Polypodiophyta


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Polypodiophyta

(pŏl'ēpō'dēŏf`ətə), division of the plant kingdom consisting of the plants commonly called fernsfern,
any plant of the division Polypodiophyta. Fern species, numbering several thousand, are found throughout the world but are especially abundant in tropical rain forests. The ferns and their relatives (e.g.
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. The ferns are vascular plants with stems, roots, and leaves. The small and inconspicuous gametophytegametophyte
, phase of plant life cycles in which the gametes, i.e., egg and sperm, are produced. The gametophyte is haploid, that is, each cell contains a single complete set of chromosomes, and arises from the germination of a haploid spore.
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 and the large spore-producing fern plant are quite independent of each other. The sporophyte plant, which is the plant form popularly recognized as a fern, may have an erect stem of more than 50 ft (16 m) in height, or a prostrate stem lying in or on the ground. The anatomy of fern stems, especially in the arrangement of the vascular bundles, differs greatly from group to group and is used as a means of interpreting the evolutionary relationships of the various groups. Typically, the leaf, or frond, is large and much divided, although many ferns have simple leaves, i.e., leaves with the blade undivided. Fern leaves generally unroll as they develop from a coiled early bud stage called the fiddlehead or crozier. Sporangia, the spore-producing structures, are generally found on the back of the leaf, but occasionally occur on special structures, which are probably evolutionarily modified leaves. In the great majority of ferns, the spore cases, or sporangia, are produced in groups, with each group called a sorus. These sori can often be seen on the back of the leaves. The sporangia in the sorus are usually protected in some manner, sometimes by an umbrellalike structure, the indusium, and sometimes by the inrolling of the leaf edge. The sporangium consists of a jacket of thin cells, partly surrounded at one side by a row of very thick-walled cells, the annulus. When the spores are mature, a springlike mechanism in the annulus serves to tear open the sporangium and eject the spores. Germinating spores produce a green, thin, sometimes heart-shaped, gametophyte, or prothallus, on the lower side of which are produced the sex organs, the sperm-producing antheridia and the egg-producing archegonia. The gametophytes are thin and delicate and thrive only in moist places; the lower side usually has a film of water, which facilitates the swimming of the motile, flagellated sperm from an antheridium to the neck of an archegonium and to the egg within it. The resulting diploid zygote develops slowly into a mature sporophyte with stem, root, and leaves. Ferns were abundant in Carboniferous times, and many are known only from fossil records. Of the 10,000 known species of living ferns, almost all belong to the order Filicales (true ferns). The grape ferns and the adder's-tongue ferns, of the order Ophioglossales, are very few in number. The genus Marsilea, of the order Marsileales, and the genera Salvinia and Azolla, both of the order Salviniales, have complex life histories. Marsilea grows in wet places, and Salvinia and Azolla float on the surfaces of ponds and lakes.

Polypodiophyta

A division of the plant kingdom, commonly called the ferns, which is widely distributed throughout the world but is most abundant and varied in moist, tropical regions, The Polypodiophyta are sometimes treated as a class Polypodiopsida of a broadly defined division Tracheophyta (vascular plants). The group consists of five living orders (Ophioglossales, Marattiales, Polypodiales, Marsileales, and Salviniales), plus several orders represented only by Paleozoic fossils. The vast majority of the nearly 10,000 species belong to the single order Polypodiales, sometimes also called Filicales.

The Polypodiophyta ordinarily have well-developed roots, stems, and leaves that contain xylem and phloem as conducting tissues. The central cylinder of vascular tissue in the stem usually has well-defined parenchymatous leaf gaps where the leaf traces depart from it. The leaves are spirally arranged on the stem and are usually relatively large, with an evidently branching vascular system. In most kinds of ferns the leaves, called fronds, are compound or dissected. See Leaf, Phloem, Root (botany), Stem, Xylem

The Polypodiophyta show a well-developed alternation of generations, both the sporophyte and the gametophyte generation being detached and physiologically independent of each other at maturity. The sporophyte is much the more conspicuous, and is the generally recognized fern plant. On some or all of its leaves it produces tiny sporangia which in turn contain spores. See Reproduction (plant)

Polypodiophyta

 

(ferns), a division of higher plants that occupies an intermediate position between Rhyniophyta and Gymnospermae. Polypodiophyta differ from Rhyniophyta chiefly by the presence of roots and leaves and from Gymnospermae by the absence of seeds. Polypodiophyta are descended from Rhyniophyta to which the most ancient Devonian Polypodiophyta were very closely related. (Some of the most primitive genera were in essence intermediate forms between Rhyniophyta and typical Polypodiophyta.)

Polypodiophyta, like other higher plants, are characterized by the alternation of asexual (sporophyte) and sexual (gameto-phyte) generations. The asexual generation is the more dominant. The sporophyte is an herbaceous or treelike plant, usually having large, dissected leaves. The young leaves are usually curled like snails. The leaves originated from dichotomous branchings of Rhyniophyta, whose flattening and limitation of growth was followed by differentiation of the upper and lower surfaces of the leaf blade. This is confirmed by the prolonged apical growth, large size, and complex dissection of leaves of Devonian Polypodiophyta. The stems of the most ancient Devonian species differed little from the typical stems of Rhyniophyta. They had a protostelic vascular system, which has been retained in some extant species (Schizaeaceae and many Glei-cheniaceae). In most extant species the vascular system consists of a siphonostele or, more often, a dictyostele. The majority lack the capacity for secondary growth of the stem. The xylem usually consists of tracheids; however, Pteridium and Marsilea also have true vessels. The plants have adventitious roots, which probably originated from the modified subterranean rhizoma-tous organs, or rhizomoids, of Rhyniophyta.

Polypodiophyta vary greatly in shape, internal structure, and size. The leaves, which vary from many-pinnate to entire, range in size from gigantic (some representatives of Marattiaceae and Cyatheaceae, 5-6 m long; the climbing leaves of Lygodium articulatum of the family Schizaeaceae, up to 30 m) to tiny (Trichomanes goebelianum of the family Hymenophyllaceae, 3-4 mm). Some small leaves are made up of a single layer of cells. The stems vary in length from a few centimeters to 20-25 m (some species of the genus Cyathea). They are subterranean or aboveground, erect or climbing, and simple or branched. In most Polypodiophyta the sporangia are located on ordinary green leaves. In some the leaves are differentiated into those that are sporebearing (sporophylls) and those that are vegetative (chlorophyll-bearing). In some plants, such as Osmunda, the same leaf may be differentiated into fertile and vegetative segments. Originally, the sporangia were borne by the apices of the leaf segments, that is, terminally; as the dichotomously branched leaf flattened, the sporangia moved to the margins. This is characteristic of plants of the families Hymenophyllaceae and Dick-soniaceae. In most extant Polypodiophyta the sporangia are located on the underside of the leaf. Originally they were borne on the leaf singly (as in the extant family Schizaeaceae), but in the course of evolution they became grouped in sori, which are naked in primitive Polypodiophyta (Marattiaceae, Osmun-daceae, and Gleicheniaceae) and have a protective covering known as an indusium in most extant species. In some genera of the family Marattiaceae the sporangia in the sorus have cohered, forming a synangium. Primitive Polypodiophyta (Ophioglos-saceae, Marattiaceae) have relatively large sporangia with a multilayer wall and a large number of spores; the majority of extant Polypodiophyta have smaller sporangia, which have a single-layered wall and produce a small number of spores. A large proportion of extant Polypodiophyta have a mechanism, usually an annulus, for opening the sporangium.

The majority of Polypodiophyta are isosporous; heterospor-ous forms, in which the spores are differentiated into microspores and megaspores, arose only in a few lines of their evolution in the Paleozoic. Among extant Polypodiophyta only three small aquatic families are heterosporous: Marsiliaceae, Sal-viniaceae, and Azollaceae. In isosporous species the spore sprouts to form a bisexual gametophyte, which in most species consists of a tender, green, short-lived thallus that is somewhat heart-shaped. The sex organs—archegonia and antheridia—are distributed on the thallus, mostly on its underside. In heterosporous species, the gametophytes are unisexual and greatly reduced. The microspores give rise to a tiny male gametophyte, and the megaspores develop into a somewhat larger female gametophyte. The spermatozoids of Polypodiophyta are multiflagellate, as in sago palms and gingkoes. Polypodiophyta multiply vegetatively by rhizomes and by brood buds that appear on the leaves.

The division Polypodiophyta includes the single class Polypodiopsida, which is divided into eight subclasses, including three that became extinct sometime between the Devonian and the Permian. The extinct subclasses are the Protopteridiidae (most similar to the Rhiniophyta), Archaeopteridiidae, and No-eggerathiidae. Extant Polypodiophyta are represented by 300 genera, embracing approximately 10,000 species. The most primitive extant representatives are the Ophioglossidae, which include 70 species in four genera, and Marattiidae, which embrace about 190 species in six genera.

The most extensive extant subclass is Polypodiidae (Filicidae), which includes about 300 genera (up to 10,000 species). Most of the families belonging to this subclass have been traced back only to the late Triassic. However, Osmundaceae have existed since the Permian, and Schizaeaceae have been traced back to the Carboniferous. Beginning in the late Triassic, almost all families of the subclass Polypodiida developed intensively; many flourished in the early Cretaceous. Species grow in all parts of the world, mainly in humid tropics and subtropics. They constitute an important part of the world’s vegetation, especially in moist ravines and mountain forests. In the temperate zone, the plants grow most often in shady forests, deep ravines, and marshes. Some of them are xerophytes that grow on rocks or on slopes between rocks. Other species grow in water; a few are encountered amid mangrove vegetation.

In tropical countries there are very many epiphytic Polypodiophyta, mainly of the families Hymenophyllaceae and Polypodiaceae. Representatives of the family Cyatheaceae are predominantly treelike, often having very tall trunks (up to 20–25 m) and a crown of twice or thrice pinnate leaves. The largest genus, Cyathea, includes more than 600 species, which grow mainly in mountains of the humid tropics and in Chile, New Zealand, and Southern Africa. Some New Zealand species grow on South Island near the Franz Josef Glacier and on the subantarctic Auckland Islands. In Tasmania, Dicksonia antarctica reaches 45° S lat. In the USSR, the subclass Polypodiidae is represented by the families Osmundaceae, Hymenophyllaceae, Pteridaceae, Aspleniaceae, Aspidiaceae, Blechniaceae, Polypodiaceae, and Plagiogyriaceae. Two other subclasses, Marsileidae and Salviniidae, are encountered in the Soviet Union. The first has a single family, Marsiliaceae, which is represented in the USSR by the genera Marsilea and Pilularia. The second subclass has two families, Salviniaceae and Asollaceae. The single genus of the family Salviniaceae, Salvinia, grows in the USSR. Both subclasses are descended from the isosporous Polypodiida and consist of aquatic plants. Marsiliaceae attach themselves by the roots to the substrate, and the sporebearing section of the leaf is converted into a closed receptacle of sori, or a sporocarp. Salviniaceae and Azollaceae float on the water, and their sori are borne by leaves immersed in the water (Salvinia) or on a segment of a bipartite leaf (Azolla).

The economic significance of Polypodiophyta is minor. Many species are raised as ornamentals, and others are cultivated for food. The leaves of a Malaysian Asplenium, the young shoots of a certain species of Pteridium, and the young leaf buds and the heartwood of some arboreal Polypodiophyta are used as vegetables. The rhizomes of species of Dryopteris, Polypodium, and other genera are used medicinally; for example, the rhizomes of D. filix-mas are used as a remedy for tapeworms. Some Polypodiophyta are poisonous.

REFERENCES

Golenkin, M. I. Kurs vysshikh rastenii. Moscow-Leningrad, 1937.
Takhtadzhian, A. L. Vysshie rasteniia, vol. 1. Moscow-Leningrad, 1956.
Krishtofovich, A. N. Paleobotanika, 4th ed. Leningrad, 1957.
Osnovy paleontologii: Vodorosli, mokhoobraznye, psilofitovye, plaunovidnye, chlenistostebel’nye, paporotniki. Moscow, 1963.
Bower, F. O. The Ferns (Filicales), vols. 1–3. Cambridge, 1923–28.
Manual of Pteridology. Edited by F. Verdoorn. The Hague, 1938.
Holttum, R. E. “The Classification of Ferns.” Biological Reviews of the Cambridge Philosophical Society, 1949, vol. 24, no. 3.
Smith, G. M. Cryptogamic Botany, 2nd ed, vol. 2. London, 1955.
Sporne, K. R. The Morphology of Pteridiophytes, 2nd ed. London, 1966.
Bierhorst, D. W. Morphology of Vascular Plants. New York-London, 1971.

A. L. TAKHTADZHIAN [19—468–1]

Polypodiophyta

[‚päl·i‚päd·ē′äf·əd·ē]
(botany)
The ferns, a division of the plant kingdom having well-developed roots, stems, and leaves that contain xylem and phloem and show well-developed alternation of generations.
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