The Encyclopedia Americana (1920)/Ferns

FERNS and FERN-ALLIES. The ferns are representatives of an extensive branch of the plant world technically known as pteridophytes, standing between the mosses and hepatics (bryophytes) on the one hand and the cycads and conifers (gymnosperms) on the other, at the head of the now obsolete subdivision of plants known as cryptogams. They resemble the former groups so closely in their method of sexual reproduction and in showing alternating phases of growth that both mosses and ferns are often united under a single primary division of plants as archegoniates, so called from the common possession of a structure described below as the archegone. The ferns differ from the mosses in their highly developed vascular or woody system seen in the fibrous strands of the leaf-stalks and the veins of the leaves, and so are still sometimes called vascular cryptogams. Besides the ferns, popularly so known, there are various groups of plants, closely allied to ferns which with them make up the branch pteridophytes. Among these fern-allies are the running-pine or ground-pine, familiar in its use for Christmas decoration, the selaginellas frequent in cultivation, the field horsetail and the scouring-rush. These will be treated in their proper sequence below.

The mature fern-plant, which may stand as a representative pteridophyte, consists of a woody axis bearing scattered or clustered leaves according as this axis is a creeping structure or an upright trunk. In our northern species this axis is either below the surface of the ground or just at the surface, but in some of the species of tropical regions it often forms a trunk from 1 foot to 50 feet in height, or even more, bearing its leaves at the summit and forming a treefern.

On the under surface of certain leaves of ordinary ferns little clusters of stalked bodies (sporangia) appear, which are variously arranged in different species, in rounded or linear masses (sori) or in a few species are scattered over the surface of the leaf. These structures consist of a membraneous wall enclosing minute bodies — the fern-spores. These are the reproductive bodies of the fern asexually produced by repeated division of the interior portion of the young sporangium, and are often popularly known as “fern-seeds.” This is a misnomer, for they rather find their homology with the pollen-grains and young embryosacs of the higher plants. In the sporangia of some ferns a series of peculiar cells serves the purpose of rupturing the sporangium wall, and by an elastic movement scatters the spores at some distance from the plant producing them. Either at once or after a period of rest the fern-spore sprouts and produces a green hepatic-like structure known as a prothallus. This is a flat expense of soft green tissue, usually more or less heart-shaped, and ranging up to a half inch or more in diameter. It forms root-hairs beneath, and grows exactly like an ordinary new plant. On its under surface among the root-hairs two sorts of structures are produced: (1) A series of rounded bodies known as antherids, in which minute motile cells are formed known as antherozoids, these being the male or sperm cells connected with the sexual reproduction of the fern; (2) a series of flask-shaped bodies (archegones) more deeply imbedded in the tissues of the prothallus, which contain a single specialized cell at the bottom of the flask, known as the egg, this being the female reproductive cell. The sperm-cells escaping from the antherid swim in the delicate film of moisture which may bathe the surface of the prothallus, swarm about the mouth of the archegone, and one of them enters the neck of the flask-like structure, penetrating the mucilaginous contents of its tube, and fuses with the egg, thus effecting a true sexual reproduction. The fertilized egg soon commences division, increases in size and develops a primary root and leaf, ultimately growing into a new fern-plant. The two alternating phases of growth in the life period of the fern are thus strongly marked; the sexual phase (prothallus) is often known as the gametophyte, and the asexual phase (the fern-plant) which follows is known as the sporophyte or spore-producing phase.

In some of the fern-allies like Marsilia and Selaginella the plants produce two kinds of spores, (1) microspores, which develop into prothalli producing only antherids, and (2) macrospores, which develop into prothalli producing only archegones. In these plants, particularly Selaginella, the prothallus is much reduced in size, and the sexual reproduction approaches that of the pines and other conifers, except that it takes place away from the plant producing the spores. The microspores of Selaginella are thus the exact counterparts of the pollen-grains of the pine, and the macrospores correspond to what is known as the embryo-sac in the young pine-cone, to that part of the ovule in which the egg-apparatus appears, and in which the seed is ultimately developed.

Ferns are distributed all over the world but diminish in the number of species, in the profusion of individuals, and in the relative proportion to the total vegetation of the locality as we pass from tropical to polar regions. Moist insular climates are best adapted to their development. Java has 575 species of ferns, while New York State with about the same area has scarcely one-tenth as many. Jamaica, with about the area of Connecticut, has nearly 500 species; Hawaii, Samoa, New Zealand and the Philippines are other prolific regions. While moist conditions favor fern-growth and development, certain species have adapted themselves to dry climates and grow on open exposed rocks of semi-arid regions. Such ferns are provided with woolly hairs, tangled scales, waxy powder or other mechanical contrivances to conserve their moisture. One species of the so-called “resurrection plants” {Selaginella lepidophytla),' of the arid regions of the Southwest conserves its moisture during the dry season by coiling its branches inward so as to form a compact ball. On the return of moisture it flattens out and exposes its fresh green surface. The “resurrection-fern” curls up in a similar way, its under surface being protected by a layer of scales. It grows on trees in the Southern States and the West Indies.

The ferns of temperate regions are terrestrial in their habit, but as we approach the tropics a large number become epiphytic, growing with mosses and orchids on the trunks and branches of trees. The filmy ferns frequently cover the trunks of tree-ferns completely.

There are over 5,000 described species of pteridophytes in the present flora of the world, which are grouped according to their characters under seven distinct orders. Of these we have nearly 290 species within the limits of the United States.

Geologically, the pteridophytes are a very old group dating back to the Devonian and reaching their first culmination during the Carboniferous Age, in which they formed the bulk of the vegetation of the coal-measures. Many of the fossil formerly described as ferns, however, are really primitive seed-plants closely allied to the ferns. (See Pteridosperms). Besides ferns, some of the fern-allies of those times attained very great size, as shown in the giant trunks of Lepidodendron and Sigillaria, related to the modern club-mosses, and in the large stems of Calamites, related to our present diminutive scouring-rushes. The various types of the modern pteridophytes and their distinctive characteristics will be treated under their respective orders, arranged, as nearly as a lineal series can show it, in their relative rank in the scale of vegetable life.

I. Filicinæ. A. Eusporangiatæ — The adder-tongues and grape-ferns are eusporangiate; that is, their sporangia develop from the interior tissues. They include plants of a fleshy texture, bearing their thick-walled sporangia in spikes or panicles. The prothallus, so far as known, is subterranean and colorless. We have in the United States seven species of adder-tongue (Ophioglossum), and 17 species of grape-ferns (Botrychium), the former having the sporangia in spikes and the latter having them in panicles. All our northern species are terrestrial plants with fleshy roots. Cheiroglossa palmatia of Florida and the West Indies is a hanging epiphyte growing on palmetto-trunks and on other trees, and the curious strap-like Ophioderma of Hawaii and other Pacific isles grows in similar situations, with its pendant leaf from two to five feet long. The order Ophioglossales represents the simplest type of fern-growth in existence, and probably the oldest type as well. The species (about 60) are not numerous, but are very widely distributed.

The Marattiaceæ include only tropical ferns of a coarse habit, ranging from 2 feet to 10 feet in height. They are eusporangiate like the last order, but have the coiled-bud form of the true ferns. They also bear their sporangia dorsally, that is, on the under surface of the leaf, but these are formed in special boat-like receptacles known as synangia. Marattia and Danæa are the two principal genera. Two species of the former and nine of the latter are found in the North American tropics. The order is a smaller one than the last and, like it, consists of a single family.

B. Leptosporangiatæ. — The ferns proper, including nearly 4,000 species, are divided unequally among the eight families of this subclass. They are leptosporangiate (that is, the sporangia develop from the epidermal tissues), and all produce green prothalli of the type described above. Except the Matoniaceæ, a small family of two species, all are represented in America. Two families are tropical only, while the remaining five are also represented in the United States.

1. The flowering ferns, so called because the sporangia are panicled, are among the conspicuous features of our northern swamp vegetation in spring and early summer. The cinnamon-fern grows in great crowns, sending up its spore-producing leaves, which have a rich cinnamon color, a short time in advance of its rich crown of foliage-leaves. The royal flowering-fern grows in similar locations, but is larger, has more compound foliage, and bears its panicle at the summit of the foliage-leaf. The flowering ferns (Osmunda), with two genera from New Zealand and Africa, make up the family Osmundaceæ with 16 species. In this family the ring of the sporangia is rudimentary and the sporangia opens longitudinally.

2. The Hartford fern or climbing fern (Lygodium) is the type of a second small family. Our species is low-growing, only two or three feet high, and twines closely about other vegetation. It is quite local, but where it grows it sometimes forms tangles. In Connecticut it is so rare that it is protected by law. The tropical species of this genus often climb trees for 40 feet or more. Another member of the same family is the curious curly-grass (Schizæa) of the pine-barrens of New Jersey. It is wholly unlike ordinary ferns, the foliage-leaves resembling curled grass-leaves and the spore-bearing leaf resembling a small sedge. The entire plant is only four or five inches high. These two species with two subtropical species of Ornithopteris form our four representatives of the family Schizæaceæ, whose 80 species are largely South American, though several species occur in the Old World. They are characterized by their pear-shaped sporangia with an apical ring. The sporangia are usually borne in spikes or panicles.

3. The Gleicheniaceæ are found in the tropics of both hemispheres, several species of Dicranopteris occurring in the uplands of the West Indies, forming almost impassable tangles or thickets. These ferns, instead of uncoiling at once, produce buds in the axils of the forked branches, and as the lower portions mature these buds develop successive upper growths. The tangles thus formed are often sufficient to bear the weight of a man, and it is possible to walk for some distance on these thickets.

4. The floating-fern (Ceratopteris), found rarely in Florida and Louisiana, and occasionally throughout the tropics of both hemispheres, is almost the only aquatic representative of the order. Its sterile leaves float on the surface of shallow waters, and its pad-like fertile ones project above the surface. The name Ceratopteris is derived from this fertile leaf, which branches like a deer's horn, and gives the name to the family Ceratopteridaceæ.

5. The tree-ferns of both hemispheres form a separate family Cyatheaceæ, though several members of this family are very diminutive, and ferns with trunks occur occasionally in other families. No tree-ferns occur in the United States, but 30 species or more are found in the higher altitudes of the West Indies, and many more occur in other tropical regions. In some portions of the mountains of Jamaica tree-ferns form half or three-fifths of the forest vegetation, the trunks ranging from 6 to 50 feet in height. A well-developed tree-fern forms one of the most beautiful types of vegetation, rivaling the palms in grace and perfection of form. Some 200 species have been described, belonging chiefly to the genera Cyathea, Alsophila, Hemitelia, Dicksonia and Cibotium.

6. The great mass of our ferns belong to the family Polypodiaceæ, often known as the true ferns, perhaps for no better reason than that they were the first to be called by the name of ferns. The members of this family have definitely stalked sporangia, always provided with an elastic vertical ring which causes the sporangium to burst transversely and thus disperses the spores. In some species, like the stag-horn fern, the sporangia are spread over the underleaf surface in a uniform layer; in others, like the spleenworts and bird's-nest fern, they are in definite lines; while in our common polypody, the maidenhair, and the wood ferns they are in small rounded masses (sori). In some species the sorus is naked, but in most it is covered by a small membrane (indusium), primarily for the purpose of protecting the young sporangia. In some ferns, like the spleenworts, the indusium develops along a vein; in others, like the Christmas-fern, it is attached at one point and covers the sorus like an umbrella; in still others, like the maidenhair, it is formed of a modified portion of the margin of the leaf folded under so as to cover the young sporangium. The nature and position of the indusial covering forms one of the leading characters for the separation of genera under the Hookerian scheme of classification, while the Preslian scheme lays stress on the character of the venation, the fundamental branching of the fibro-vascular system of the plant. A more rational and natural scheme combines with these the habitual and biological characters that serve to group in each genus ferns that really have a natural relation to one another.

In so large a group of plants as that presented by this family we must expect to find the greatest diversity of size and structure, dependent on the geographical distribution, the altitudinal distribution, and on the ecological conditions under which the various members of the family have become developed. Marsh, swamp, rocky hillside, cliff and moist ravine each have their characteristic species in every country, while the trees of every tropical region support an extensive epiphytic series which varies with every considerable change of altitude. Even exposed cliffs in semi-arid regions support their own peculiar ferns, which have become adapted to conditions that at first would appear wholly unsuited to fern-growth. Obviously only a few of the more common species which grow wild or in cultivation can be mentioned here.

The sword-fcrn (Nephrolepis), some forms of which are known in cultivation as the “Boston Fern,” is a native of tropical America, occurring frequently in peninsular Florida. In its native haunts it is more commonly found on the trunks of palm-trees, though it often grows on the ground or on rotten wood. It is a favorite in cultivation, either in hanging baskets or in jardinières, and requires a rich soil and moist surroundings to thrive best. The sori are provided with a kidney-shaped indusium.

The maidenhairs (Adiantum) are perhaps the most graceful of the herbaceous ferns. Over 60 species are known, many of which have long been in cultivation. The luxuriant split-leaved A. Farleyense and the numerous fine-leaved varieties of A. cuneatum and A. gracillimum are among the most graceful in cultivation. Owing to their readiness to wilt most maidenhairs thrive best in Wardian cases.

In late years Japanese fern-balls have become popular in cultivation, consisting of the slender rootstocks of a species of Davallia wrapped about a mass of peat-enclosed soil, and held in form by wires. The Davallias are graceful ferns mostly confined to the Old World, though related genera are found in the American tropics. Numerous species are found in cultivation in the larger conservatories. Among our wild species the ostrich-fern (Matteuccia) is one of the best for out-of-door cultivation, to which it readily yields. It grows in graceful crowns from an erect rootstock, and has a bright green color. Its spore-bearing leaves are curiously rolled together so as to have the appearance of pods containing the copious sporangia. It requires a moist soil for its growth, and when growing wild thrives best in low ground. Other ferns suited for bed-cultivation are the graceful wood-ferns (Dryopteris spinulosa, and related species) and the Christmas-fern (Polystichum), both with dark foliage and crown-like habit, and the hay-fern (Dennstædtia) with a foliage of lighter green and a more scattered habit of growth resulting from its creeping rootstocks. The wood-ferns and Christmas-fern are firm in texture, and remain green throughout the winter; the hay-fern dies down with the early frosts. Another delicate pale-green species suitable for cultivation is the New York fern (Dryopteris Noveboracensis), which, however, has a range from New England to Alabama.

For rockeries the bladder-fern (Filix fragilis), the smaller spleenworts (Asplenium trichomanes and A. platyneuron), the rock-brake (Pellæa), and the curious walking-leaf are common among the best native species; and all thrive best if the rocks are part limestone. The last-named fern has a peculiar habit among our native species, shared, however, by many exotic ferns. The leaf, which is simple and tapers to a slender point from a rounded lobed base, takes root at the apex and forms a new plant, whose leaves may again take root and form still other plants. This peculiarity of taking steps away from the mother plant has given the name walking-leaf to this small fern, whose leaves range from 4 to 10 inches in length.

Aside from decorative value, ferns have little utility. The matted tufts of soft hair-like scales of some of the Hawaiian tree-ferns (Cibotium) are collected in commercial quantities and used for filling mattresses under the name “pulu.” The male-fern (Dryopteris filixmas) and some of its near allies are used in medicine as a vermifuge, and the soft parts of certain species are used for food. The mission of the fern, however, seems to be an æsthetic one, to minister grace and beautify the duller and more prosaic sides of nature.

7. In point of grace and delicacy none of the ferns as ordinarily known can equal the filmies, which flourish best at the higher elevations of tropical lands. These ferns differ from the true ferns not only in the form of the sporangium, and its attachment to a threadlike receptacle, but also in the texture of the leaf, whose blade is often only a single cell thick, so that the leaf is strongly translucent. The filmy ferns belong chiefly to the genera Trichomanes and Hymenophyllum, and although many are apparently so simple, the Hymenophyllaceæ form one of the most highly differentiated families among the ferns. A few others simulate them in texture, like the delicate New Zealand species of Leptopteris and some species of Asplenium in the American tropics, but these are representatives of two distinct families. The genus Loxsoma represents a structural connecting-link between the genus Davallia and the filmy ferns, and this is apparently their only point of kinship. Two species of Trichomanes occur under moist rocks in some of our Gulf States, one of them scarcely over a half inch in height.

C. Salviniales. — Next to the ferns are the members of this order, in which spores of two sorts are produced in special conceptacles resembling small pods or spheres. There are two types of habit represented, one series (Salviniaceæ) floating on the surface of water, and the other (Marsiliaceæ) rooting in mud. Salvinia, with oval leaves, is occasionally seen in cultivation but is rare in a wild state. Azolla is found in great abundance in Florida, sometimes covering the surface of ponds with its delicate branching small-leaved stems. A second species occurs in California. Marsilia is the type of the second family, and has a four-parted leaf resembling that of Oxalis. It grows on the wet borders of ponds, or if the water becomes high, it elongates its leaf-stalks and the leaves float on the surface like four-leaf clovers. Its conceptacles are oval and are borne on short stalks. One species grows abundantly at Bantam Lake, Connecticut, and others in Florida and in the Far Northwest where they are more common. Pilularia is a second genus of the family with thread-like leaves, and spherical pill-like conceptacles. Our single species occurs in California, Oregon and Arkansas.

II. Equisetinæ. — About 25 species forming a single genus and family make up this order. There are two types of growth, both common species of Equisteum, which have received different popular names. One is the field horsetail, which appears in low sandy ground early in spring, sending up a flesh-colored stalk terminating in a cone, followed by the “horsetails,” which are the vegetative parts of the plant, consisting of a central hollow stalk with angular whorled branches. The leaves are reduced to sheaths encircling the stem. Under the surface-shields that cover the cone, the spores, which in mass resemble a fluffy mold, are borne in sporangia. The peculiar appearance of the spores is due to the fact that each is surrounded by an outer covering which splits spirally into four branches arranged cross-like at one end of the spore. These are exceedingly hygroscopic, and they absorb water and coil up at the slightest breath; then uncoiling as soon as the moisture dries, they jostle each other about. The purpose of this mechanism is to scatter the spores gradually from the sporangia at maturity. The second type of Equisetum grows on banks with stiff, jointed, rush-like, often clustered stems surrounded at the joints by a light-colored sheath. These also bear cones in late spring similar to those of the field-horsetail. These plants are sometimes called “scouring-rushes,” from the fact that their roughness caused by silica in the epidermis led to their use for scouring floors and woodenware.

III. Lycopodinæ. A. Isoetaceæ. — This order likewise contains a single genus (Isoetes) and family. The plants are aquatic, either growing submerged or on the shores of ponds and rivers. They consist of a short fleshy base, bearing a cluster of leaves which are broadened at the base and contain an axillary sporangium, and end in awl-like joints. The position of the order is problematical, as these obscure plants seem to show unexpected affinities with several other groups. They are cusporangiate and produce spores of two sorts.

B. Lycopodiaceæ. — This order contains the ground-pines or club-mosses and the selaginellas or little club-mosses, which are alike in producing their spores in the axils of leaves, but differ in the spore-characters. The club-mosses (Lycopodium) have yellow dust-like spores, which are used commercially under the name of lycopodium powder for the relief of chafing and for producing flash-lights, since they contain a large amount of oil and are highly inflammable. The lycopods are familiar as Christmas greens, but are rarely seen in cultivation. Over 100 species are known of which at least 12 are found within the limits of the United States.

C. Selaginellaceæ. — This order, whose sole extant representatives belong to the genus Selaginella, is distinguished from all extant Lycopodaceæ by the possession of two kinds of spores — microspores, from which develop the male prothalli, and macrospores, from which develop the female prothalli. Selaginella is frequent in cultivation, some of the species being exceedingly delicate and graceful; but most of our native species are inconspicuous and little known. The “resurrection-plant” has already been mentioned. Some of the tropical species attain enormous size as climbing-plants. Over 600 species have been described, of which at least 12 are found in the United States.

IV. Psilotaceæ. — This group of Pteridophytes, which is represented by Psilotum, a leafy epiphyte in our Southern States, used formerly to be assigned to the Lycopedinæ, but it is now recognized that its closest allies are to be found in the Spenaphyllinæ, an extinct class of Pteridophytes intermediate betwen the Lycopodinæ and the Equisetinæ.

Fossil Pteridophytes are abundant in the Carboniferous period, when indeed they dominated the entire terrestrial flora, assuming arboreal proportions. Equisetinæ, Lycopodinæ and Filicinæ all have their representatives. Many groups possessed two types of spores whose present relatives have but one. See Palæobotany.

Affinities. — We have already seen that the Cycadofilices form a transition between the Pteridophytes and the flowering plants. The origin of the Pteridophytes is perhaps to be sought in some such liverwort as Anthoceras, whose sporophyte begins to show signs of an independent existence, and in which the spore-bearing tissue gives indications of a segmentation and a superficial position.