Equisetopsida
Encyclopedia
Equisetopsida, or Sphenopsida, is a class
of plant
s with a fossil record going back to the Devonian
. They are commonly known as horsetails. Living species typically grow in wet areas, with needle-like leaves radiating at regular intervals from a single vertical stem.
The Equisetopsida were formerly regarded as a separate division of spore plants and also called Equisetophyta, Arthrophyta or Sphenophyta; today they have been recognized as rather close relatives of the typical ferns (Pteridopsida) and form a specialized lineage of the Pteridophyta.
. In the only extant genus Equisetum, these are small leaves (microphyll
s) with a singular vascular trace. However, sphenophyte leaves probably arose by the reduction of a megaphyll, as evidenced by early fossil forms such as Sphenophyllum
, in which the leaves are broad with branching veins. The plumbing of these leaves is interesting: the vascular
traces trifurcate at the junctions, with one thread going to the microphyll, and the other two moving left and right to merge with the new branches of their neighbours. The vascular system itself curiously resembles that of the vascular plants' eustele, which evolved convergently. A primary xylem
contains carinal canals; in the Calamitaceae
, secondary xylem (but not secondary phloem
) can be secreted as the cambium
grows outwards, producing a woody stem, and allowing the plants to grow as high as 10m. The cortex itself contains valecular canals; due to the softer nature of the phloem, these are very rarely seen in fossil instances.
The plant does not bear a coherent root system but underground rhizome
s, from which roots and aerial axes emerge.
The plant contains an intercalary meristem
: that is to say, each segment of the stem grows as the plant gets taller. This contrasts with the seed plants, which contain an apical meristem - i.e. new growth comes only from growing tips (and widening of stems). Growth was determinate - i.e. the plants' phenotype dictated a maximum height, which the plant would grow to then get no higher.
Sphenophytes bear cones (technically strobili, sing. strobilus) at the tips of some stems. These cones comprise spirally arranged sporophores, which bear spores in four clusters, and in extant sphenophytes cover the spores externally - like four sacs hanging from an umbrella, with its handle embedded in the central cone body. In extinct groups, further protection was afforded to the spores by the presence of whorls of bract
s - big pointy microphylls protruding from the cone.
The spores themselves bear characteristic elater
s, 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. Dispersal is aided in the first instance by laterally dehiscing
sporangia, which pop open and scatter spores.
The extant horsetails are mostly homosporous, but this is conspicuously not the case in the past.
. Second, the Sphenophyllales
which were a dominant member of the Carboniferous understory, and prospered until the mid and early Permian respectively. 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, 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 .
to division. When recognized as a separate division, the literature uses many possible names, including Arthrophyta, Sphenophyta, or Equisetophyta. Other authors have 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.
Recent phylogenetic analysis
has produced evidence that this group of plants belongs firmly within the fern
clade
of vascular plants. A 2006 classification by Smith et al. places the class Equisetopsida within an unranked clade of broadly defined ferns, as a sister to two classes more traditionally called ferns, Marattiopsida and Polypodiopsida.
The probable relationships within Equisetopsida are shown in the cladogram below:
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s with a fossil record going back to the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
. They are commonly known as horsetails. Living species typically grow in wet areas, with needle-like leaves radiating at regular intervals from a single vertical stem.
The Equisetopsida were formerly regarded as a separate division of spore plants and also called Equisetophyta, Arthrophyta or Sphenophyta; today they have been recognized as rather close relatives of the typical ferns (Pteridopsida) and form a specialized lineage of the Pteridophyta.
Morphology
The Sphenophytes comprise photosynthesising, "segmented", hollow stems, sometimes filled with pith. At the junction ("node", see diagram) between each segment is a whorl of leavesLeaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
. In the only extant genus Equisetum, these are small leaves (microphyll
Microphyll
The terminology of fossil plants is in places a little confusing. In the discipline's 200+ year history, certain concepts have become entrenched, even though improved understanding has threatened the foundations upon which they are based...
s) with a singular vascular trace. However, sphenophyte leaves probably arose by the reduction of a megaphyll, as evidenced by early fossil forms such as Sphenophyllum
Sphenophyllum
Sphenophyllum is a genus in the order Sphenophyllales....
, in which the leaves are broad with branching veins. The plumbing of these leaves is interesting: the vascular
Vascular
Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
traces trifurcate at the junctions, with one thread going to the microphyll, and the other two moving left and right to merge with the new branches of their neighbours. The vascular system itself curiously resembles that of the vascular plants' eustele, which evolved convergently. A primary xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
contains carinal canals; in the Calamitaceae
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...
, secondary xylem (but not secondary phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
) can be secreted as the cambium
Cambium (botany)
A cambium , in botany, is a tissue layer that provide undifferentiated cells for plant growth. It forms parallel rows of cells, which result in secondary tissues....
grows outwards, producing a woody stem, and allowing the plants to grow as high as 10m. The cortex itself contains valecular canals; due to the softer nature of the phloem, these are very rarely seen in fossil instances.
The plant does not bear a coherent root system but underground rhizome
Rhizome
In botany and dendrology, a rhizome is a characteristically horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes...
s, from which roots and aerial axes emerge.
The plant contains an intercalary meristem
Meristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....
: that is to say, each segment of the stem grows as the plant gets taller. This contrasts with the seed plants, which contain an apical meristem - i.e. new growth comes only from growing tips (and widening of stems). Growth was determinate - i.e. the plants' phenotype dictated a maximum height, which the plant would grow to then get no higher.
Sphenophytes bear cones (technically strobili, sing. strobilus) at the tips of some stems. These cones comprise spirally arranged sporophores, which bear spores in four clusters, and in extant sphenophytes cover the spores externally - like four sacs hanging from an umbrella, with its handle embedded in the central cone body. In extinct groups, further protection was afforded to the spores by the presence of whorls of bract
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 often different from foliage leaves. They may be smaller, larger, or of a different color, shape, or texture...
s - big pointy microphylls protruding from the cone.
The spores themselves bear characteristic elater
Elater
An elater is 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...
s, 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. Dispersal is aided in the first instance by laterally dehiscing
Dehiscence
Dehiscence can refer to:*Dehiscence is the spontaneous opening at maturity of a plant structure, such as a fruit, anther, or sporangium, to release its contents.*Wound dehiscence is a previously closed wound reopening....
sporangia, which pop open and scatter spores.
Fossil record
The extant horsetails represent a tiny fraction of Sphenophyte diversity in the past. There were three orders of Equisetopsid; the Pseudoborniales, which first appeared in the late DevonianDevonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
. Second, the Sphenophyllales
Sphenophyllales
Sphenophyllales is an extinct order of articulate land plants and a sister group to the present-day Equisetales . They are known known from fossils dating from the Devonian to the Triassic...
which were a dominant member of the Carboniferous understory, and prospered until the mid and early Permian respectively. The Equisetales
Equisetales
The Equisetales is an order of pteridophytes with only one living genus Equisetum , of the family Equisetaceae. The fossil record includes additional extinct species in the Equisetaceae and the extinct families Calamitaceae and Archaeocalamitaceae....
existed alongside the Sphenophyllales
Sphenophyllales
Sphenophyllales is an extinct order of articulate land plants and a sister group to the present-day Equisetales . They are known known from fossils dating from the Devonian to the Triassic...
, 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, a time when land plants were undergoing a rapid diversification, with roots, seeds and leaves having only just evolved. (See Evolutionary history of plants
Evolutionary history of plants
The evolution of plants has resulted in increasing levels of complexity, from the earliest algal mats, through bryophytes, lycopods, ferns to the complex gymnosperms and angiosperms of today...
) However, plants had already been on the land for almost a hundred million years, with the first evidence of land plants dating to .
Systematics
The horsetails and their fossil relatives have long been recognized as distinct from other seedless vascular plants. 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 have assigned to the horsetails varies from orderOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
to division. When recognized as a separate division, the literature uses many possible names, including Arthrophyta, Sphenophyta, or Equisetophyta. Other authors have 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.
Recent phylogenetic analysis
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
has produced evidence that this group of plants belongs firmly within the fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of vascular plants. A 2006 classification by Smith et al. places the class Equisetopsida within an unranked clade of broadly defined ferns, as a sister to two classes more traditionally called ferns, Marattiopsida and Polypodiopsida.
The probable relationships within Equisetopsida are shown in the cladogram below: