The lycophytes, when broadly
circumscribed
In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius.
Not every polyg ...
, are a
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 ...
(tracheophyte) subgroup of the kingdom
Plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
ae. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest lineages of
extant
Extant is the opposite of the word extinct. It may refer to:
* Extant hereditary titles
* Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English
* Extant taxon, a taxon which is not extinct, ...
(living) vascular plants; the group contains extinct plants that have been dated from the
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
(ca. 425 million years ago).
Lycophytes were some of the dominating plant species 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 ...
period, and included tree-like species, although extant lycophytes are relatively small plants.
The scientific names and the informal English names used for this group of plants are ambiguous. For example, "Lycopodiophyta" and the shorter "Lycophyta" as well as the informal "lycophyte" may be used to include the extinct
zosterophyll
The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosteroph ...
s or to exclude them.
Description
Lycophytes reproduce by
spore
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, f ...
s and have
alternation of generations
Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of Biological life cycle, life cycle in plants and algae. It consists of a Multicellular organism, multicellular haploid sexual phase, the gametophy ...
in which (like other vascular plants) the
sporophyte
A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase.
Life cycle
The sporophyte develops from the zygote pr ...
generation is dominant. Some lycophytes 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, ...
while others are
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 ...
.
[Eichhorn, Evert, and Raven (2005). ''Biology of Plants'', Seventh Edition. 381-388.] When broadly
circumscribed
In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius.
Not every polyg ...
, the lycophytes represent a line of evolution distinct from that leading to all other
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, the
euphyllophyte
The euphyllophytes are a clade of plants within the tracheophytes (the vascular plants). The group may be treated as an unranked clade, a division under the name Euphyllophyta or a subdivision under the name Euphyllophytina. The euphyllophytes ar ...
s, such as
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,
gymnosperm
The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, ''Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμνό ...
s and
flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s. They are defined by two
synapomorphies
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
: lateral rather than terminal
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 ...
(often kidney-shaped or reniform), and
exarch
An exarch (;
from Ancient Greek ἔξαρχος ''exarchos'', meaning “leader”) was the holder of any of various historical offices, some of them being political or military and others being ecclesiastical.
In the late Roman Empire and ea ...
protostele 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 b ...
s, in which the protoxylem is outside the metaxylem rather than vice versa. The extinct
zosterophylls
The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosteroph ...
have at most only flap-like extensions of the stem ("enations") rather than leaves, whereas extant lycophyte species have
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, leaves that have only a single vascular trace (vein), rather than the much more complex
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 of other vascular plants. The extinct genus ''
Asteroxylon
''Asteroxylon'' ("star-shaped xylem") is an extinct genus of vascular plants of the Division Lycopodiophyta known from anatomically preserved specimens described from the famous Early Devonian Rhynie chert and Windyfield chert in Aberdeenshire, S ...
'' represents a transition between these two groups: it has a vascular trace leaving the central protostele, but this extends only to the base of the enation.
[ See .
Zosterophylls and extant lycophytes are all relatively small plants, but some extinct species, such as the ]Lepidodendrales
Lepidodendrales (from the Greek for "scale tree") were primitive, vascular, heterosporous, arborescent (tree-like) plants related to present day lycopsids. Members of Lepidodendrales are the best understood of the fossil lycopsids due to the vast ...
, were tree-like, and formed extensive forests that dominated the landscape and contributed to the formation of coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when dea ...
.[
]
Taxonomy
Classification
In the broadest circumscription
Circumscription may refer to:
*Circumscribed circle
*Circumscription (logic)
*Circumscription (taxonomy)
* Circumscription theory, a theory about the origins of the political state in the history of human evolution proposed by the American anthrop ...
of the lycophytes, the group includes the extinct zosterophyll
The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosteroph ...
s as well as the extant (living) lycophytes and their closest extinct relatives. The names and ranks used for this group vary considerably. Some sources use the names "Lycopodiophyta" or the shorter "Lycophyta" to include zosterophylls as well as extant lycophytes and their closest extinct relatives, while others use these names to exclude zosterophylls.[ The name "Lycopodiophytina" has also been used in the inclusive sense.][ English names, such as "lycophyte", "lycopodiophyte" or "lycopod", are similarly ambiguous, and may refer to the broadly defined group or only to the extant lycophytes and their closest extinct relatives.
The consensus classification produced by the ]Pteridophyte Phylogeny Group
The Pteridophyte Phylogeny Group, or PPG, is an informal international group of systematic botanists who collaborate to establish a consensus on the classification of pteridophytes (lycophytes and ferns) that reflects knowledge about plant relation ...
classification in 2016 (PPG I) places all extant (living) lycophytes in the class Lycopodiopsida
Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...
.[ There are around 1,290 to 1,340 such species.][ For more information on the classification of extant lycophytes, see .
]
Phylogeny
A major cladistic
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
study of land plants was published in 1997 by Kenrick and Crane.[ In 2004, Crane et al. published some simplified ]cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
s, based on a number of figures in Kenrick and Crane (1997). Their cladogram for the lycophytes is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram).
In this view, the "zosterophylls" comprise a paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
group, ranging from forms like '' Hicklingia'', which had bare stems,[ to forms like '']Sawdonia
Sawdonia is an extinct genus of early vascular plants, known from the Upper Silurian to the Lower Carboniferous (). ''Sawdonia'' is best recognized by the large number of spikes (enations) covering the plant. These are vascular plants that do n ...
'' and '' Nothia'', whose stems are covered with unvascularized spines or enations.[ The genus '']Renalia
''Renalia'' is a genus of extinct vascular plants from the Early Devonian (around ). It was first described in 1976 from compressed fossils in the Battery Point Formation (Gaspé, Québec, Canada). It is difficult to reconstruct the original for ...
'' illustrates the problems in classifying early land plants. It has characteristics both of the non-lycophyte rhyniophyte
The rhyniophytes are a group of extinct early vascular plants that are considered to be similar to the genus ''Rhynia'', found in the Early Devonian (around ). Sources vary in the name and rank used for this group, some treating it as the class R ...
s – terminal rather than lateral sporangia – and of the zosterophylls – kidney-shaped sporangia opening along the distal margin.
A rather different view is presented in a 2013 analysis by Hao and Xue. Their preferred cladogram shows the zosterophylls and associated genera basal to both the lycopodiopsids and the euphyllophytes, so that there is no clade corresponding to the broadly defined group of lycophytes used by other authors.[
Some extinct orders of lycophytes fall into the same group as the extant orders. Different sources use varying numbers and names of the extinct orders. The following phylogram shows a likely relationship between some of the proposed Lycopodiopsida orders.
]
Evolution of microphylls
Within the broadly defined lycophyte group, species placed in the class Lycopodiopsida
Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...
are distinguished from species placed in the Zosterophyllopsida by the possession of 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. Some zosterophylls, such as 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 ...
'' Zosterophyllum myretonianum'', had smooth stems (axes). Others, such as ''Sawdonia ornata
Sawdonia is an extinct genus of early vascular plants, known from the Upper Silurian to the Lower Carboniferous (). ''Sawdonia'' is best recognized by the large number of spikes ( enations) covering the plant. These are vascular plants that do n ...
'', had flap-like extensions on the stems ("enations"), but without any vascular tissue. ''Asteroxylon
''Asteroxylon'' ("star-shaped xylem") is an extinct genus of vascular plants of the Division Lycopodiophyta known from anatomically preserved specimens described from the famous Early Devonian Rhynie chert and Windyfield chert in Aberdeenshire, S ...
'', identified as an early lycopodiopsid, had vascular traces that extended to the base of the enations. Species in the genus '' Leclercqia'' had fully vascularized microphylls. These are considered to be stages in the evolution of microphylls.
Gallery
File:Lycopodites.JPG, ''Lycopodites'', an early lycopod-like fossil
File:LepidodendronOhio.jpg, External mold of ''Lepidodendron
''Lepidodendron'' is an extinct genus of primitive vascular plants belonging to the family Lepidodendraceae, part of a group of Lycopodiopsida known as scale trees or arborescent lycophytes, related to Isoetes, quillworts and Lycopodiopsida, lyco ...
'' from the Upper Carboniferous
Upper may refer to:
* Shoe upper or ''vamp'', the part of a shoe on the top of the foot
* Stimulant
Stimulants (also often referred to as psychostimulants or colloquially as uppers) is an overarching term that covers many drugs including those ...
of Ohio
Ohio () is a state in the Midwestern region of the United States. Of the fifty U.S. states, it is the 34th-largest by area, and with a population of nearly 11.8 million, is the seventh-most populous and tenth-most densely populated. The sta ...
.
File:Lycopod bark.jpg, Lycopod bark showing leaf scars, from the Middle Devonian of Wisconsin.
File:Lycopsid joggins mcr1.JPG, Fossil ''in situ'' lycopsid
Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...
, probably ''Sigillaria'', with attached stigmarian roots.
File:Lycopsid mcr2.jpg, Base of a fossil lycopsid
Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...
showing connection with stigmarian roots.
File:Zosterophyllum sp. - MUSE cropped.jpg, Reconstruction of a Silurian ''Zosterophyllum
''Zosterophyllum'' was a genus of Silurian-Devonian vascular land plants with naked branching axes on which usually kidney-shaped sporangia were arranged in lateral positions. It is the type genus for the group known as zosterophylls, thought to ...
''
File:Nothia.png, Reconstruction of '' Nothia aphylla''
File:Lepidodendron.png, Reconstruction of ''Lepidodendron
''Lepidodendron'' is an extinct genus of primitive vascular plants belonging to the family Lepidodendraceae, part of a group of Lycopodiopsida known as scale trees or arborescent lycophytes, related to Isoetes, quillworts and Lycopodiopsida, lyco ...
''
File:Lycopod axis.jpg, Lycopod axis (branch) from the Middle Devonian of Wisconsin.
File:Lycopodium dendroideum.JPG, '' Lycopodium dendroideum'', a modern member of the Lycopodiales
The Lycopodiaceae (class Lycopodiopsida, order Lycopodiales) are an old family of vascular plants, including all of the core clubmosses and firmosses, comprising 16 accepted genera and about 400 known species. This family originated about 380 mil ...
File:Closeup of Black-spored Quillwort (Isoetes melanospora).jpg, ''Isoetes melanospora
''Isoetes melanospora'', commonly known as black-spored quillwort or black-spored Merlin's grass, is a rare and endangered aquatic lycophyte endemic to the U.S. states of Georgia and South Carolina. It grows exclusively in shallow, temporary pool ...
'', a modern member of the Isoetales
Isoetales, sometimes also written Isoëtales, is an order of plants in the class Lycopodiopsida.
There are about 140-150 living species, all of which are classified in the genus ''Isoetes'' (quillworts), with a cosmopolitan distribution, but ...
File:Pleuromeia restoration.png, Restoration of '' Pleuromeia'', an extinct Isoetales genus from the Early Triassic
References
External links
Lycophytes
Fossil Groves
Paleo Plants
{{Taxonbar, from=Q215370
Cryptogams
Plant divisions
Wenlock first appearances
Extant Silurian first appearances