Botany - B.Ye. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter VII. Systematics of Higher Plants
7.7. Division Polypodiophyta

Ferns (Polypodiophyta) belong to the most ancient groups of vascular plants and share approximately the same geological age as horsetails. Emerging in the middle Paleozoic, they played a vital role in Earth's plant cover throughout the Carboniferous period. Although their ecological significance is somewhat diminished in modern flora, ferns are distributed globally and inhabit a wide range of environments. Their highest species diversity is concentrated in tropical forests. Adapting to diverse environmental conditions, ferns have evolved various life forms whose external and internal structures are more complex than those of earlier groups of lower spore-bearing plants. Their size ranges from tree-like forms to tiny herbaceous plants. In temperate climates, they exist as perennial herbs. The sporophyte dominates the developmental cycle; it features adventitious roots, and the stem may be either aerial or subterranean, taking the form of a rhizome. The Vascular System is structured as a siphonostele.

Unlike the microphylls (phylloids) of clubmosses, fern leaves (fronds) are of stem origin and morphologically correspond to fused telomes of their likely ancestors, the Rhyniophytes.

In many ferns, sori develop on the lower surface of the leaves, representing clusters of sporangia covered by a protective membrane known as an indusium. Strobili are absent. Spores mature within the sporangia, initiating the haploid phase of the fern life cycle. This phase concludes with The formation of Gametes produced on the gametophyte (prothallium), which develops from a germinated spore.

Fern gametophytes exhibit great diversity. In homosporous species, gametophytes are multicellular, bisexual, subterranean or aerial, and last for one or two years. In heterosporous species, they are microscopic, unisexual, reduced, and develop entirely within the micro- and megaspores. Water is strictly required for Fertilization. A zygote develops into an embryo, which subsequently grows into the mature sporophyte.

According to current scientific understanding, the division Polypodiophyta is divided into seven classes. Four of these—Anerophytopsida, Archaeopteridopsida, Cladoxylopsida, and Zygopteridopsida—are exclusively extinct forms. The remaining three—Ophioglossopsida (adder's-tongues), Marattiopsida, and Polypodiopsida—comprise extant ferns, encompassing 300 genera and over 12,000 species. Ukraine's native flora includes 57 species.

Fossil ferns are known from the late Early to the mid-Devonian periods; some possessed dichotomous branching and lacked true leaves, whereas later forms already featured a cambium and true dorsiventral leaves. Certain representatives (Archaeopteris) were heterosporous.

Class Ophioglossopsida (Ophioglossopsida). Extant ferns originating from homosporous Paleozoic fossil forms. These are perennial aerial plants, occasionally epiphytes. The rhizome is short, and the leaves lack the circinate vernation (coiling) typical of fern buds; they grow slowly and, when fully expanded, are dichotomously divided into a vegetative (sterile) and a spore-bearing (fertile) part. The fertile segments bear sporangia that fuse into synangia.

The walls of the sporangia are thick, developing from groups of epidermal or subepidermal Cells, lack an annulus, and dehisce via Valves.

Spores germinate after a specific dormancy period. The gametophytes are subterranean, perennial, bisexual, and obtain Nutrition through an endophytic fungus; sex Organs are scattered across their surface. Fertilization depends on water.

The most widespread genera of these ferns are Ophioglossum (adder's-Tongue) and Botrychium (moonwort). In Ukraine, they are rare and require conservation.

Class Marattiopsida (Marattiopsida). During the Carboniferous and Permian periods, they occupied vast areas of the globe and locally dominated the vegetation cover. Tree-like forms with fronds reaching 4–5 meters in length were common. Extant species are few. These are typically tropical, terrestrial plants of striking exotic beauty. Their stems are mostly fleshy and thick, taking the form of tubers, rhizomes, or short trunks. The leaves are pinnate, and the sporangia are thick-walled, lacking annuli, and opening via pores or slits. The gametophyte reaches up to 2–3 cm in diameter and is perennial. Typical genera include Marattia, Angiopteris, and Danaea.

Class Polypodiopsida. These are modern ferns whose geological history begins in the Carboniferous period. They are predominantly perennial plants represented by small trees, shrubs, herbs, lianas, and epiphytes. Young fronds are spirally coiled prior to expansion. Sporangia are gathered into sori on the lower surface of the leaves and protected by an indusium. The walls of the sporangia develop from a single epidermal Cell, resulting in a single-layered wall equipped with a mechanical annulus composed of thicker-walled cells, which AIDS in opening the sporangium and shedding spores. The Polypodiopsida include both homosporous and heterosporous species, the latter possessing a distinct Structure of sporangia and sori.

The class is divided into three subclasses: Polypodiidae, Marsileidae, and Salviniidae.

The subclass Polypodiidae is represented by homosporous species distributed across tropical, subtropical, and temperate latitudes. In the tropics and subtropics, all life forms are present—ranging from trees to lianas and epiphytes—whereas in temperate zones, they are perennial herbs characterized mostly by a subterranean rhizome from which large, often pinnate leaves emerge. Sori are located on the underside of the leaves, and their shape and coloration serve as systematic diagnostic traits. Upon maturation, the spores are released and, upon reaching moist soil, germinate into a gametophyte (prothallium). The prothallium is green, aerial, plate-like, and anchored to the soil by a tuft of rhizoids. Antheridia and archegonia are located on the underside of the thallus; the multiflagellate spermatozoa reach the archegonium exclusively via water. A zygote develops into an embryo and subsequently into the sporophyte. In addition to sexual reproduction and asexual propagation, some species can form gemmae (brood buds).

In the flora of Ukraine, the most frequent representatives belong to the genera Dryopteris (wood-ferns), Pteridium aquilinum (bracken), Athyrium filix-femina (lady fern), and others.

Heterosporous forms belong to the subclasses Marsileidae and Salviniidae.

Subclass Marsileidae comprises perennial plants whose sporophytes feature a rhizome with adventitious roots and bilobed or acicular leaves. Primarily aquatic, these plants are characterized by specialized spore-bearing organs called sporocarps, which possess a very dense wall and enclose sori. There are two types of sori: microsori containing microsporangia (each holding 64 microspores) and megasori developing megasporangia, each carrying only a single megaspore. The spores are released into water, where unisexual gametophytes develop within a day. As in heterosporous lycophytes, the gametophytes develop entirely within the spore wall: the microspore develops into a microscopic male gametophyte, and the megaspore into a female one. Spore walls rupture only after gamete formation. Fertilization takes place in water.

Ukraine has only two representatives of this class: Marsilea quadrifolia (water clover) and Pilularia globulifera (pillwort). Both species are listed in the Red Data Book.

Subclass Salviniidae consists entirely of aquatic plants. Ukraine is home to a single species, Salvinia natans (floating fern), which inhabits stagnant water bodies or slow-flowing rivers. The sporophyte floats on the water's surface and bears Two Types of leaves: submerged leaves dissected into long linear segments that function as roots, and oval, green aerial leaves. In late summer, sori resembling small spheres form at the Base of the submerged leaves. There are two types of sori: microsori, which bear microsporangia containing numerous microspores, and megasori, featuring megasporangia that mature a single megaspore. The sori possess a double wall; they sink to the bottom of the water body, their walls decay in the spring, and the spores float to the surface. Gametophyte development occurs inside the spores, and the spore wall ruptures following the formation of antheridia and archegonia. As in all heterosporous species, the gametophytes are highly reduced.

Thus, ferns represent the macrophyllous evolutionary Lineage of lower spore-bearing plants that are exceptionally well adapted to terrestrial life. They possess true leaves of telomal origin and a more advanced vascular system, enabling them to occupy diverse ecological niches, establish new life forms, and solidify heterospory and cross-fertilization.