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See also: Equisetopsida sensu lato

Equisetidae
Temporal range: Late Devonian[1] to Recent
An erect plant with unbranched segmented stems. Whorls of small leaves sprout from each segment, thicker at the top end and absent in the lower portion of the stem, giving it the appearance of a bottle brush or a horse's tail.
Equisetum telmateia
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Division: Polypodiophyta
Class: Polypodiopsida
Subclass: Equisetidae
Warm.
Orders
Synonyms

See text.

Equisetidae is one of the four subclasses of Polypodiopsida (ferns), a group of vascular plants with a fossil record going back to the Devonian. They are commonly known as horsetails.[2] They typically grow in wet areas, with whorls of needle-like branches radiating at regular intervals from a single vertical stem.

The Equisetidae were formerly regarded as a separate division of spore plants and called Equisetophyta, Arthrophyta, Calamophyta or Sphenophyta. When treated as a class, the names Equisetopsida s.s. and Sphenopsida have also been used. They are now recognized as rather close relatives of the ferns (Polypodiopsida) of which they form a specialized lineage.[3] However, the division between the horsetails and the other ferns is so ancient that many botanists, especially paleobotanists, still regard this group as fundamentally separate at the higher level.

Description

[edit]

The horsetails comprise photosynthesising, "segmented", hollow stems, sometimes filled with pith. At the junction ("node", see diagram) between each segment is a whorl of leaves. In the only extant genus Equisetum, these are small leaves (microphylls) with a singular vascular trace, fused into a sheath at each stem node. However, the leaves of Equisetum probably arose by the reduction of megaphylls, as evidenced by early fossil forms such as Sphenophyllum, in which the leaves are broad with branching veins.[4]

The vascular bundles trifurcate at the nodes, with the central branch becoming the vein of a microphyll, and the other two moving left and right to merge with the new branches of their neighbours.[5] The vascular system itself resembles that of the vascular plants' eustele, which evolved independently and convergently.[5] Very rapid internode elongation results in the formation of a pith cavity and a ring of carinal canals formed by disruption of the primary xylem. Similar spaces, the vallecular canals are formed in the cortex.[5] Due to the softer nature of the phloem, these are very rarely seen in fossil instances.[citation needed] In the Calamitaceae, secondary xylem (but not secondary phloem) was secreted as the cambium grew outwards, producing a woody stem, and allowing the plants to grow as high as 10m. All extant species of Equisetum are herbaceous, and have lost the ability to produce secondary growth.[5]

The underground parts of the plants consist of jointed rhizomes, from which roots and aerial axes emerge. The plants have intercalary meristems in each segment of the stem and rhizome that grow as the plant gets taller. This contrasts with most seed plants, which grow from an apical meristem - i.e. new growth comes only from growing tips (and widening of stems).

Horsetails bear cones (technically strobili, sing. strobilus) at the tips of some stems. These cones comprise spirally arranged sporangiophores, which bear sporangia at their edges, and in extant horsetails cover the spores externally - like sacs hanging from an umbrella, with its handle embedded in the axis of the cone. In extinct groups, further protection was afforded to the spores by the presence of whorls of bracts - big pointed microphylls protruding from the cone.

The extant horsetails are homosporous, but extinct heterosporous species such as Calamostachys casheana appear in the fossil record.[6] The sporangia open by lateral dehiscence to release the spores. The spores bear characteristic elaters, distinctive spring-like attachments which are hygroscopic: i.e. they change their configuration in the presence of water, helping the spores move and aiding their dispersal.

Vegetative stem:
N = node,
I = internode,
B = branch in whorl,
L = fused microphylls
Cross-section through a strobilus; sporangiophores, with attached sporangia (spore capsules) full of spores, can be discerned.
Strobilus of E. braunii, terminal on an unbranched stem

Taxonomy

[edit]

Classification

[edit]

The horsetails and their fossil relatives have long been recognized as distinct from other seedless vascular plants, such as the ferns (Polypodiopsida).[7] Before the advent of modern molecular studies, the relationship of this group to other living and fossil plants was considered problematic.[8] Because of their unclear relationships, the rank botanists assigned to the horsetails varied from order to division. When recognized as a separate division, the literature uses many possible names, including Arthrophyta,[8] Calamophyta, Sphenophyta,[1][9] or Equisetophyta. Other authors regarded the same group as a class, either within a division consisting of the vascular plants or, more recently, within an expanded fern group. When ranked as a class, the group has been termed the Equisetopsida[10] or Sphenopsida.[5]

Modern phylogenetic analysis, back to 2001, demonstrated that horsetails belong firmly within the fern clade of vascular plants.[11][12] Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era, and considered the ferns (monilophytes), to comprise four classes, with the horsetails as class Equisetopsida sensu stricto.[3] (This distinction is necessary because of the alternative usage of Equisetopsida sensu lato as a synonym for all land plants (Embryophyta) with rank of class.[13]) Chase and Reveal (2009) treated the horsetails as subclass Equisetidae of class Equisetopsida sensu lato. The consensus classification produced by the Pteridophyte Phylogeny Group in 2016 also places horsetails in the subclass Equisetidae, but in the class Polypodiopsida (ferns broadly defined).[2]

Phylogeny

[edit]

The following diagram shows a likely phylogenic relationship between subclass Equisetidae and the other fern subclasses according to the Pteridophyte Phylogeny Group.[2]

Polypodiopsida

Equisetidae (horsetails)

A 2018 study by Elgorriaga et al. suggests the relationships within the Equisetidae are as shown in the following cladogram.[14]

†Sphenophyllales

Equisetales

†Archaeocalamitaceae

A.G. clade (†Paracalamitina, †Cruciaetheca)

†Calamitaceae

According to the study, the age of the crown group of Equisetum dates at least to the Early Cretaceous, and most probably up to the Jurassic.[14]

Subdivision

[edit]

Subclass Equisetidae contains a single extant order, Equisetales. This order consists of a single monotypic family, Equisetaceae, with one genus Equisetum. Equisetum has about 20 species.[12][2]

Fossil record

[edit]

The extant horsetails represent a tiny fraction of horsetail diversity in the past. There were three orders of the Equisetidae. The Pseudoborniales first appeared in the late Devonian.[1] The Sphenophyllales were a dominant member of the Carboniferous understory, and prospered until the mid and early Permian. The Equisetales existed alongside the Sphenophyllales, but diversified as that group disappeared into extinction, gradually dwindling in diversity to today's single genus Equisetum.

The organisms first appear in the fossil record during the late Devonian,[1] a time when land plants were undergoing a rapid diversification, with roots, seeds and leaves having only just evolved. (See Evolutionary history of plants) However, plants had already been on the land for almost a hundred million years, with the first evidence of land plants dating to 475 million years ago.[15]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Equisetidae

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from Grokipedia
Equisetidae is a subclass of the class Polypodiopsida, comprising the order and several extinct orders, with living members restricted to the family and genus , commonly known as horsetails. These vascular plants are distinguished by their upright, articulate (jointed) stems that are typically hollow and ridged, with silica deposits in the providing a rough texture, and small, scalelike leaves fused into toothed sheaths at the nodes. Reproduction occurs via spores produced in terminal, cone-like strobili on specialized fertile stems, marking them as spore-bearing tracheophytes without seeds or flowers. The subclass Equisetidae has an extensive fossil record dating back to the Late Devonian period, approximately 380 million years ago, with diverse extinct forms such as the giant, tree-like of the period that could exceed 10 meters in height and played key roles in ancient swamp ecosystems. In contrast, modern species—numbering 15 to 20 worldwide—exhibit a distribution, occurring in nearly cosmopolitan but disjunct populations across temperate and regions of , , Asia, and parts of the , often in moist, disturbed, or habitats such as streambanks, meadows, and forests. These plants are rhizomatous perennials capable of forming dense colonies through extensive , and some species, like , are noted for their weedy or invasive tendencies in agricultural settings due to their deep root systems and ability to tolerate poor soils. Ecologically, horsetails contribute to and are historically significant for their abrasive stems used in scouring, while their silica content and spore-based life cycle highlight their primitive yet resilient evolutionary lineage among ferns.

Morphology and Reproduction

Vegetative Structure

Equisetidae plants exhibit a distinctive vegetative characterized by hollow, jointed stems that provide both and efficient water conduction. The stems consist of underground rhizomes and annual aerial shoots, both featuring prominent nodes and internodes that give them an articulated appearance. These stems are ribbed longitudinally, with the ridges and furrows aiding in mechanical strength, and are reinforced by silica deposits in the epidermal cell walls, which contribute to their hardness and abrasiveness. At each node, whorls of reduced leaves, known as microphylls, are arranged in a circular pattern and fused proximally to form toothed sheaths that encircle the stem. These leaves are scale-like, non-photosynthetic, and serve primarily a protective role rather than contributing significantly to . Branching occurs in whorls at the nodes, with branches emerging directly from the stem rather than from leaf axils, and the vascular connections at nodes form a zigzag pattern due to . In extant forms, such as those in the genus , branching is often profuse in vegetative shoots, enhancing surface area for light capture. Many extant Equisetidae species display stem dimorphism, with green, branched, photosynthetic vegetative shoots contrasting with non-photosynthetic, unbranched fertile stems that are typically brownish and shorter-lived. Vegetative stems in species like and are herbaceous, reaching heights of 15–60 cm, though some tropical species such as Equisetum giganteum can grow up to 5 m tall, with related species reaching 8 m. Roots arise adventitiously from rhizome nodes, forming extensive underground networks that anchor the and facilitate vegetative spread. In extinct members of Equisetidae, such as those in the Calamitaceae family (e.g., ), vegetative structures were more robust and arborescent, forming woody trees up to 20 m in height with stem diameters exceeding 60 cm. These stems retained the jointed, ribbed morphology but included secondary for added support, arising from subterranean rhizomes or lateral buds to create multibranched crowns. Leaves in whorls were more varied, including lanceolate forms (Annularia) or needle-like types (Asterophyllites), fused into sheaths similar to extant species. The vascular system in Equisetidae stems is organized as a siphonostele, with alternating bundles of and arranged in a around a central cavity, interrupted by carinal canals for water transport. At nodes, these bundles anastomose to form a continuous ring, while internodes feature discrete bundles aligned with the ribs. possess a simpler protostele, typically triarch or tetrarch, embedded in a thick cortex. This arrangement supports efficient resource allocation in both extant herbaceous forms and extinct woody lineages.

Reproductive Biology

Equisetidae exhibit homospory in all extant species, producing a single type of that develops into bisexual gametophytes. are generated in terminal strobili, compact cone-like structures that bear sporangia on specialized sporangiophores arranged in whorls. These strobili are unisexual, producing that develop into gametophytes bearing either antheridia, archegonia, or both. Each features four ribbon-like elaters, hygroscopic appendages that uncoil in dry conditions to aid dispersal by facilitating movement across surfaces or brief jumps up to 1 cm. The life cycle of Equisetidae follows an characteristic of vascular plants, with a dominant diploid phase and a reduced haploid phase./3.2.02:_Early_Land_Plants/3.2.2.03:_Seedless_Vascular_Plants/3.2.2.3.02:_Polypodiopsida) Spores germinate into small, green, photosynthetic prothalli that are thalloid and typically subterranean or surface-dwelling, bearing both antheridia and . Fertilization requires moist conditions, as multiflagellated must swim through films to reach the egg within the ; Equisetidae lack seeds and rely entirely on this free-living stage for . In contrast, some extinct Equisetidae displayed , producing distinct microspores and megaspores that gave rise to male and female s, respectively. For instance, the genus Calamostachys included heterosporous species like C. casheana, where sporangia contained dimorphic spores adapted for separate gametophyte development. This trait represents an evolutionary precursor to seed plants in some sphenophyte lineages, though it was lost in modern forms. The ultrastructure of Equisetidae spores has evolved from complex, multilayered exospores in calamites to simpler, laevigate walls in extant Equisetum, reflecting adaptations to changing dispersal environments over geological time. Early spores featured ornate perispore layers with alveolar structures, while modern ones retain basic trilete marks and elaters but with reduced ornamentation for enhanced wind dispersal.

Ecology and Distribution

Habitat Preferences

Equisetidae, commonly known as horsetails and represented primarily by the genus , exhibit a strong preference for moist, nutrient-rich soils found in wetlands, stream banks, meadows, and disturbed areas. These plants tolerate a range of from acidic (around 4.0) to neutral (up to 7.0), and they can withstand periodic flooding, which supports their establishment from spores in damp environments. In their ecological roles, Equisetidae contribute to through extensive networks that bind substrates and prevent , particularly in riparian zones. They act as in , colonizing disturbed, wet sites and facilitating nutrient cycling, including the uptake and deposition of silica that influences . Additionally, these plants exhibit allelopathic effects, releasing compounds that inhibit the and growth of nearby plant species, thereby reducing in their habitats. Adaptations such as high silica content in their tissues provide mechanical deterrence against herbivores by abrading mouthparts and enhance structural integrity, while some species demonstrate tolerance to through efficient water storage in rhizomes. Extant species often thrive in partial shade, allowing them to occupy forest edges and positions alongside open areas. Their deep rhizomes, extending up to 6 feet underground, contribute to invasive potential in agricultural settings, where they can persist and spread despite management efforts. As non-flowering plants, Equisetidae lack interactions with pollinators, and they form limited or atypical associations with mycorrhizal fungi compared to other vascular plants.

Global Distribution

Equisetum, the sole extant genus in Equisetidae, comprises approximately 15 species with a nearly , though it is absent from , , and in their native flora. The majority of species exhibit dominance in the , particularly in temperate and boreal regions, with examples such as E. arvense occurring widely across and . Southern extensions are limited but notable, including native occurrences in the of (e.g., E. bogotense in high-altitude Andean regions and the ). Disjunct distribution patterns characterize the genus, with Equisetum (including like E. arvense and E. fluviatile) largely confined to temperate zones of the , while Hippochaete (e.g., E. hyemale) shows a broader range spanning both hemispheres, including parts of and . Equisetum are rare in tropical lowlands and occur primarily at higher elevations in tropical regions, reflecting a general preference for higher latitudes and elevations. Dispersal occurs primarily through wind-borne spores for long-distance colonization and extensive systems for local vegetative spread, enabling persistence in fragmented habitats. Human-mediated introduction has facilitated establishment as invasives outside native ranges, such as E. hyemale and E. arvense in and parts of Africa (e.g., ), often via ornamental plantings or contaminated substrates. Fossil evidence points to Gondwanan origins for the Equisetum lineage, with early fossils from paleofloristic provinces in the supporting an ancient southern cradle before northward migration. Recent biogeographic analyses estimate the crown age diversification in the (approximately 175 million years ago), aligning with post-Pangaean patterns that shaped modern disjunctions. Endemism is low across the genus, with most species exhibiting broad ranges rather than narrow restrictions. E. fluviatile exemplifies this, holding one of the widest distributions as a circumboreal species spanning and , from the to mid-latitudes.

Evolutionary History

Phylogeny

Equisetidae occupies a basal position within the Polypodiopsida (ferns), as part of the monilophyte that also encompasses Psilotopsida and Ophioglossidae. Recent plastid phylogenomic analyses recover Equisetidae as sister to Ophioglossidae with strong bootstrap support (95%), forming a that is sister to Marattiidae, which in turn is sister to the core leptosporangiate ferns (Polypodiidae). This topology confirms Equisetidae's integration as an ancient lineage rather than a separate division, aligning with updates from molecular studies between 2019 and 2021 that resolve its deep relationships using multi-locus datasets. Cladistic analyses depict Equisetidae diverging from other monilophytes approximately 375–400 million years ago during the , marking an early split in evolution. The crown group of , the sole extant genus, is estimated to have originated in the around 175 million years ago, though some fossil-calibrated molecular clocks suggest an diversification between 100–145 million years ago, consistent with Jurassic precursors in the phylogeny. Within Equisetidae, a 2018 combined molecular-morphological phylogeny positions and the extinct Neocalamites as a basal sister to other equisetaleans, including Calamitaceae and regional horsetail lineages. Molecular evidence from chloroplast DNA sequences, particularly the rbcL and trnL-F loci, strongly supports the of subgenera, with and Hippochaete forming distinct clades based on 5,086 aligned sites across all 15 species. A 2021 phylogenomic study using four plastid loci and fossil calibrations further resolves intra-generic relationships, confirming the basal position of subgenus Paramochaete and highlighting 's depauperate diversification since the . Genome sizes in Equisetum have remained relatively stable since the lineage's origin, ranging from 11.90 to 31.34 pg (1C-values), with subgenus Hippochaete exhibiting larger genomes on average than Equisetum. Biogeographic patterns inferred from analyses trace ancestral distributions to Angaran (Laurasian) and Gondwanan realms, with the first divergences among extant species coinciding with Pangaea's breakup around 170 million years ago and subsequent dispersals across hemispheres.

Fossil Record

The fossil record of Equisetidae, also known as sphenopsids, begins in the Late period around 360 million years ago, marking the emergence of early vascular with jointed stems and whorled appendages during a time of rapid land plant diversification. This group achieved peak diversity during the , particularly in coal swamp environments of the Late Carboniferous (Pennsylvanian), where they formed a significant component of paleotropical swamp forests alongside lycophytes and ferns. By this period, three major orders had evolved: the Pseudoborniales, characterized by primitive, shrubby forms; the Sphenophyllales, featuring vinelike or herbaceous with wedge-shaped leaves; and the , which included both herbaceous and arborescent taxa. These orders collectively represented a diverse array of habits, from shrubs to dominant canopy trees, contributing to the complex ecosystems of habitats. Extinct families within Equisetidae highlight the group's former ecological dominance, with the Calamitaceae producing woody, arborescent forms that reached heights of up to 20 meters, featuring ribbed trunks and whorled branches. Similarly, the Archaeocalamitaceae comprised early, more primitive arborescent members that appeared in the Early Carboniferous. Diversity declined sharply after the Permian, coinciding with global climatic shifts and the rise of seed plants, leading to the extinction of all woody forms by the Early Triassic; only herbaceous lineages in the genus Equisetum persisted into the modern era. Key Mesozoic fossils include Equisetites from Triassic and Jurassic deposits, which exhibit jointed stems similar to modern horsetails, and Cretaceous stems resembling Equisetum in anatomy and branching. Over 60 extinct genera are documented across the group's history, underscoring its once-vast morphological and ecological breadth. Spore wall ultrastructure provides evidence of evolutionary transitions within Equisetidae, with early Carboniferous forms like Calamites producing laevigate (smooth-walled) spores of the Calamospora type, which gradually evolved into the echinate (spiny) spores characteristic of modern Equisetum by the Triassic and Jurassic. Recent discoveries continue to refine this record, including a 2022 report of Miocene giant horsetail fossils from Patagonia, Argentina, representing the first clear evidence of large-statured Equisetum and likely ancestral to the modern E. giganteum. In 2023, fossils from the middle Siwalik sediments (Late Miocene) in Himachal Pradesh, India, described as E. siwalikum sp. nov., provided the oldest record of Equisetum in the Indian Cenozoic, indicating humid, swampy paleoenvironments. Additionally, a 2020 study identified three new Equisetum species from Neogene deposits in southwestern China and northern Vietnam, based on well-preserved rhizomes and stems that bridge gaps in the Cenozoic fossil history.

Taxonomy and Classification

Historical and Modern Classification

The classification of Equisetidae has evolved significantly from artificial systems based on morphology to modern phylogenetic frameworks informed by molecular data. In the , horsetails were recognized as a distinct group of vascular , classified as the division Sphenophyta or Equisetophyta, separate from ferns (Pterophyta) due to their jointed stems, whorled branches, and reduced leaves. This separation reflected early botanical efforts to group by shared vegetative and reproductive traits, treating Equisetidae as an independent lineage of spore-bearing plants akin to but divergent from leptosporangiate ferns. By the late 20th century, systematists like Arthur Cronquist incorporated Equisetidae into broader schemes, designating them as the class Equisetopsida within the division Pterophyta in his 1981 classification system. This placement emphasized their primitive vascular features while maintaining distinction from other pteridophyte classes, such as Filicopsida for ferns. Synonyms for Equisetopsida at the class level include Sphenopsida, reflecting historical rooted in their wedge-shaped (sphenoid) stems. A pivotal shift occurred in the early 2000s with the advent of molecular phylogenetics, which revealed Equisetidae as part of the monilophyte clade—a monophyletic group encompassing ferns, horsetails, and whisk ferns—rather than a isolated division. This recognition marked a transition from artificial classifications, which prioritized superficial similarities, to phylogenetic ones based on DNA sequence data, positioning horsetails as the sister group to all other monilophytes and closely related to seed plants. The current classification, as outlined by the Pteridophyte Phylogeny Group (PPG I) in 2016, places as a subclass within the class Polypodiopsida (leptosporangiate ferns), integrating horsetails firmly into the fern lineage under the division Polypodiophyta. This system recognizes as comprising a single order () and family (), with as the sole extant genus. Major taxonomic revisions at the infrageneric level occurred in 2019 with typification studies that also established three subgenera within , refining both nomenclatural stability and phylogenetic structure. Nomenclaturally, Equisetum serves as the type genus for Equisetidae, originally described by in 1753 under , where it was based on seven species including the lectotypified . This foundational description underpins the subclass's , with Equisetidae itself typified by the order .

Subdivision and Diversity

Equisetidae encompasses a single extant order, , which contains one family, , and a sole living genus, Equisetum, representing the only surviving lineage from a once-diverse group of vascular plants. The genus Equisetum comprises 18 species (as of 2025), all of which are homosporous herbs characterized by jointed, hollow stems with whorled leaf sheaths and terminal spore-bearing cones. These species exhibit limited morphological variation, primarily distinguished by stem branching, sheath dentition, and stomatal arrangement, but show uniformity in their reproductive strategy and growth habit as rhizomatous perennials. The extant diversity of is organized into three monophyletic subgenera: , Hippochaete, and Paramochaete, a division supported by molecular phylogenetic analyses including a comprehensive 2021 study using DNA sequences that confirmed the of these groups with high posterior probabilities. includes nine with annual, branched aerial stems and scattered stomata, enabling through branching whorls. In contrast, subgenus Hippochaete encompasses eight featuring , mostly unbranched aerial stems with sunken stomata, adapted for more robust, persistence in challenging environments. Paramochaete is monotypic, comprising only E. bogotense, a South American sister to the other subgenera. Representative species highlight the genus's range of forms and adaptations. Equisetum arvense, the common horsetail, is a widespread member of subgenus Equisetum, known for its dimorphic stems and ability to thrive in temperate wetlands across the Northern Hemisphere. Equisetum giganteum, from subgenus Hippochaete, stands out as the giant horsetail, native to South America, where it can reach heights of up to 8 meters in moist, subtropical habitats. Hybrids are relatively common within Equisetum, particularly between closely related species, resulting in fertile nothospecies due to the uniform chromosome number (2n ≈ 216) across the genus; a notable example is E. × litorale, the hybrid of E. arvense and E. fluviatile, which occurs in overlapping ranges and exhibits intermediate morphology. No other genera survive in Equisetaceae today, underscoring the remarkably low modern diversity of Equisetidae compared to its extensive fossil record, which documents hundreds of species across multiple families during the and eras.

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  52. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=111898
  53. https://www.britannica.com/plant/Equisetum-giganteum
  54. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=233500613
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