Sporophyte-Dominant Life Cycle in Spore Plants

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Sporophyte-dominant life cycle (sporophyte-dominant cycle) in spore plants is a type of life cycle typical of most vascular spore plants (ferns, horsetails, club mosses), in which the diploid phase – the sporophyte – is the dominant, large, long‑lived and completely independent organism, while the haploid phase – the gametophyte (prothallus) – is represented by a small, short‑lived but also free‑living organism, dependent on external conditions (especially the presence of liquid water) for sexual reproduction (Raven et al., 2013; Mauseth, 2017).

In contrast to bryophytes, where the gametophyte dominates and the sporophyte is parasitic on it, in ferns (Monilophyta) the sporophyte is completely autotrophic and morphologically complex. It possesses true roots, stems, and large leaves (fronds) on which the organs of asexual reproduction – sporangia – are formed (Schneider, 2013). The gametophyte (prothallus) is a small (usually a few millimetres), heart‑shaped or ribbon‑like thallus that lives independently but, unlike the sporophyte, lacks vascular tissues and requires a water film for sperm movement.

It is precisely the ferns (in a broad sense, including horsetails and adder’s‑tongues) that serve as the classic model for studying the sporophyte‑dominant cycle, because here the alternation of generations is most contrasting and all stages are easily observable in nature and in laboratory culture (Raven et al., 2013). In this article we will examine this type of cycle using the example of ferns (class Polypodiopsida), which constitute the vast majority of modern spore‑bearing vascular plants (more than 10,000 species) and exhibit all the key adaptations associated with sporophyte dominance.

1. General Characteristics of the Life Cycle of Ferns (Alternation of Generations)

Life cycle of Ceratopteris richardii — CC BY 4.0
Life cycle of a homosporous fern (using <span lang="la" class="biological-name">Ceratopteris richardii</span> as an example)
The diagram shows alternation of generations: the diploid sporophyte (2n) produces sporangia with spores, from which haploid gametophytes (n) develop. After fertilisation, a new sporophyte is formed. Note the dimorphism of gametophytes – hermaphroditic (with meristem) and male (without meristem).

The life cycle of ferns, like that of all higher plants, represents a heteromorphic alternation of generations – a regular succession of two fundamentally different forms: the diploid (2n) asexual generation, the sporophyte, and the haploid (n) sexual generation, the gametophyte (Lersten, 2004; Raven et al., 2013). Ferns are characterised by a clear alternation of nuclear phases: the diploid sporophyte produces haploid spores by meiosis, from which the haploid gametophyte develops; the gametophyte produces gametes by mitosis, and after fertilisation (fusion of gametes) the diploid chromosome number is restored in the zygote, which gives rise to a new sporophyte (Raven et al., 2013; Mauseth, 2017).

The fundamental difference between ferns and bryophytes is that in ferns the sporophyte dominates (Raven et al., 2013). It is the sporophyte – the large, complexly organised, long‑lived and fully autotrophic plant – that we commonly call a “fern”. The gametophyte is a small (usually 2–10 mm) free‑living prothallus that, although capable of photosynthesis, lacks true vascular tissues and lives only a few weeks or months (Pinson et al., 2017).

Schematically, the cycle can be represented as a sequence:

The mature diploid sporophyte (fern) forms sporangia on the underside of its leaves (fronds).
In the sporangia, haploid spores are produced by meiosis.
The spores are dispersed and, when they land in a moist environment, germinate into a haploid gametophyte – the prothallus.
On the prothallus, sexual organs develop: antheridia (male, producing multiflagellate spermatozoids) and archegonia (female, containing an egg cell).
Fertilisation requires liquid water, in which the spermatozoid swims to the archegonium and fuses with the egg cell, forming a diploid zygote.
From the zygote, a young sporophyte develops on the prothallus; it initially receives nourishment from the gametophyte but soon roots and becomes independent. The gametophyte then dies off (Raven et al., 2013; Mauseth, 2017).

Thus, in the life cycle of ferns, the sporophyte and gametophyte exist as independent organisms, but the sporophyte greatly surpasses the gametophyte in size, structural complexity and lifespan. This sporophyte‑dominant type of development was an evolutionary prerequisite for further land colonisation and the emergence of seed plants (Schneider, 2013).

2. The Sporophyte – the Dominant Generation

In ferns, the sporophyte is not only the dominant but also evolutionarily the most “advanced” phase of the life cycle. It is the sporophyte that possesses a full set of vegetative organs, a vascular system, and the capacity for prolonged indeterminate growth (Raven et al., 2013; Mauseth, 2017). Sporophyte dominance is expressed in three main aspects:

Morphological dominance: the sporophyte is a large plant, often reaching from several decimetres to many metres in height (in tree ferns), while the gametophyte is a microscopic or nearly microscopic thallus (Schneider, 2013).
Physiological dominance: the sporophyte is fully autotrophic, with a developed conducting system (xylem and phloem), stomata and mechanical tissues, allowing it to efficiently colonise diverse, including dry, habitats (Mauseth, 2017).
Temporal dominance: the sporophyte is a perennial plant, living from several years to many decades (in tree forms). The gametophyte lives only a few weeks or months (Pinson et al., 2017).

Such dominance became possible through the evolutionary reduction of the gametophyte and simultaneous elaboration of the sporophyte, which reduced the dependence of reproduction on water – although water is still required for fertilisation (Raven et al., 2013).

2.1. Structure of the Sporophyte (Morphologically)

Fertile frond of Polypodium — commons.wikimedia.org CC BY 4.0
Fertile frond of the fern <span lang="la" class="biological-name">Polypodium virginianum</span> with numerous sori
On the underside of the frond, rounded sori – clusters of sporangia – are clearly visible. Each sorus contains dozens of sporangia in which spores develop. Leaves of this type, bearing sporangia, are called _sporophylls_.

The mature sporophyte of ferns (using typical representatives of the class Polypodiopsida, e.g. shield ferns – Dryopteris, or bracken – Pteridium) is morphologically divided into three main organs: root, stem and leaf (Raven et al., 2013). In the vast majority of temperate ferns, the stem is a rhizome – an underground or creeping perennial shoot (Mauseth, 2017). The rhizome bears numerous adventitious roots that anchor the plant and provide water and mineral nutrition. In tree ferns (e.g. Cyathea), the stem is erect and woody but lacks secondary growth (Schneider, 2013).

Fern leaves are called fronds (from French). They are usually large, pinnately or multi‑pinnately divided blades that grow by their tips and are coiled like a fiddlehead (circinate vernation) when young (Raven et al., 2013). This form of coiling protects the delicate apical meristematic margin during growth. Fronds perform two main functions: photosynthesis and spore production. In most ferns, fronds are monomorphic – they both photosynthesise and bear sporangia. However, in some species, leaf dimorphism occurs: sterile (trophophylls) – purely vegetative, and fertile (sporophylls) – specialised for spore formation (e.g. in the ostrich fern – Matteuccia struthiopteris) (Mauseth, 2017; Raven et al., 2013).

On the underside of the fronds (rarely on the margins or upper side) are sporangia – organs of asexual reproduction. Often sporangia are grouped into compact clusters – sori, which in many species are covered by a protective membrane – the indusium (Raven et al., 2013). The shape, arrangement of sori and presence/absence of an indusium are important taxonomic characters of ferns (Schneider, 2013).

Thus, the fern sporophyte is a complex vascular plant, morphologically comparable in organisation to seed plants, but lacking secondary growth (in herbaceous forms) and seeds. This morphological complexity is a direct expression of sporophyte dominance in the life cycle.

2.2. Spore Formation (Meiosis)

Fern sorus under microscope — commons.wikimedia.org CC BY-SA 4.0
Sorus of the fern <span lang="la" class="biological-name">Polypodium aureum</span> under the microscope
The sorus consists of a group of sporangia covered by an indusium. On each sporangium, the annulus – a specialised structure that ensures spore catapulting at maturity – is clearly visible.

The key event in the life of the sporophyte is sporogenesis, i.e. the formation of haploid spores in the sporangia. In ferns, sporangia are of two types: leptosporangia (characteristic of most ferns of the class Polypodiopsida) or eusporangia (in some primitive groups, e.g. Osmundales, Marattiales) (Raven et al., 2013; Mauseth, 2017). In the vast majority of ferns, which will serve as our main example, leptosporangia develop: each sporangium arises from a single superficial cell, has a thin one‑layered wall and a stalk, and inside it forms 16–64 spores (Raven et al., 2013). In eusporangiate ferns, sporangia are more massive, develop from a group of cells and contain hundreds of spores (Schneider, 2013).

Fern leptosporangium with annulus — ©David Monniaux commons.wikimedia.org CC BY-SA 3.0
Fern leptosporangium with an annulus
The leptosporangium has a characteristic structure: the annulus – a row of cells with thickened walls – serves to catapult spores when mature. As the annulus cells dry out, they contract, the sporangium wall ruptures, and the spores are ejected.

Inside the sporangium is the spore‑forming tissue (archesporium). The cells of this tissue (sporocytes, or spore mother cells) have a diploid chromosome number (2n). It is in these cells that meiosis (reduction division) occurs, resulting in a tetrad of four haploid spores (n) from each diploid mother cell (Raven et al., 2013; Mauseth, 2017). This process marks the transition from the diploid phase (sporophyte) to the haploid phase (gametophyte).

In most ferns, all spores in a sporangium are equal in size and shape – these are homosporous (isosporous) plants (Raven et al., 2013). Only in a few aquatic ferns (Salviniales) is heterospory expressed: in some sporangia numerous small microspores are formed, while in others a few (often 1–4) large megaspores are produced. However, in the overwhelming majority of ferns (over 99%), spores are isomorphic. Fern spores have a tough outer coat (exine or perine), often with a sculptured surface, which aids in preservation and wind dispersal (Raven et al., 2013).

It is important to note that the sporophyte‑dominant cycle is inextricably linked with meiosis, which occurs not at fertilisation (as in many algae) but at the asexual reproductive stage, as is characteristic of all higher plants (Lersten, 2004).

2.3. Spore Dispersal and Germination Conditions

After meiosis is completed, the sporangium opens. In leptosporangiate ferns, opening is brought about by a specialised structure – the annulus – a strip of cells with thickened walls on the sporangium wall. As it dries, the annulus ruptures the sporangium and catapults the spores (Mauseth, 2017). In eusporangiate ferns, opening occurs through ruptures in the wall without a catapulting mechanism (Raven et al., 2013).

Spores are mainly dispersed by wind (anemochory) and, thanks to their small size (usually 30–70 µm) and air‑filled cavities, can be carried over long distances (Schneider, 2013). Once they land on a suitable substrate, the spore must germinate.

Three main conditions are required for spore germination:

Liquid water – the spore must absorb water to activate metabolism (Raven et al., 2013).
Light – in most ferns spores germinate only in the light (although some species can germinate in the dark in the presence of additional substances – antheridiogens) (Hornych et al., 2021; Atallah & Banks, 2015).
Suitable temperature (usually 20–25 °C) and air humidity (not below 70–80%) (Mauseth, 2017).

Germination begins with spore swelling and rupture of the exine. The first to emerge is a rhizoid (a colourless cell that serves for attachment), followed by a protonemal (or pre‑thallus) cell, which starts dividing and forms a filamentous stage (protonema). From the protonema, the adult gametophyte – the prothallus – develops (Raven et al., 2013). It is important to emphasise that the spore is the first cell of the haploid generation, and its germination marks the beginning of the gametophyte phase of the life cycle.

3. The Gametophyte (Prothallus) – a Temporary and Environment‑Dependent Generation

In contrast to the sporophyte, the fern gametophyte (also called the prothallus) is a small, usually short‑lived and simply organised plant. It represents the haploid (n) phase of the life cycle, on which sexual organs are formed and fertilisation occurs (Raven et al., 2013). Although the gametophyte is photosynthetic and lives independently of the sporophyte (unlike the gametophyte of seed plants), its size and organisational complexity are incomparable to the sporophyte. This makes it a “temporary” and ecologically more vulnerable generation, critically dependent on external conditions – above all, the presence of liquid water for fertilisation and shaded, moist substrates for growth (Pinson et al., 2017).

Ferns show considerable diversity in gametophyte forms: from typical heart‑shaped (in most terrestrial Polypodiopsida) to ribbon‑like, filamentous or even subterranean non‑chlorophyllous types (in some primitive groups, e.g. Psilotales, Ophioglossales) (Schneider, 2013; Pinson et al., 2017). However, the classic example is the green heart‑shaped prothallus, easily observable in spore sowings of ferns.

3.1. Spore Germination and Prothallus Structure

The development of the gametophyte begins with the germination of the haploid spore. After swelling and emergence of the rhizoid, the first vegetative cell – the protonemal cell – divides, first forming a filamentous structure (protonema), which then, by a change in division planes, becomes a thin (often one‑cell‑thick) plate (Raven et al., 2013; Mauseth, 2017). This plate grows rapidly by cell divisions and acquires the characteristic heart‑shaped (cordate) form (Pinson et al., 2017).

The adult prothallus of ferns of the class Polypodiopsida is a green thallus, usually one cell thick (except for the central cushion). It attaches to the substrate by numerous rhizoids – colourless outgrowths that function as anchors and partly absorb water and minerals (Raven et al., 2013). On the underside (ventral side) of the prothallus, among the rhizoids, the sexual organs develop: antheridia (male) and archegonia (female) (Mauseth, 2017). The upper (dorsal) side bears stomata and actively photosynthesises.

A key feature of the gametophyte is the absence of true vascular tissues (xylem and phloem). This limits its size (usually 2 to 10 mm in diameter) and makes it dependent on direct diffusion transport (Raven et al., 2013). In addition, the prothallus lacks roots, and its cuticle is thin or absent, so it can survive only in moist environments. The lifespan of the gametophyte is short: after fertilisation and formation of the young sporophyte, the prothallus gradually dies, although in some species (especially epiphytes) gametophytes may persist longer and even reproduce vegetatively – for example, through gemmae (bud‑like propagules) (Pinson et al., 2017; Atallah & Banks, 2015).

Thus, the fern gametophyte, although free‑living and photosynthetic, is a temporary, ecologically sensitive generation. Its simplicity and small size are the result of evolutionary reduction, which allowed the sporophyte to become the dominant form in the life cycle (Raven et al., 2013).

3.2. Sexual Organs on the Gametophyte

Gametophyte of Asplenium ruta-muraria with antheridia and archegonia — CC BY 4.0
Fern gametophyte with sexual organs
(a) Male gametophyte bearing numerous antheridia (arrows). (b) Female gametophyte with archegonia. The heart‑shaped form of the prothallus and the position of the sexual organs on the ventral side are clearly visible.

On the fully formed prothallus (usually a few weeks after spore germination), multicellular sexual organs – gametangia – develop. In ferns, as in all higher plants (except flowering plants), gametangia are divided into male (antheridia) and female (archegonia) (Raven et al., 2013). This separation of sexes ensures oogamy – a type of sexual process in which a large, immobile egg cell fuses with a small, motile spermatozoid.

Antheridia (male sexual organs) are rounded or oval sac‑like structures, often on a short stalk. They are located mainly on the ventral side of the prothallus, usually among the rhizoids, closer to the margins or the base (Mauseth, 2017). The antheridial wall is one‑layered. Inside the antheridium, a large number (up to several hundred) of small, multiflagellate, spirally coiled spermatozoids (antherozoids) are produced. The structure of the spermatozoids is an important taxonomic character: in most ferns (class Polypodiopsida), spermatozoids bear 30 to 50 flagella, allowing them to move actively in water (Raven et al., 2013; Schneider, 2013). At maturity, the antheridial wall ruptures and the spermatozoids are released.

Archegonia (female sexual organs) have the classic flask‑shaped form for higher plants: a narrowed neck (canal) and a swollen venter, containing a large, immobile egg cell (one per archegonium) (Raven et al., 2013; Lersten, 2004). In the neck, canal cells are present; at maturity they become mucilaginous, creating a path for the spermatozoid. Archegonia usually develop later than antheridia and are located closer to the central notch (apical indentation) of the prothallus, often on its ventral side but in a more protected zone (Mauseth, 2017).

An important feature: in homosporous ferns, prothalli are usually hermaphroditic (bisexual), i.e. they bear both antheridia and archegonia. However, many species have mechanisms to prevent self‑fertilisation: antheridia mature earlier than archegonia (protandry) or vice versa, or chemical regulation (antheridiogens) operates (Atallah & Banks, 2015). In some species (e.g. many epiphytic ferns), gametophytes may be unisexual (Pinson et al., 2017).

Fertilisation occurs only in the presence of liquid water (dew, rain, a film of water on the prothallus surface). Spermatozoids released from the antheridium actively swim towards the archegonial neck, guided by chemical substances (e.g. malic acid) released by the maturing archegonium (Raven et al., 2013). One spermatozoid penetrates through the neck into the venter and fuses with the egg cell, forming a diploid (2n) zygote. This profound dependence on free water is the main limitation for fern distribution, retained from their aquatic ancestors.

After fertilisation, the zygote immediately begins to develop into a multicellular embryo (young sporophyte), which initially obtains nutrients from the gametophyte through a specialised structure – the haustorium (foot) – and then becomes independent (Raven et al., 2013). The gametophyte then gradually dies off, having fulfilled its function.

Thus, despite its simplicity, the fern gametophyte bears complex multicellular sexual organs, and the fertilisation process remains water‑dependent – a key difference from seed plants, which do not require water.

4. Fertilisation and Formation of a New Sporophyte

Fertilisation in ferns is possible only in the presence of liquid water, which serves as the medium for the movement of male gametes – spermatozoids (Raven et al., 2013; Mauseth, 2017). This stage is critical because it links the haploid (gametophyte) and diploid (sporophyte) phases of the life cycle.

4.1. Mechanism of Fertilisation

The mature antheridium absorbs water, its wall ruptures, and numerous multiflagellate spermatozoids (antherozoids) are released. They actively move in the water film covering the prothallus, chemotactically responding to substances released by the archegonial neck (e.g. malic acid and malates) (Raven et al., 2013). The canal cells of the archegonial neck have by then become mucilaginous, forming a passage for the spermatozoid. One spermatozoid passes through the neck into the venter and fuses with the egg cell. As a result of gamete fusion, the diploid chromosome number (2n) is restored, and a zygote is formed (Raven et al., 2013; Lersten, 2004).

Many fern species have mechanisms that prevent self‑fertilisation (inbreeding). For example, in most homosporous ferns, antheridia and archegonia on the same prothallus do not mature simultaneously (protandry – antheridia first), or sexual development is regulated by hormone‑like substances – antheridiogens – secreted by the prothalli (Atallah & Banks, 2015). Antheridiogens released by older (hermaphroditic or female) prothalli induce the development of male gametophytes from neighbouring spores, increasing the likelihood of cross‑fertilisation (Atallah & Banks, 2015; Hornych et al., 2021).

4.2. Formation of the Young Sporophyte (Embryogenesis)

After fertilisation, the zygote immediately begins to divide – without a resting period (unlike seed plants) (Raven et al., 2013). The first division of the zygote is usually transverse, giving rise to a basal cell (forming the foot or suspensor) and an apical cell (from which the embryo proper develops). The embryo differentiates into a young sporophyte with rudimentary organs: a primary root, a stemlet and the first leaf (cotyledon) (Mauseth, 2017). In ferns, the embryo does not have the distinct bipolar organisation typical of seed plants – the primary root is short‑lived and soon replaced by adventitious roots from the stem (Schneider, 2013).

In the early stages of development, the young sporophyte (embryo) is embedded in the gametophyte tissue and receives nutrients from it through a specialised structure – the haustorium (foot). As its own roots grow and the first green leaf begins photosynthesis, the young sporophyte switches to autotrophic nutrition. The gametophyte (prothallus) gradually dies off, its function having been fulfilled (Raven et al., 2013; Mauseth, 2017).

Thus, the new diploid sporophyte forms on the haploid gametophyte but soon becomes independent. This stage completes the cycle: from the sporophyte, sporangia are again formed, meiosis occurs, and the cycle repeats. Sporophyte dominance is expressed here in that it is the diploid generation that gives rise to the new plant, which will be large, complex and long‑lived.

5. Evolutionary Significance of Sporophyte Dominance (Comparison with Mosses)

To understand the evolutionary significance of sporophyte dominance, it is necessary to compare the life cycles of ferns and bryophytes (mosses, liverworts). These two groups represent two different evolutionary “experiments” in land colonisation.

In bryophytes (Bryophyta s.l.), the gametophyte (the green “moss” plant) dominates the life cycle. The sporophyte in mosses is reduced, lacks chlorophyll, is unable to photosynthesise, and is parasitic on the gametophyte throughout its life, obtaining nourishment from it (Raven et al., 2013; Яковлев et al., 2003). The gametophyte, however, is a relatively large, often leafy, but vascular‑tissue‑lacking plant. Because they lack an efficient conducting system and because fertilisation depends on liquid water, mosses remain small (usually 1–10 cm) and confined to moist habitats (Яковлев et al., 2003).

In ferns, by contrast, the sporophyte dominates – a large vascular plant with true roots, stems and leaves. The gametophyte is reduced to a small, short‑lived prothallus (see sections 3–4). A comparison of the main characteristics of the two cycle types is given in the table.

Table. Comparison of the life cycles of bryophytes and ferns (based on Raven et al., 2013; Mauseth, 2017; Яковлев et al., 2003)

Trait	Bryophytes (e.g. Polytrichum)	Ferns (e.g. Polypodium)
Dominant generation	Gametophyte (green plant)	Sporophyte (rhizomatous plant with fronds)
Sporophyte	Short‑lived, non‑independent, parasitic on gametophyte	Long‑lived (perennial), fully independent, autotrophic
Gametophyte	Large (several cm), often with leaves and rhizoids, autotrophic	Small (2–10 mm), heart‑shaped thallus, autotrophic but short‑lived
Vascular system of sporophyte	Absent or primitive (hydroids)	True vascular (xylem with tracheids, phloem)
Water dependence for fertilisation	High	High
Maximum size	Up to 20–30 cm (rarely 50 cm)	Up to 20–25 m (tree forms)

Evolutionary advantages of sporophyte dominance

The shift to sporophyte dominance was a key event in the evolution of higher plants, conferring the following advantages:

Resistance to mutations. The diploid sporophyte carries a double set of chromosomes. Harmful recessive mutations do not manifest in the phenotype because they are “masked” by normal alleles on the homologous chromosome. In the haploid gametophyte, any mutation is immediately expressed (Raven et al., 2013). This allows the sporophyte to accumulate genetic diversity without immediate risk of elimination.
Increased organisational complexity and organ specialisation. Dominance of the diploid phase created the opportunity for the development of complex tissues (xylem, phloem, mechanical tissues) and organs (roots, stems, leaves) (Mauseth, 2017; Schneider, 2013). The haploid gametophyte retained primitive features (absence of vessels, one‑layered structure), limiting its size.
Heterosis and genetic recombination. The diploid sporophyte can combine different alleles from two parents, enhancing viability and adaptive potential (heterosis) (Raven et al., 2013). Meiosis in sporangia, preceding spore formation, provides genetic recombination, producing genetically diverse offspring.
Expansion into new habitats. Thanks to a powerful conducting system and mechanical strength, the fern sporophyte can reach considerable sizes and colonise both shaded moist forests and, in some species, even relatively dry habitats (e.g. Cheilanthes, Pellaea) (Raven et al., 2013; Mauseth, 2017).

Why did ferns not outcompete seed plants?

Despite all the advantages of the sporophyte‑dominant cycle, ferns retained two archaic traits: (1) motile spermatozoids requiring liquid water for fertilisation, and (2) a free‑living gametophyte that is easily damaged by desiccation (Pinson et al., 2017). These limitations prevented ferns from becoming completely independent of water. Seed plants (gymnosperms and angiosperms) went further: their gametophyte is reduced to a few cells and is entirely protected by the sporophyte (in the pollen grain and embryo sac), and fertilisation occurs via a pollen tube, completely eliminating dependence on free water and allowing colonisation of the most diverse habitats (Raven et al., 2013). Thus, sporophyte dominance in ferns was an important evolutionary step, but not the final one; it paved the way for the emergence of seed plants.

6. Differences from Other Spore Plants (Club Mosses, Horsetails)

Ferns (Monilophyta) are not the only group of spore‑bearing vascular plants with a dominant sporophyte. A similar type of life cycle occurs in two other modern divisions: club mosses (Lycopodiophyta) and horsetails (Equisetophyta) (Raven et al., 2013; Яковлев et al., 2003). All have an independent, long‑lived, vascular sporophyte and a reduced free‑living gametophyte. However, there are important differences between these groups, related to sporophyte morphology, leaf structure and details of spore production. The table below summarises the key distinctions.

Table. Comparative characteristics of spore‑bearing vascular plants with sporophyte dominance (based on Raven et al., 2013; Mauseth, 2017; Яковлев et al., 2003)

Trait	Club mosses (Lycopodiophyta)	Horsetails (Equisetophyta)	Ferns (Monilophyta)
Leaves	Microphylls (small, with one vein, no leaf gap)	Microphylls (reduced, scale‑like, fused into a sheath)	Megaphylls (large, with branched venation and leaf gap)
Stem	Creeping (in most), dichotomously branched, protostele	Articulate (with nodes and internodes), hollow inside, siphonostele	Rhizome (in most), rarely erect (tree forms), siphonostele or dictyostele
Sporangia	In axils of sporophylls, often aggregated into strobili (cones)	On sporangiophores (shield‑shaped structures), aggregated into strobili	In sori (groups) on the underside of fronds, often with an indusium
Spore type	Predominantly homosporous (Lycopodiaceae); heterosporous (Selaginellaceae, Isoetaceae)	Homosporous	Predominantly homosporous; heterosporous only in aquatic ferns (Salviniales)
Gametophyte (prothallus)	Subterranean or semi‑subterranean, often mycorrhizal, some green	Terrestrial, green, cushion‑shaped or lobed	Terrestrial, green, heart‑shaped or ribbon‑like (in epiphytes)

Key differences between ferns and club mosses/horsetails:

Leaf type. Ferns are a megaphyllous lineage. Their large, dissected leaves (fronds) with branched venation arose from flattened and fused branch systems (telomes) (Raven et al., 2013; Mauseth, 2017). Club mosses and horsetails are microphyllous, their small leaves with a single vein having originated from superficial outgrowths (enations) of the stem (Яковлев et al., 2003). This is a fundamental difference in leaf origin.

Stem structure. Horsetails have an articulate stem with whorls of reduced leaves and sporangiophores – a unique structure not found in other plants (Mauseth, 2017).

Position of sporangia. In club mosses, sporangia are solitary, in the axils of sporophylls (often in strobili); in horsetails, they are on sporangiophores (also in strobili); while in ferns, they are in sori on the underside of ordinary leaves (or on specialised sporophylls) (Raven et al., 2013).

Heterospory. Among club mosses, there are both homosporous (Lycopodiaceae) and heterosporous (Selaginellaceae, Isoetaceae) groups. All modern horsetails are homosporous. Among ferns, heterospory occurs only in aquatic forms (Salviniales) (Raven et al., 2013).

Despite these differences, all three groups exhibit essentially the same type of life cycle: sporophyte dominance, gametophyte reduction, dependence of fertilisation on liquid water. This similarity points to their common origin from ancient vascular plants (rhyniophytes or their close relatives) and to common evolutionary trends in land colonisation (Schneider, 2013; Яковлев et al., 2003). Ferns, as the most diverse and widespread group of spore‑bearing vascular plants, serve as the best model for studying this type of life cycle.

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