Equisetidae is one of the four
subclasses of
Polypodiopsida
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except th ...
(ferns), a group of
vascular plant
Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...
s with a fossil record going back to the
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
. They are commonly known as horsetails.
[ 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
Division or divider may refer to:
Mathematics
*Division (mathematics), the inverse of multiplication
*Division algorithm, a method for computing the result of mathematical division
Military
*Division (military), a formation typically consisting ...
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 fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except t ...
s (Polypodiopsida) of which they form a specialized lineage.[ 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
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
. In the only extant genus ''Equisetum'', these are small leaves (microphyll
In plant anatomy and evolution a microphyll (or lycophyll) 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 ...
s) with a singular vascular trace, fused into a sheath at each stem node. However, the leaves of ''Equisetum'' probably arose by the reduction of megaphyll
In plant anatomy and evolution a microphyll (or lycophyll) 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 ...
s, as evidenced by early fossil forms such as ''Sphenophyllum
''Sphenophyllum'' is a genus in the order Sphenophyllales
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 Triassi ...
'', in which the leaves are broad with branching veins.[
The ]vascular bundle
A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will inclu ...
s 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.[ The vascular system itself resembles that of the vascular plants' ]eustele In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost bo ...
, which evolved independently and convergently.[ 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
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
. Similar spaces, the vallecular canals are formed in the cortex.[ Due to the softer nature of the phloem, these are very rarely seen in fossil instances. In the ]Calamitaceae
Calamitaceae is an extinct family of plants related to the modern horsetails. Some members of this family attained tree-like stature during the Carboniferous Period (around ) and in Permian Period, reaching heights of up to 20 meters. The famil ...
, secondary xylem (but not secondary phloem
Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...
) was secreted as the cambium
A cambium (plural cambia or cambiums), in plants, is a tissue layer that provides partially undifferentiated cells for plant growth. It is found in the area between xylem and phloem. A cambium can also be defined as a cellular plant tissue from w ...
grew outwards, producing a woody stem, and allowing the plants to grow as high as 10m. All extant species of ''Equisetum
''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds.
''Equisetum'' is a "living fossil", the only living genus of the entire subclass Eq ...
'' are herbaceous, and have lost the ability to produce secondary growth.[
The underground parts of the plants consist of jointed ]rhizome
In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...
s, 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
A strobilus (plural: strobili) is a structure present on many land plant species consisting of sporangia-bearing structures densely aggregated along a stem. Strobili are often called cones, but some botanists restrict the use of the term cone to th ...
'', sing. strobilus
A strobilus (plural: strobili) is a structure present on many land plant species consisting of sporangia-bearing structures densely aggregated along a stem. Strobili are often called cones, but some botanists restrict the use of the term cone to th ...
) at the tips of some stems. These cones comprise spirally arranged sporangiophores, which bear sporangia
A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
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 bract
In botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves. They may be smaller, larger, or of ...
s - big pointed microphylls protruding from the cone.
The extant horsetails are homosporous
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...
, but extinct heterosporous
Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isos ...
species such as ''Calamostachys casheana'' appear in the fossil record.[ The sporangia open by lateral dehiscence to release the spores. The spores bear characteristic ]elater
An elater is a cell (or structure attached to a cell) that is hygroscopic, and therefore will change shape in response to changes in moisture in the environment. Elaters come in a variety of forms, but are always associated with plant spores. In ...
s, distinctive spring-like attachments which are hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
: i.e. they change their configuration in the presence of water, helping the spores move and aiding their dispersal.
Taxonomy
Classification
The horsetails and their fossil relatives have long been recognized as distinct from other seedless vascular plant
Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...
s, such as the fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except t ...
s (Polypodiopsida). Before the advent of modern molecular studies, the relationship of this group to other living and fossil plants was considered problematic. Because of their unclear relationships, the rank botanists assigned to the horsetails varied from order to division
Division or divider may refer to:
Mathematics
*Division (mathematics), the inverse of multiplication
*Division algorithm, a method for computing the result of mathematical division
Military
*Division (military), a formation typically consisting ...
. When recognized as a separate division, the literature uses many possible names, including Arthrophyta, Calamophyta, Sphenophyta, 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 or Sphenopsida.[
Modern ]phylogenetic analysis
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
, back to 2001, demonstrated that horsetails belong firmly within the fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except t ...
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
of vascular plant
Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...
s. Smith et al. (2006) carried out the first higher-level pteridophyte
A pteridophyte is a vascular plant (with xylem and phloem) that disperses spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden. Ferns, ...
classification published in the molecular phylogenetic
Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
era, and considered the ferns (monilophytes), to comprise four classes, with the horsetails as class Equisetopsida ''sensu stricto
''Sensu'' is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, semiotics, and law. Commonly it refers to how strictly or loosely an expression is used in describing any particular co ...
''.[ (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.]) Chase
Chase or CHASE may refer to:
Businesses
* Chase Bank, a national bank based in New York City, New York
* Chase Aircraft (1943–1954), a defunct American aircraft manufacturing company
* Chase Coaches, a defunct bus operator in England
* Chase Co ...
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).
Phylogeny
The following diagram shows a likely phylogenic relationship between subclass Equisetidae and the other fern subclasses according to the Pteridophyte Phylogeny Group.[
A 2018 study by Elgorriaga et al. suggests the relationships within the Equisetidae are as shown in the following cladogram.]
According to the study, the age of the crown group of ''Equisetum'' dates at least to the Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
, and most probably up to the Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
.
Subdivision
Subclass Equisetidae contains a single extant order, Equisetales
Equisetales is an order of subclass Equisetidae with only one living family, Equisetaceae, containing the genus ''Equisetum'' (horsetails).
Classification
In the molecular phylogenetic classification of Smith et al. in 2006, Equisetales, in its ...
. This order consists of a single monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...
family, Equisetaceae
Equisetaceae, sometimes called the horsetail family, is the only extant family of the order Equisetales, with one surviving genus, ''Equisetum'', which comprises about twenty species.
Evolution and systematics
Equisetaceae is the only survivin ...
, with one genus ''Equisetum
''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds.
''Equisetum'' is a "living fossil", the only living genus of the entire subclass Eq ...
''. ''Equisetum'' has about 20 species.[
]
Fossil record
The extant horsetails represent a tiny fraction of horsetail diversity in the past. There were three orders of the Equisetidae. The Pseudoborniales
''Pseudobornia'' is a genus of plants known only from fossils found from the Upper Devonian. It contains a single species ''Pseudobornia ursina'', and is the earliest fossil assigned with certainty to the Equisetopsida.
The first fossils of '' ...
first appeared in the late Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
.[ The ]Sphenophyllales
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 ...
were a dominant member of the Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
understory, and prospered until the mid and early Permian 01 or '01 may refer to:
* The year 2001, or any year ending with 01
* The month of January
* 1 (number)
Music
* '01 (Richard Müller album), 01'' (Richard Müller album), 2001
* 01 (Son of Dave album), ''01'' (Son of Dave album), 2000
* 01 (Urban ...
. The Equisetales
Equisetales is an order of subclass Equisetidae with only one living family, Equisetaceae, containing the genus ''Equisetum'' (horsetails).
Classification
In the molecular phylogenetic classification of Smith et al. in 2006, Equisetales, in its ...
existed alongside the Sphenophyllales
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 ...
, but diversified as that group disappeared into extinction, gradually dwindling in diversity to today's single genus ''Equisetum
''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds.
''Equisetum'' is a "living fossil", the only living genus of the entire subclass Eq ...
''.
The organisms first appear in the fossil record during the late Devonian,[ a time when land plants were undergoing a rapid diversification, with roots, seeds and leaves having only just evolved. (See ]Evolutionary history of plants
The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats, through multicellular marine and freshwater green algae, terrestrial bryophytes, lycopods and ferns, to the complex gymnosperms and angiosperms (fl ...
) However, plants had already been on the land for almost a hundred million years, with the first evidence of land plants dating to .
References
{{Taxonbar, from1=Q5384600, from2=Q134677, from3=Q3055913, from4=Q23044259
Plant subclasses
Extant Late Devonian first appearances
Late Devonian plants