Polysporangiophyte
Encyclopedia
Polysporangiophytes, also called polysporangiates or more formally Polysporangiophyta, are plants in which the spore-bearing generation (sporophyte
) has a structure of branching stems (axes) terminating in sporangia
(hence the name of the group which literally means "many sporangia plant"). The clade
includes most land plants (embryophyte
s) except for the bryophyte
s (liverworts, mosses and hornworts) whose sporophytes are always unbranched. The definition is independent of the presence of vascular tissue
. All living polysporangiophytes also have vascular tissue, i.e. are vascular plant
s or tracheophytes. Fossil polysporangiophytes are known which do not have vascular tissue and so are not tracheophytes.
s, either single or in groups. Megafossils are preserved parts of plants which are large enough to show structure, such as stem cross-sections or branching patterns.
Dawson
, a Canadian geologist and paleobotanist, was the first to discover and describe a megafossil of a polysporangiophyte. In 1859 he published a reconstruction of a Devonian
plant, collected as a fossil from the Gaspé region
of Canada, which he named Psilophyton
princeps. The reconstruction shows horizontal and upright stem-like structures; no leaves or roots are present. The upright stems or axes branch dichotomously and have pairs of spore-forming organs (sporangia
) attached to them. Cross-sections of the upright axes showed that vascular tissue
was present. He later described other specimens. Dawson's discoveries initially had little scientific impact; Taylor et al. speculate that this may have been because his reconstruction looked very unusual and the fossil was older than was expected.
From 1917 onwards, Kidston and Lang published a series of papers describing fossil plants from the Rhynie chert
– a fine-grained sedimentary rock found near the village of Rhynie, Aberdeenshire, Scotland, now dated to the Pragian of the Lower Devonian (around ). The fossils were better-preserved than Dawson's, and showed clearly that these early land plants did indeed consist of generally naked vertical stems arising from similar horizontal structures. The vertical stems were dichotomously branched with some branches ending in sporangia.
Since these discoveries, similar megafossils have been discovered in rocks of Silurian
to mid-Devonian age throughout the world, including Arctic Canada, the eastern USA, Wales, the Rhineland of Germany, Kazakhstan, Xinjiang and Yunnan in China, and Australia.
is the possession of a sporophyte which branches and bears multiple sporangia. This distinguishes polysporangiophytes from liverworts
, moss
es and hornwort
s which have unbranched sporophytes each with a single sporangium. Polysporangiophytes may or may not have vascular tissue
; those which do are vascular plants or tracheophytes.
At first most of the early polysporangiophytes were placed in a single order
, Psilophytales, in the class
Psilophyta, established in 1917 by Kidston and Lang. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida.
As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants. In 1975 Banks expanded on his earlier 1968 proposal which split it three groups, which he put at the rank
of subdivision. These groups have since been treated at the ranks of division, class and order. A variety of names have been used, which are summarized in the table below.
For Banks, rhyniophytes comprised simple leafless plants with terminal sporangia (e.g. Cooksonia
, Rhynia
); zosterophylls comprised plants with lateral sporangia which split distally (i.e. away from their attachment) to release their spores and which had exarch strands of xylem
(e.g. Gosslingia
); and trimerophytes comprised plants with large clusters of downwards curving terminal sporangia which split along their length to release their spores and which had centrarch xylem strands (e.g. Psilophyton
).
The research by Kenrick and Crane which established the polysporangiophytes concluded that none of Banks' three groups were monophyletic
. The rhyniophytes included 'protracheophytes' which were precursors to vascular plants (e.g. Horneophyton
, Aglaophyton
); basal tracheophytes (e.g. Stockmansella
, Rhynia
gwynne-vaughanii); and plants allied to the lineages that led to the living club-mosses and allies as well as ferns and seed plants (e.g. Cooksonia
species). The zosterophylls did contain a monophyletic clade, but some genera previously included in the group fell outside this clade (e.g. Hicklingia
, Nothia
). The trimerophytes were paraphyletic stem groups to both the crown group
fern
s and the crown group seed plants.
Many researchers have urged caution in the classification of early polysporangiophytes. Taylor et al. note that basal groups, such as early land plants, are inherently difficult to characterize since they will share many characters with all later-evolving groups (i.e. will have multiple plesiomorphies). In discussing the classification of the trimerophytes, Berry and Fairon-Demaret say that reaching a meaningful classification requires "a breakthrough in knowledge and understanding rather than simply a reinterpretation of the existing data and the surrounding mythology". Kenrick and Crane's cladograms have been questioned – see the Evolution section below.
, there appears to be no complete Linnean (i.e. rank-based) classification for early polysporangiophytes which is consistent with Kenrick and Crane's cladistic analysis and subsequent research, although Cantino et al. have published a Phylocode
classification. Banks' three groups continue to be used for convenience.
study of land plants was published in 1997 by Kenrick and Crane; this both established the concept of the polysporangiophytes and presented a view of their phylogeny
. Since 1997 there have been continual advances in understanding plant evolution, using RNA and DNA genome sequences and chemical analyses of fossils (e.g. Taylor et al. 2006), resulting in revisions to this phylogeny.
In 2004, Crane et al. published a simplified cladogram
for the polysporangiophytes (which they call polysporangiates), based on a number of figures in Kenrick and Crane (1997). Their cladogram is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram). Their analysis is not accepted by other researchers; for example Rothwell and Nixon say that the broadly defined fern group (moniliforms or monilophytes) is not monophyletic.
More recently, Gerrienne and Gonez have suggested a slightly different characterization of the early diverging polysporangiophytes.
The paraphyletic protracheophytes, such as Aglaophyton, have water-conducting vessels like those of mosses, i.e. without cells containing thickened cell walls. The paratracheophytes, a name intended to replace Rhyniaceae or Rhyniopsida, have 'S-type' water-conducting cells, i.e. cells whose walls are thickened but in a much simpler fashion than those of the true vascular plants, the eutracheophytes.
, a clade at the 'protracheophyte' grade that is sister to all other polysporangiophytes. They had essentially an isomorphic alternation of generations
(meaning that the sporophytes and gametophytes were equally free living), which might suggest that both the gametophyte-dominant life style of bryophytes and the sporophyte-dominant life style of vascular plants evolved from this isomorphic condition. They were leafless and did not have true vascular tissues. In particular, they did not have tracheid
s: elongated cells that aid in the transport of water and mineral salts and which develop a thick lignified
wall at maturity, thus providing mechanical strength. Unlike plants at the bryophyte
grade, their sporophytes were branched.
According to the cladogram, the genus Rhynia
illustrates two steps in the evolution of modern vascular plants. Plants have vascular tissue, albeit significantly simpler than modern vascular plants. Their gametophytes are distinctly smaller than their sporophytes (but have vascular tissue, unlike almost all modern vascular plants).
The remainder of the polysporangiophytes divide into two lineages, a deep phylogenetic split that occurred in the early to mid Devonian, around 400 million years ago. Both lineages have developed leaves, but of different kinds. The lycophytes, which make up less than 1% of the species of living vascular plants, have small leaves (microphyll
s or more specifically lycophylls), which develop from an intercalary meristem
(i.e. the leaves effectively grow from the base). The euphyllophytes are by far the largest group of vascular plants, in terms of both individuals and species. Euphyllophytes have large 'true' leaves (megaphylls), which develop through marginal or apical meristems (i.e. the leaves effectively grow from the sides or the apex).
Both the cladogram derived from Kenrick and Crane's studies and its implications for the evolution of land plants have been questioned by others. A 2008 review by Gensel notes that recently discovered fossil spores suggest that tracheophytes were present earlier than previously thought; perhaps earlier than supposed stem group members. Spore diversity suggests that there were many plant groups of which no other remains are known. Some early plants may have had heteromorphic alternation of generations, with later acquisition of isomorphic gametophytes in certain lineages.
The cladogram above shows the 'protracheophytes' diverging earlier than the lycophytes; however, lycophytes were present in the Ludfordian stage of the Silurian around , long before the 'protracheophytes' found in the Rhynie chert
, dated to the Pragian stage of the Devonian around . However, it has been suggested that the poorly preserved Eohostimella
, found in deposits of Early Silurian age (Llandovery, around ), may be a rhyniophyte.
Boyce has shown that the sporophytes of some Cooksonia species and allies ('cooksonioids') had stems which were too narrow to have supported sufficient photosynthetic activity to have allowed them to be independent of their gametophytes, inconsistent with their position in the cladogram.
The evolutionary history of plants is far from settled.
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
) has a structure of branching stems (axes) terminating in sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
(hence the name of the group which literally means "many sporangia plant"). The 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...
includes most land plants (embryophyte
Embryophyte
The land plants or embryophytes, more formally Embryophyta or Metaphyta, are the most familiar group of plants. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees,...
s) except for the bryophyte
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...
s (liverworts, mosses and hornworts) whose sporophytes are always unbranched. The definition is independent of the presence of vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
. All living polysporangiophytes also have vascular tissue, i.e. are vascular plant
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s or tracheophytes. Fossil polysporangiophytes are known which do not have vascular tissue and so are not tracheophytes.
History of discovery
Paleobotanists distinguish between micro- and megafossils. Microfossils are primarily sporeSpore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s, either single or in groups. Megafossils are preserved parts of plants which are large enough to show structure, such as stem cross-sections or branching patterns.
Dawson
John William Dawson
Sir John William Dawson, CMG, FRS, FRSC , was a Canadian geologist and university administrator.- Life and work :...
, a Canadian geologist and paleobotanist, was the first to discover and describe a megafossil of a polysporangiophyte. In 1859 he published a reconstruction of a 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...
plant, collected as a fossil from the Gaspé region
Gaspé Peninsula
The Gaspésie , or Gaspé Peninsula or the Gaspé, is a peninsula along the south shore of the Saint Lawrence River in Quebec, Canada, extending into the Gulf of Saint Lawrence...
of Canada, which he named Psilophyton
Psilophyton
Psilophyton is a genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age . Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China...
princeps. The reconstruction shows horizontal and upright stem-like structures; no leaves or roots are present. The upright stems or axes branch dichotomously and have pairs of spore-forming organs (sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
) attached to them. Cross-sections of the upright axes showed that vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
was present. He later described other specimens. Dawson's discoveries initially had little scientific impact; Taylor et al. speculate that this may have been because his reconstruction looked very unusual and the fossil was older than was expected.
From 1917 onwards, Kidston and Lang published a series of papers describing fossil plants from the Rhynie chert
Rhynie chert
The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness . It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away...
– a fine-grained sedimentary rock found near the village of Rhynie, Aberdeenshire, Scotland, now dated to the Pragian of the Lower Devonian (around ). The fossils were better-preserved than Dawson's, and showed clearly that these early land plants did indeed consist of generally naked vertical stems arising from similar horizontal structures. The vertical stems were dichotomously branched with some branches ending in sporangia.
Since these discoveries, similar megafossils have been discovered in rocks of Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
to mid-Devonian age throughout the world, including Arctic Canada, the eastern USA, Wales, the Rhineland of Germany, Kazakhstan, Xinjiang and Yunnan in China, and Australia.
Taxonomy
The concept of the polysporangiophytes, more formally called Polysporangiophyta, was first published in 1997 by Kenrick and Crane. (The taxobox at the right represents their view of the classification of the polysporangiophytes.) The defining feature of the cladeClade
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...
is the possession of a sporophyte which branches and bears multiple sporangia. This distinguishes polysporangiophytes from liverworts
Marchantiophyta
The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....
, moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es and hornwort
Hornwort
Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...
s which have unbranched sporophytes each with a single sporangium. Polysporangiophytes may or may not have vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
; those which do are vascular plants or tracheophytes.
At first most of the early polysporangiophytes were placed in a single order
Order (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...
, Psilophytales, in the class
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...
Psilophyta, established in 1917 by Kidston and Lang. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida.
As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants. In 1975 Banks expanded on his earlier 1968 proposal which split it three groups, which he put at the rank
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
of subdivision. These groups have since been treated at the ranks of division, class and order. A variety of names have been used, which are summarized in the table below.
| Division | Subdivision | Class | Order | Informal |
|---|---|---|---|---|
| Rhyniophyta | Rhyniophytina | Rhyniopsida (Rhyniophytopsida) | Rhyniales | rhyniophyte |
| Zosterophyllophyta | Zosterophyllophytina | Zosterophyllopsida | Zosterophyllales | zosterophyll (zosterophyllophyte) |
| Trimerophyta (Trimerophytophyta) | Trimerophytina (Trimerophytophytina) | Trimeropsida (Trimerophytopsida) | Trimerophytales | trimerophyte |
For Banks, rhyniophytes comprised simple leafless plants with terminal sporangia (e.g. Cooksonia
Cooksonia
Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian ; the group continues to be an important component of the flora until the Early Devonian, a total time span of...
, Rhynia
Rhynia
Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes, and was basal to modern vascular plants or eutracheophytes.-Description:R...
); zosterophylls comprised plants with lateral sporangia which split distally (i.e. away from their attachment) to release their spores and which had exarch strands of 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...
(e.g. Gosslingia
Gosslingia
Gosslingia was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Gosslingia in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
); and trimerophytes comprised plants with large clusters of downwards curving terminal sporangia which split along their length to release their spores and which had centrarch xylem strands (e.g. Psilophyton
Psilophyton
Psilophyton is a genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age . Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China...
).
The research by Kenrick and Crane which established the polysporangiophytes concluded that none of Banks' three groups were monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
. The rhyniophytes included 'protracheophytes' which were precursors to vascular plants (e.g. Horneophyton
Horneophyton
Horneophyton, a member of the Horneophytopsida, was an early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert.-Description:...
, Aglaophyton
Aglaophyton
Aglaophyton major was the sporophyte generation of a diplohaplontic, pre-vascular, axial, free-sporing land plant of the Lower Devonian . It had anatomical features intermediate between those of the bryophytes and vascular plants or tracheophytes.A. major was first described by Kidston and Lang in...
); basal tracheophytes (e.g. Stockmansella
Stockmansella
Stockmansella is a genus of extinct plants of the Middle Devonian , fossils of which have been found in north-west Germany. The sporophyte generation consists of prostrate dichotomizing stems up to 10cm long and around 3mm wide, which at intervals produce narrower smooth upright stems...
, Rhynia
Rhynia
Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes, and was basal to modern vascular plants or eutracheophytes.-Description:R...
gwynne-vaughanii); and plants allied to the lineages that led to the living club-mosses and allies as well as ferns and seed plants (e.g. Cooksonia
Cooksonia
Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian ; the group continues to be an important component of the flora until the Early Devonian, a total time span of...
species). The zosterophylls did contain a monophyletic clade, but some genera previously included in the group fell outside this clade (e.g. Hicklingia
Hicklingia
Hicklingia is a genus of extinct plants of the Middle Devonian . Compressed specimens were first described in 1923 from the Old Red Sandstone of Scotland...
, Nothia
Nothia
Nothia was a genus of Early Devonian vascular plants whose fossils were found in the Rhynie chert in Scotland. It had branching horizontal underground stems and leafless aerial stems bearing lateral and terminal spore-forming organs . Its aerial stems were covered with small 'bumps' , each...
). The trimerophytes were paraphyletic stem groups to both the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
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...
s and the crown group seed plants.
Many researchers have urged caution in the classification of early polysporangiophytes. Taylor et al. note that basal groups, such as early land plants, are inherently difficult to characterize since they will share many characters with all later-evolving groups (i.e. will have multiple plesiomorphies). In discussing the classification of the trimerophytes, Berry and Fairon-Demaret say that reaching a meaningful classification requires "a breakthrough in knowledge and understanding rather than simply a reinterpretation of the existing data and the surrounding mythology". Kenrick and Crane's cladograms have been questioned – see the Evolution section below.
, there appears to be no complete Linnean (i.e. rank-based) classification for early polysporangiophytes which is consistent with Kenrick and Crane's cladistic analysis and subsequent research, although Cantino et al. have published a Phylocode
PhyloCode
The International Code of Phylogenetic Nomenclature, known as the PhyloCode for short, is a developing draft for a formal set of rules governing phylogenetic nomenclature...
classification. Banks' three groups continue to be used for convenience.
Phylogeny
A major cladisticCladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
study of land plants was published in 1997 by Kenrick and Crane; this both established the concept of the polysporangiophytes and presented a view of their phylogeny
Phylogenetic tree
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
. Since 1997 there have been continual advances in understanding plant evolution, using RNA and DNA genome sequences and chemical analyses of fossils (e.g. Taylor et al. 2006), resulting in revisions to this phylogeny.
In 2004, Crane et al. published a simplified cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
for the polysporangiophytes (which they call polysporangiates), based on a number of figures in Kenrick and Crane (1997). Their cladogram is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram). Their analysis is not accepted by other researchers; for example Rothwell and Nixon say that the broadly defined fern group (moniliforms or monilophytes) is not monophyletic.
More recently, Gerrienne and Gonez have suggested a slightly different characterization of the early diverging polysporangiophytes.
The paraphyletic protracheophytes, such as Aglaophyton, have water-conducting vessels like those of mosses, i.e. without cells containing thickened cell walls. The paratracheophytes, a name intended to replace Rhyniaceae or Rhyniopsida, have 'S-type' water-conducting cells, i.e. cells whose walls are thickened but in a much simpler fashion than those of the true vascular plants, the eutracheophytes.
Evolution
If the cladogram above is correct it has implications for the evolution of land plants. The earliest diverging polysporangiophytes in the cladogram are the HorneophytopsidaHorneophytopsida
Horneophytopsida is a class of extinct plants which consisted of branched stems without leaves, true roots or vascular tissue, found from the Late Silurian to the Early Devonian . They are the simplest known polysporangiophytes, i.e. plants with sporophytes bearing many spore-forming organs on...
, a clade at the 'protracheophyte' grade that is sister to all other polysporangiophytes. They had essentially an isomorphic alternation of generations
Alternation of generations
Alternation of generations is a term primarily used in describing the life cycle of plants . A multicellular sporophyte, which is diploid with 2N paired chromosomes , alternates with a multicellular gametophyte, which is haploid with N unpaired chromosomes...
(meaning that the sporophytes and gametophytes were equally free living), which might suggest that both the gametophyte-dominant life style of bryophytes and the sporophyte-dominant life style of vascular plants evolved from this isomorphic condition. They were leafless and did not have true vascular tissues. In particular, they did not have tracheid
Tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...
s: elongated cells that aid in the transport of water and mineral salts and which develop a thick lignified
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
wall at maturity, thus providing mechanical strength. Unlike plants at the bryophyte
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...
grade, their sporophytes were branched.
According to the cladogram, the genus Rhynia
Rhynia
Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes, and was basal to modern vascular plants or eutracheophytes.-Description:R...
illustrates two steps in the evolution of modern vascular plants. Plants have vascular tissue, albeit significantly simpler than modern vascular plants. Their gametophytes are distinctly smaller than their sporophytes (but have vascular tissue, unlike almost all modern vascular plants).
The remainder of the polysporangiophytes divide into two lineages, a deep phylogenetic split that occurred in the early to mid Devonian, around 400 million years ago. Both lineages have developed leaves, but of different kinds. The lycophytes, which make up less than 1% of the species of living vascular plants, have 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 or more specifically lycophylls), which develop from 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....
(i.e. the leaves effectively grow from the base). The euphyllophytes are by far the largest group of vascular plants, in terms of both individuals and species. Euphyllophytes have large 'true' leaves (megaphylls), which develop through marginal or apical meristems (i.e. the leaves effectively grow from the sides or the apex).
Both the cladogram derived from Kenrick and Crane's studies and its implications for the evolution of land plants have been questioned by others. A 2008 review by Gensel notes that recently discovered fossil spores suggest that tracheophytes were present earlier than previously thought; perhaps earlier than supposed stem group members. Spore diversity suggests that there were many plant groups of which no other remains are known. Some early plants may have had heteromorphic alternation of generations, with later acquisition of isomorphic gametophytes in certain lineages.
The cladogram above shows the 'protracheophytes' diverging earlier than the lycophytes; however, lycophytes were present in the Ludfordian stage of the Silurian around , long before the 'protracheophytes' found in the Rhynie chert
Rhynie chert
The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness . It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away...
, dated to the Pragian stage of the Devonian around . However, it has been suggested that the poorly preserved Eohostimella
Eohostimella
Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils of Early Silurian age . The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae...
, found in deposits of Early Silurian age (Llandovery, around ), may be a rhyniophyte.
Boyce has shown that the sporophytes of some Cooksonia species and allies ('cooksonioids') had stems which were too narrow to have supported sufficient photosynthetic activity to have allowed them to be independent of their gametophytes, inconsistent with their position in the cladogram.
The evolutionary history of plants is far from settled.
External links
- Cladogram from
