Equisetidae Temporal range:Late Devonian[1] to Recent PreꞒ Ꞓ O S D C P T J K Pg N
Equisetum telmateia
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Division:	Polypodiophyta
Class:	Polypodiopsida
Subclass:	Equisetidae Warm.
Orders
Equisetales † Archaeocalamitaceae † Calamitaceae Equisetaceae † Pseudoborniales † Sphenophyllales
Synonyms
See text.

Equisetidae is one of the foursubclasses ofPolypodiopsida (ferns), a group ofvascular plants with a fossil record going back to theDevonian. They are commonly known ashorsetails.^[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 separatedivision of spore plants and calledEquisetophyta,Arthrophyta,Calamophyta orSphenophyta. When treated as a class, the namesEquisetopsida s.s. andSphenopsida have also been used. They are now recognized as rather close relatives of theferns (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.

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

Thevascular 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 andconvergently.^[5] Very rapid internode elongation results in the formation of a pith cavity and a ring ofcarinal canals formed by disruption of the primaryxylem. Similar spaces, thevallecular 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 theCalamitaceae, secondary xylem (but not secondaryphloem) was secreted as thecambium grew outwards, producing a woody stem, and allowing the plants to grow as high as 10m. All extant species ofEquisetum are herbaceous, and have lost the ability to produce secondary growth.^[5]

The underground parts of the plants consist of jointedrhizomes, from which roots and aerial axes emerge. The plants haveintercalary 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 (technicallystrobili, sing.strobilus) at the tips of some stems. These cones comprise spirally arrangedsporangiophores, which bearsporangia 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 ofbracts - big pointed microphylls protruding from the cone.

The extant horsetails arehomosporous, but extinctheterosporous species such asCalamostachys casheana appear in the fossil record.^[6] The sporangia open by lateraldehiscence to release the spores. The spores bear characteristicelaters, distinctive spring-like attachments which arehygroscopic: i.e. they change their configuration in the presence of water, helping the spores move and aiding their dispersal.

The horsetails and their fossil relatives have long been recognized as distinct from other seedlessvascular plants, such as theferns (Polypodiopsida).^[7] Before the advent of modernmolecular 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 fromorder todivision. 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]

Modernphylogenetic analysis, back to 2001, demonstrated that horsetails belong firmly within thefernclade ofvascular plants.^[11][12] Smith et al. (2006) carried out the first higher-levelpteridophyte classification published in themolecular phylogenetic era, and considered the ferns (monilophytes), to comprise fourclasses, with the horsetails as class Equisetopsidasensu stricto.^[3] (This distinction is necessary because of the alternative usage ofEquisetopsidasensu lato as a synonym for all land plants (Embryophyta) with rank of class.^[13])Chase andReveal (2009) treated the horsetails as subclass Equisetidae of class Equisetopsidasensu 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]

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

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

According to the study, the age of the crown group ofEquisetum dates at least to theEarly Cretaceous, and most probably up to theJurassic.^[14]

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

The extant horsetails represent a tiny fraction of horsetail diversity in the past. There were three orders of the Equisetidae. ThePseudoborniales first appeared in the lateDevonian.^[1] TheSphenophyllales were a dominant member of theCarboniferous understory, and prospered until the mid andearly Permian. TheEquisetales existed alongside theSphenophyllales, but diversified as that group disappeared into extinction, gradually dwindling in diversity to today's single genusEquisetum.

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. (SeeEvolutionary 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 to475 million years ago.^[15]

Data related toEquisetopsida at Wikispecies

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• Horsetails
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• Extant Late Devonian first appearances
• Late Devonian plants

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