Equisetidae

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Equisetidae
Temporal range: Late Devonian [1] to Recent
Equisetum telmateia Fallatsche 20200526 2.jpg
Equisetum telmateia
Scientific classification OOjs UI icon edit-ltr.svg
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.

Contents

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

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 Horsetail vegeative stem.JPG
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. Equisetum strob xs.jpg
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 Equisetum braunii (strobilus), Portland, Oregon.jpg
Strobilus of E. braunii, terminal on an unbranched stem

Taxonomy

Classification

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

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

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

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]

Related Research Articles

<span class="mw-page-title-main">Fern</span> Class of vascular plants

The ferns are a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissues that conduct water and nutrients, and in having life cycles in which the branched sporophyte is the dominant phase.

<span class="mw-page-title-main">Lycopodiopsida</span> Class of vascular plants

Lycopodiopsida is a class of vascular plants also known as lycopods or lycophytes. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching stems bearing simple leaves called microphylls and reproduce by means of spores borne in sporangia on the sides of the stems at the bases of the leaves. Although living species are small, during the Carboniferous, extinct tree-like forms (Lepidodendrales) formed huge forests that dominated the landscape and contributed to coal deposits.

<i>Equisetum</i> Genus of vascular plants in the family Equisetaceae

Equisetum is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.

<span class="mw-page-title-main">Osmundaceae</span> Family of ferns

Osmundaceae is a family of ferns containing four to six extant genera and 18–25 known species. It is the only living family of the order Osmundales in the class Polypodiopsida (ferns) or in some classifications the only order in the class Osmundopsida. This is an ancient and fairly isolated group that is often known as the "flowering ferns" because of the striking aspect of the ripe sporangia in Claytosmunda, Osmunda, Osmundastrum, and Plensium. In these genera the sporangia are borne naked on non-laminar pinnules, while Todea and Leptopteris bear sporangia naked on laminar pinnules. Ferns in this family are larger than most other ferns.

In plant anatomy and evolution a microphyll is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of a microphyll, the leaf vein emerges from the protostele without leaving a leaf gap. Leaf gaps are small areas above the node of some leaves where there is no vascular tissue, as it has all been diverted to the leaf. Megaphylls, in contrast, have multiple veins within the leaf and leaf gaps above them in the stem.

<span class="mw-page-title-main">Equisetaceae</span> Family of vascular plants known as horsetails

Equisetaceae, also known as the horsetail family, is a family of ferns and the only surviving family of the order Equisetales, with one surviving genus, Equisetum, comprising about twenty species.

<span class="mw-page-title-main">Equisetales</span> Order of ferns

Equisetales is an order of subclass Equisetidae with only one living family, Equisetaceae, containing the genus Equisetum (horsetails), as well as a variety of extinct groups, including the tree-like Calamitaceae.

<span class="mw-page-title-main">Salviniales</span> Order of plants

The order Salviniales is an order of ferns in the class Polypodiopsida.

<span class="mw-page-title-main">Gleicheniales</span> Order of ferns

Gleicheniales is an order of ferns in the subclass Polypodiidae. The Gleicheniales have records potentially as early as the Carboniferous, but the oldest unambiguous records date to the Permian.

<span class="mw-page-title-main">Cyatheales</span> Order of ferns

The order Cyatheales, which includes most tree ferns, is a taxonomic order of the fern class, Polypodiopsida. No clear morphological features characterize all of the Cyatheales, but DNA sequence data indicate the order is monophyletic. Some species in the Cyatheales have tree-like growth forms from a vertical rhizome, others have shorter or horizontal expanding rhizomes.

<i>Calamites</i> Extinct genus of vascular plants

Calamites is a genus of extinct arborescent (tree-like) horsetails to which the modern horsetails are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights of 30–50 meters. They were components of the understories of coal swamps of the Carboniferous Period.

<span class="mw-page-title-main">Pteridophyte</span> Group of plants that reproduce by spores

A pteridophyte is a vascular plant that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden.

<span class="mw-page-title-main">Psilotaceae</span> Family of ferns

Psilotaceae is a family of ferns consisting of two genera, Psilotum and Tmesipteris with about a dozen species. It is the only family in the order Psilotales.

<i>Equisetum bogotense</i> Species of fern

Equisetum bogotense, the Andean horsetail, is a herbaceous perennial that reproduces through spores. It has thicker, less bushy whorled branches, and a silica rich rhizomatous stem, which roots grow out of, under ground. This stem is a dull dark brown color with glabrous growth aside from the sheathed segments. The plant has a history as a traditional herbal remedy, and a study of its diuretic effects on humans showed significant increases in urinary sodium, potassium, and chloride. Human and animal trials of indicate that E. bogotense has "high" efficacy as a diuretic. It is used in several modern herbal supplements. The species epithet refers to Bogotá, the capital of Colombia.

<span class="mw-page-title-main">Eusporangiate fern</span> Common name for a group of ferns

Eusporangiate ferns are vascular spore plants, whose sporangia arise from several epidermal cells and not from a single cell as in leptosporangiate ferns. Typically these ferns have reduced root systems and sporangia that produce large amounts of spores.

<span class="mw-page-title-main">Leptosporangiate fern</span> Subclass of ferns

The Polypodiidae, commonly called leptosporangiate ferns, formerly Leptosporangiatae, are one of four subclasses of ferns, the largest of these being the largest group of living ferns, including some 11,000 species worldwide. The group has also been treated as the class Pteridopsida or Polypodiopsida, although other classifications assign them a different rank. Older names for the group include Filicidae and Filicales, although at least the "water ferns" were then treated separately.

<i>Equisetum palustre</i> Species of vascular plant in the horsetail family Equisetaceae

Equisetum palustre, the marsh horsetail, is a perennial herbaceous pteridophyte belonging to the subclass of horsetails (Equisetidae). It is widespread in cooler regions of Eurasia and North America.

<span class="mw-page-title-main">Fern ally</span>

Fern allies are a diverse group of seedless vascular plants that are not true ferns. Like ferns, a fern ally disperses by shedding spores to initiate an alternation of generations.

<span class="mw-page-title-main">Sphenophyllales</span> Order of plants

Sphenophyllales is an extinct order of articulate land plants and a sister group to the present-day Equisetales (horsetails). They are fossils dating from the Devonian to the Triassic. They were common during the Late Pennsylvanian to Early Permian, with most of the fossils coming from the Carboniferous period.

<span class="mw-page-title-main">Ophioglossidae</span> Subclass of ferns

Ophioglossidae is one of the four subclasses of Polypodiopsida (ferns). This subclass consists of the ferns commonly known as whisk ferns, grape ferns, adder's-tongues and moonworts. It is equivalent to the class Psilotopsida in previous treatments, including Smith et al. (2006). The subclass contains two orders, Psilotales and Ophioglossales, whose relationship was only confirmed by molecular phylogenetic studies.

References

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