have only one kind of spore. The prothallium developed from this is in some cases monœcious, bearing both antheridia and archegonia; but in others there is a distinct tendency for each prothallium to bear only antheridia or archegonia (diœcious)—true Ferns and Lycopodium.
In Equisetum there is only one kind of spore, but two kinds of prothallia are developed, one of which bears only antheridia (male), the other only archegonia (female); but the one that bears antheridia may be transformed into the one that bears archegonia and vice versa.
II. In the higher group, heterosporous Vascular Cryptogams (Selaginella and Isoëtes, etc.), there are two distinct kinds of spores, the small, microspores, and the large, macrospores. The microspores are male, and produce prothallia which bear only antheridia. The macrospores are female, and produce prothallia which bear only archegonia.
Corresponding to this difference in the spores, there is also found a difference in the development of the prothallium. In the Isosporeæ the prothallium is large, and either green, leaf-like, and provided with rhizoids (most of the Ferns, Horsetails, etc.), or subterranean, pale-coloured, and globular (Ophioglossum, Lycopodium). It lives vegetatively for a fairly long time, and generally produces a large and varying number of archegonia and antheridia. The prothallium in the Heterosporeæ is gradually more and more reduced, its independent and vegetative life becomes of less and less importance, it becomes more dependent on the mother-plant, and projects from the spore very slightly, or not at all. The antheridia and archegonia become reduced in number to one, and also degenerate in point of development.
It may here be remarked that the gradual development of the asexual generation, the development of the two kinds of spores, and the progressive reduction of the prothallium and sexual organs which is found in this Division, is continued to the Gymnosperms and Angiosperms. The microspores are in these called pollen-grains, and the male prothallium is very rudimentary. The macrospores are termed embryo-sacs, and the female prothallium, the endosperm.
The asexual generation, sporophyte. When the oosphere, which in this case as in all others is a primordial cell, is fertilised, it surrounds itself with a cell-wall and commences to divide into a number of cells, to form the embryo.
The first dividing wall (basal wall) is nearly horizontal, and in the direction of the longitudinal axis of the archegonium. The next wall is vertical, and the next perpendicular to the other two. The oosphere, therefore, is now divided into eight octants by these three walls. The basal wall divides the embryo into a hypobasal and an epibasal half. From the first one, by continued divisions, the first root is developed; from the latter, the stem and leaves. After the formation of the octants the development proceeds in somewhat different ways in the various classes. In addition to the stem, leaf, and root, a “foot” is developed from the hypobasal half which remains enclosed in the prothallium, and conveys nourishment from the prothallium to the young plant until it is able to sustain itself (Fig. 202). The formation of these members in the embryo depends on the position of the oosphere in the archegonium and prothallium, and is independent of gravity.
Fig. 202.—Adiantum capillus veneris. Vertical section through a prothallium (f f), with a young plant attached on its under side (mag. about 10 times); r the first root, and b the first leaf of the young Fern-plant; m the foot. In the angle between m and b lies the apex of the stem: h the rhizoids of the prothallium; æ æ unfertilised archegonia.
In the Mosses the asexual generation is the sporogonium, which is limited in its development and in a great measure dependent upon the sexual generation, upon which it is situated; but in the Pteridophyta this generation is an independent and highly developed plant, provided with stem, leaf, and true roots, and has in many instances an unlimited development. The Pteridophyta are the lowest Division with true roots. The root which is first formed is very similar in nature to the primary root of the Monocotyledons; it very soon dies and is replaced by others which are more permanent, and developed upon the stem (adventitious roots); roots are wanting in Salvinia, Psilotum, and some Hymenophyllaceæ. The differentiation is, however, not so complete as in the Flowering-plants, and so many leafy forms are not found. The various members of these plants are anatomically much higher than in the Mosses, having an epidermis, a ground tissue with variously differentiated cells, and a highly developed vascular system. The vascular bundles, like those in the Monocotyledons, are without cambium, and closed; they are therefore incapable of any increase in thickness. In general the bundles are concentric, with the bast round the wood (Fig. 203). The wood is almost entirely made up of scalariform tracheides.
In Isoëtes a secondary thickening takes place by a cambium, which is formed inside the cortex, constructing secondary cortex to the exterior, and secondary wood towards the interior.—Botrychium has also a thickening growth. Collateral vascular bundles occur in Osmundaceæ, Equisetaceæ, and the leaves of many Polypodiaceæ, etc.
Fig. 203.—Portion of the stem of a Fern. Above is seen the transverse section, with vascular bundles of different form and size. The rhombic figures on the side of the stem are leaf-scars.
It is a point of special interest, that the gigantic forms of Ferns, Equisetums, and Club-Mosses (which flourished in earlier geological periods, when these classes attained their highest development) possessed some means of increasing in thickness.
The sporangia are in all cases capsule-like, and burst open when ripe to eject the spores. They are nearly always situated on the leaves (in Lycopodiaceæ, in the axils of the leaves, or above these, on the stems themselves). In some forms (Leptosporangiatæ), the sporangia are developed from a single epidermal cell; in others (Eusporangiatæ), from a group of epidermal cells, or from cells which lie beneath the epidermis. In the first group a primitive mother-cell (archesporium) is formed, which divides commonly into sixteen special mother-cells. In the latter group, on the other hand, a number of primitive spore-mother-cells are developed. In each sporangium three different tissues are generally developed; an innermost sporogenous one (s in Fig. 204 A), which arises from the archesporangium; an outermost one, which forms the wall (a), and may be one or, more rarely, several layers in thickness; and an intermediate one, the tapetum (Fig. 204 A, B, b t), which is rich in protoplasm, and whose cells are dissolved so that the spores float freely in the fluid thus provided. The spores arise as in the Mosses (in tetrads), by the cross-division of the special mother-cells, and according to the manner in which they are arranged in the mother-cell have either a tetrahedral form, with a large base resembling a segment of a ball, or are oblong (bilateral spores). Their construction is the same as in the Mosses (p. 187).
Fig. 204.—Selaginella inæqualifolia. A A young sporangium, which may develope either into a macro-, or a microsporangium. B A microsporangium.
The spore-formation in its earliest commencement takes place in the same way in the Isosporous and the Heterosporous Vascular Cryptogams; but from a certain point, after the tetrahedral division, a difference occurs with regard to the macrosporangia. All the spores formed in the microsporangium may complete their development; but those which are formed in the macrosporangium are generally aborted, with the exception of one or four, and these consequently attain a much larger size (see Fig. 239.—The series to the left are microsporangia; those to the right, macrosporangia).
Apogamy. In some Ferns (Pteris cretica; Aspidium filix mas, var. cristatum; A. falcatum; Todea africana) the young plant is not developed as a consequence of fertilisation, but as a bud from the prothallium. This is known as apogamy, or loss of the power of sexual reproduction. The antheridia are generally more or less developed; archegonia are entirely wanting in Asp. filix mas, var. cristatum. This variety has probably only become apogamous through cultivation. Many specimens of Isoëtes lacustris, in a lake in the Vosges mountains, produce in the place where the sporangia are usually found, a vegetative shoot which grows into a new plant, so that the sexual generation is wanting in this case. Some specimens have sporangia on some leaves, and shoots on others.
Apospory, or the formation of prothallia instead of sporangia and spores on the leaves, is found in Athyrium filix femina, var. clarissimum. In this case the development of the sporangia proceeds only to a certain point, and from these arrested sporangia the prothallia are produced. Normal sporangia are entirely wanting in this variety, and in Aspidium angulare, var. pulcherrimum, sporangia are completely wanting. Compare the Mosses (page 188).
The Vascular Cryptogams are divided into three large classes, in each of which a progressive development can be traced from the isosporous to the heterosporous forms, but some of these are now only known as fossils.
Class 1. Filicinæ (Ferns).—The stem is small in comparison with the leaves, and branches only seldom, and then by lateral shoots. The leaves are scattered, large, often deeply divided, and of various highly developed forms. The undeveloped leaves are rolled up in the bud, having what is termed circinate venation. The sporangia are situated on the edge or on the lower side of the leaves, those on which the sporangia are borne (sporophylls) being often the ordinary foliage-leaves; but in a few cases the fertile differ from the barren ones (a higher stage in development). The fertile leaves are not confined to definite parts of the shoot, and do not limit its growth. The archesporium is most frequently unicellular.
A. Isosporous: Sub-Class 1. Filices (True Ferns).
B. Heterosporous: Sub-Class 2. Hydropterideæ (Water Ferns).
Class 2. Equisetinæ (Horsetails), in its widest meaning.—The leaves in this class are small in comparison with the stem. They are arranged in whorls, and unite to form a sheath. The sporangia are situated on specially modified, shield-like leaves, which are closely packed together and form a “cone.” The cone is borne terminally, and limits the growth of the shoot. The sporangia are developed from a large group of epidermal cells, the archesporium being unicellular. The branches are arranged in whorls, and develope acropetally.
A. Isosporous: Sub-Class 1. Equisetaceæ. Existing forms.
B. Heterosporous: Sub-Class 2. Extinct forms.
Class 3. Lycopodinæ (Club-Mosses).—Roots generally branching dichotomously. The leaves are scattered or opposite, and in proportion to the stem very small, undivided, and simple. They are scale-like and triangular, tapering from a broad base to a point. The sporangia are situated singly (except in Psilotaceæ), and almost in every case on the upper side of the leaf or in the axil of a leaf; but in some cases they are borne on the stem, just above the leaf-axil. The sporangia arise from groups of epidermal cells. The sporophylls are often modified, and differ from the foliage-leaves; they are then arranged in cones placed terminally on branches, thus limiting their growth.
A. Isosporous: Sub-Class 1. Lycopodieæ.
B. Heterosporous: Sub-Class 2. Selaginelleæ.
Class 1. Filicinæ (Ferns).
The characteristics of this class have already been given on page 204.
The class is divided into two sub-classes:—
1. The True Ferns, Filices, have one kind of spore which generally developes monœcious prothallia, relatively large and green. The sporangia are most frequently situated in groups (sori), which are often covered but not enclosed by an indusium.
2. Water Ferns, Hydropteridæ, have microsporangia with many (4 × 16) microspores, and macrosporangia, each with one macrospore. The prothallium is small, and projects but slightly from the germinating spore. The sporangia are situated in groups (sori), which are either enclosed by an indusium, or enveloped in a portion of a leaf, to form “fruits” termed sporocarps.
The old name for the Hydropterideæ, “Rhizocarpeæ,” i.e. the “root-fruited,” originated from the erroneous supposition that the sporocarps were borne on the roots.
Sub-Class 1. Filices (the True Ferns).
Of the eight orders (with about 4,000 species) comprised in this sub-class, the Polypodiaceæ is the largest (having about 2,800 species) and the most familiar; for this reason it will be taken as typical.
The sexual generation. When the spore germinates, the external covering (exospore) is ruptured, as in the Mosses. The internal cell-wall (endospore) grows out as a filament, which soon divides and gives rise to the prothallium, a flat, cellular expansion resembling the thallus of a Liverwort. In its fully developed state the prothallium is generally heart-shaped, dark green, and provided with root-hairs, and it attains a diameter of about one centimetre (Fig. 205). It is formed of one layer of cells, except along the central line near the anterior depression, where it becomes several layers of cells in thickness, forming the “cushion,” on the lower side of which the archegonia are developed. The antheridia are first formed; they are thus found on the oldest parts of the prothallium, on its edge, or among the root-hairs. The archegonia are developed later, and are therefore found near the apex. Several tropical Ferns have prothallia[18] deviating from this typical form; Trichomanes (Order Hymenophyllaceæ) has filamentous, branched prothallia, which resemble the protonema of a Moss. Others, again, have strap-shaped prothallia, which resemble the thallus of certain Liverworts.
Fig. 205.—Prothallium (p p) of Maiden hair (Adiantum capillus veneris) with a young plant attached: b first leaf; w′ primary root; w″ adventitious roots; h h root-hairs of the prothallium (× abt. 30).
Fig. 206.—Antheridia of Maiden-hair (× 550). A Unripe; B ripe, but unopened; C open and ejecting the spermatozoids (s). Those which have been last ejected are still lying enclosed in their mother-cells, the others are coiled up and drag with them the cytoplasmic remains (b); f cells of the prothallium.
The ARCHEGONIA have been already mentioned (p. 199, Fig. 201). The ANTHERIDIA are hemispherical or slightly conical bodies (Fig. 206). They consist, as in the Mosses, of a wall formed by one layer of cells, which encloses a number of spermatozoid-mother-cells (A and B). The antheridia when ripe absorb water, and are ruptured, and the spirally-coiled spermatozoids liberated (Fig. 206 S). The spermatozoids have been observed to pass down the neck of the archegonium, and to fuse with the oosphere.
The asexual generation. The first leaf, the “cotyledon,” of the embryo developed from the oospore (Figs. 202, 205) is always small, and has a very simple shape. The leaves which occur later become more perfect, stage by stage, until the permanent form of leaf has been attained.—The STEM is most frequently a subterranean or a semi-aerial rhizome; it is only in the tropical, palm-like Tree-Ferns, that the stem raises itself high in the air and resembles that of a tree, with leaf-scars or with the remains of leaves attached (Figs. 207, 203); in certain species the stem is encased in a thick mat of aerial roots (Dicksonia antarctica). When the rhizome is horizontal the internodes are frequently elongated, and the leaves are arranged in two rows, as in Polypodium vulgare and in the Bracken-Fern (Pteridium aquilinum), etc.; it is also generally dorsiventral, having a dorsal side on which the leaves are situated, and a ventral side, different from the former, on which the roots are borne. When the stem ascends in an oblique direction, or is nearly vertical, its internodes are extremely short, and the leaves are arranged in a spiral line with a complicated phyllotaxis, e.g. in Athyrium filix-fœmina, Aspidium filix-mas, etc. The BRANCHING upon the whole is extremely slight, and is generally confined to the petiole (e.g. Aspid. filix-mas), or to the stem near the insertion of the leaves. Several species normally form buds on different parts of the lamina. The buds which are formed on the stem are not confined to the leaf-axil as in the higher plants. The Tree-Ferns, generally, do not branch at all.
The VASCULAR BUNDLES are concentric, with the wood surrounded by the soft bast. In transverse section they are seen as circles or irregularly-shaped figures (Fig. 203), the name of “King Charles and the Oak” (Bracken-Fern) having originated from the appearance which the bundles present in oblique section. In Osmunda they are collateral and resemble those of the Flowering-plants. Round each individual bundle is often a sheath of thick-walled, hard, brown, sclerenchymatous cells, which act as a mechanical tissue; similar strands are also found in other parts of the stem.
Fig. 207.—Various Ferns (1, 2, 3, 4).
The LEAVES in nearly all species are only foliage-leaves, borne in a spiral. They have an apical growth which continues for a long time, and some require several years for their complete development. In the buds they are rolled up (circinate); not only the midrib, but also all the lateral veins, and even the terminal portions of a leaf are sometimes rolled up together, the tissues of the leaf being already fully developed and only waiting to expand. The leaves are often excessively divided and compound, with pinnate branches, and have an epidermis with stomata and a well-developed system of venation. Stipules are only found in Marattiaceæ and Ophioglossaceæ.
Very often peculiar hairs or scales (paleæ, ramenta), dry, brown, flat and broad, are found on stem and leaf.
The SPORANGIA are small, round capsules, which, in a very large number of Ferns, are formed on the back, but more rarely on the edge of the ordinary foliage-leaves. It is very seldom that there is any difference in form between the barren foliage-leaves and the fertile leaves, as is found for example in Blechnum spicant or Struthiopteris; or that the fertile part of the leaf is differently constructed from the barren portion of the same leaf, as in the Royal-Fern (Osmunda). In such instances the mesophyll of the fertile parts is poorly developed.
The sporangia in the Polypodiaceæ are lens-shaped, with long stalk (Fig. 211 D): their wall consists of one cell-layer on which a single row of cells, passing vertically over the top (that is along the edge of the sporangium), is developed into the “ring” (annulus). The cells of the annulus are very much thickened on the inner and side walls, and are yellowish-brown. The thickened cells, however, do not entirely encircle the sporangium, and on one side, near the stalk, they pass over into large, flat, thin-walled cells. These form a weak point in the wall, and it is here that the sporangium is opened diagonally by the elongation of the annulus. The sporangium of the Polypodiaceæ opens as it dries. The cells of the annulus are very hygroscopic, and in straightening, the annulus bends back with a jerk, thus ejecting the spores to considerable distances. The cells of the annulus absorb water with great readiness. [The sporangium arises as a single epidermal cell, from which a basal stalk-cell is cut off. Three oblique cell-walls, intersecting near the base, are next formed in the upper cell, and a fourth between these and parallel to the free surface; an inner tetrahedral cell enclosed by four others is thus formed, the outer cells become the wall of the sporangium, while the inner cell, by a series of walls, parallel to its sides, cuts off a layer of cells which eventually form the tapetum, the remaining central cell constituting the archesporium.]
The SPORES are either oblong and bilateral, or they are tetrahedric with curved sides, depending upon the way in which the tetrad division has taken place.
The sporangia are almost always situated on the nerves and gathered into groups, sori, which differ in form in the various genera. The sori, in many genera, may be covered by a scale-like structure, the indusium (Figs. 211 B, 212).
In the majority of cases, each sorus is situated on a small papilla (placenta, or receptacle), which is supplied by a small vascular bundle. Between the sporangia, hairs (paraphyses) are often situated, which spring either from the placenta or from the stalks of the sporangia.
Systematic Division. The Ferns may be divided into two groups, characterized by the structure and development of the sporangia. The sporangia in the Eusporangiatæ take their origin from a group of epidermal cells, and their walls are formed by several layers of cells. The archesporium is the (not tetrahedric) hypodermal terminal cell of the axial row of cells which give rise to the sporangium. In the Leptosporangiatæ the sporangia are developed from single epidermal cells, and their walls are uni-layered. The archesporium is a central, often tetrahedric cell, from which sixteen spore-mother-cells are developed.[19] It is difficult to say which form is the oldest (according to Prantl, those which have the sori on the nerve-endings); however, the Eusporangiatæ would seem to have made their appearance long before the others, and also well defined Marattiaceæ and Ophioglossaceæ occur in the Kulm and Coal period, before the true Polypodiaceæ.
About 4,000 species of Ferns are now existing, and they are found especially in tropical and sub-tropical forests.
Family 1. Eusporangiatæ.
Order 1. Ophioglossaceæ. The prothallium differs from that of all other Ferns in being subterranean, free from chlorophyll, pale and tuberous. The stem is extremely short, with short internodes, most frequently unbranched, vertical, and entirely buried in the ground (Fig. 208 st). In several species (among which are the native ones) one leaf is produced every year, which has taken three to four years for its development. In Botrychium a closed, sheath-like basal part of each leaf covers the subsequent leaves during their development. In Ophioglossum and others each leaf has at its base an intrapetiolar, cap-like sheath, which protects the succeeding leaf. The leaves are of two kinds: (a) foliage, which in Ophioglossum vulgatum are lanceolate and entire, but in Botrychium however, are pinnate (b in Fig. 208 A, B); and (b) fertile, which are found facing the upper side of the foliage-leaves. These latter in Ophioglossum are undivided and spike-like (Fig. 209 A), but pinnate in Botrychium (Fig. 208 B). Each foliage and fertile leaf are branches from the same petiole. The large sporangia are placed laterally, and open by two valves. No annulus is formed (Fig. 209).—Ophioglossum reproduces vegetatively by adventitious buds on the roots.
Fig. 208.—A Ophioglossum vulgatum (Adder’s-tongue); B Botrychium lunaria (Moonwort), both natural size; r-r roots; bs leaf-stalk; st stem; b foliage-leaf; f fertile leaf.
Fig. 209.—Fertile leaf of Ophioglossum.
Three genera with about twelve species.
Order 2. Marattiaceæ are tropical Ferns, whose gigantic leaves resemble those of the Polypodiaceæ, but have stipules in addition. The sporangia are grouped in sori, situated on the lower side of the leaves, the sporangia in each sorus being arranged either in two rows or in a ring. In Angiopteris they are isolated (Fig. 210 A), but in the other species (Kaulfussia, Danæa, Marattia), they are united, and form “synangia” divided into a number of chambers corresponding to the sporangia. These open by clefts or pores. Marattia presents the highest development, as its sporangia are completely united in a capsule-like synangium, which is closed until maturity, and then opens by two valves. In each valve there is a row of three to eleven sporangia, each opening by a slit towards the inside (Fig. 210 B, C). An indusium encloses the sorus, except in Kaulfussia; it is formed of flat and lobed hairs, which resemble the hairs of the other portions of the leaves. In Angiopteris and Marattia the indusium is very rudimentary; in Danæa it forms a kind of cupule.
The numerous fossil Marattiaceæ (15 genera, with 98 species) present similar differences to those now living, but more various forms are found, for example, with solitary free sporangia. Those now living are the last small remnant (4 genera with only 23 species) of a once dominant family, which existed from very early times, and whose culminating point was reached in the Kulm and Coal periods.
The Ophioglossaceæ appear also in the Kulm and Coal periods, and were about as numerous as at the present time (presumably 2 genera, with 19 species). Leptosporangiate Ferns appear however to have occurred first of all in the Trias-formation.
Fig. 210.—Sporangia of the Marattiaceæ: A Angiopteris; B and C Marattia; C is a half sorus with nine sporangia, each of which has opened by a longitudinal cleft.
Family 2. Leptosporangiatæ.
Order 1. Polypodiaceæ. Sporangia on the lower side of the leaves, stalked and provided with a vertical, incomplete annulus; dehiscing by a transverse cleft (Fig. 211 D).—The genera are distinguished by the form of the indusium and the position of the sori, etc.
1. The sporangia cover the entire lower surface of the leaf (Tropical America and Asia). Acrostichum, Platycerium.
2. Sori without indusia, circular or oval. Polypodium (Fig. 211 A). The leaves are most frequently situated in two rows on the dorsal side of the creeping rhizome, and fall off leaving a smooth scar behind.—P. vulgare, common in woods, on stones. (Phegopteris also has no indusium; see page 214).
3. The sporangia are situated in continuous lines just inside the margin of the leaf.—Pteris[20]: the sporangia form a continuous line along the entire margin of the leaf (Fig. 211 C), which bends over and covers the sporangia, forming a “false-indusium.” Pteridium has linear sori situated on a marginal vascular bundle, covered by two linear basal indusia, of which the outer is bent over like the edge of a leaf.—P. aquilinum (Bracken) has a wide-spreading rhizome with large alternate leaves, placed on opposite sides, at some distance apart. Only one leaf is developed from each branch every year.
Fig. 211.—Portions of leaves with sori. A Polypodium. B Aspidium. C Pteridium. D A sporangium of one of the Polypodiaceæ: r the annulus; s spores.
Adiantum (Maiden-hair): sori on the underside of small portions of the edge of the leaf, which are bent over (false indusium). Cryptogramme (Allosorus), Cheilanthes.
4. The sori are oval or linear, situated on one side of the vascular bundle.—Asplenium (Fig. 212 A): sori linear; indusium with one of its edges attached at the external side. A. ruta muraria (Wall-Rue); A. septentrionale; A. trichomanes.—Athyrium: sori linear or curved; A. filix-fœmina (Lady-Fern).—Scolopendrium (Fig. 212 B): sori as in Asplenium, but situated in pairs across the lanceolate, entire leaves. Each sorus is covered on the external side by an indusium, whose free edges are parallel and approach each other. S. vulgare (Hart’s-tongue).—Blechnum (B. spicant, Hard Fern; the fertile leaves differ from the barren, the pinnæ being narrower, while the underside is almost entirely covered with sori, and hence they are of a much darker brownish hue than the barren ones).—Ceterach: indusium rudimentary or absent.
5. Sori circular and covered by a shield-like, or reniform indusium.—Aspidium (Fig. 211 B); the leaves wither away and leave no scar upon the root-stock. A. filix-mas (Male-Fern); A. spinulosum.—Phegopteris has no indusium, the withered bases of the leaf-stalks are persistent; P. dryopteris and P. polypodioides.
6. The indusium is situated below the sori, and has the shape of a one-sided scale (Cystopteris, Struthiopteris), or of a cup or cupule, which in Woodsia is sometimes fimbriate (Fig. 212 C, D).
Fig. 212.—A Asplenium. B Scolopendrium. C Woodsia; D single sorus of the same. E Cyathea: the sporangia have fallen off in the upper sori. (All magnified.)
7. The sori are situated on the margin of the leaf, and at the end of a vascular bundle. Indusium, semi-cupular. Davallia. Principally tropical species. 1 in S. Europe.
This order is the greatest, comprising about 2,800 species, the majority being perennial plants. A few are large, and known as Tree-Ferns.
As plants in conservatories and rooms the following are cultivated: species of Gymnogramme (tropical America), Lomaria, Nephrolepis, Pteris (P. serrulata, cretica).
Officinal. Aspidium filix-mas, rhizome and the withered petioles.—Species of Alsophila and Cibotium give Penghawar Djambi. The rhizome of Pteridium aquilinum, var. esculentum, contains so much starch that it is used as food.
The other orders of true Ferns deviate from the Polypodiaceæ, especially in the formation of the annulus, the bursting of the sporangium and its mode of attachment and development, and in the differences in the formation of the prothallium, etc. The principal are:—
Order 2. Hymenophyllaceæ. To this order belong the lowest and most Moss-like Ferns; the leaves, with the exception of the veins, are most frequently formed of only one layer of cells, and consequently stomata are wanting; the formation of the prothallium also somewhat resembles the Mosses. Sori marginal, on the extremities of the vascular bundles, and surrounded by a cupular indusium. The sporangia are sessile, with equatorial annulus. Hymenophyllum (H. tunbridgense, European). Trichomanes (T. speciosum, European). Species about 200, which live especially on rocks and trees in damp and shady tropical forests. Some have no roots.
Order 3. Cyatheaceæ. Annulus complete and oblique. To this order belong, principally, the tree-like Ferns with palm-like habit. The number of species is about 200, they are all tropical and form forests in some regions of Australia. Cibotium and Dicksonia have marginal sori, with cupular, basal indusium. (The stem of D. antarctica is covered with aerial roots.) Alsophila (without indusium); Cyathea with cupular, inferior indusium (Fig. 212 E).
Fig. 213.—Gleichenia: A part of a leaf with sori; B a single sorus.
Order 4. Gleicheniaceæ. Sporangia with equatorial annulus, and longitudinal dehiscence, most frequently groups of 3–4 in sori without indusium (Fig. 213). Gleichenia: the apical growth of the leaves continues for a long time.
Order 5. Schizæaceæ. Annulus apical. To this order belongs Aneimia, which is so commonly cultivated in conservatories. The two lowest pinnæ are metamorphosed, having no leaf parenchyma and being covered with sporangia. Schizæa. Mohria. Lygodium, a climber, whose leaves have unlimited growth and attain a length of several metres. About 70 species. Tropical.
Order 6. Osmundaceæ. The sporangia have at the apex a lateral group of strongly thickened cells, which gradually pass over into the ordinary cells. The sporangia open by a longitudinal cleft. Indusium wanting. Osmunda bears the sporangia upon peculiar, branched pinnæ, without parenchyma (the uppermost in the leaf). O. regalis (Royal-Fern): European.
Sub-Class 2. Hydropterideæ (formerly Rhizocarpeæ), Water Ferns.
The following further characteristics must be added to those given on page 205:—