| Lycopodiopsida | |
|---|---|
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| Palhinhaea cernua with close-up of branch | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Lycophytes |
| Class: | Lycopodiopsida Bartl. |
| Orders | |
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| Synonyms | |
SeeTable 1. | |
Lycopodiopsida is a class ofvascular plants also known aslycopsids,[1]lycopods, orlycophytes. Members of the class are also calledclubmosses,firmosses,spikemosses andquillworts. They have dichotomously branching stems bearing simple leaves calledmicrophylls and reproduce by means ofspores borne insporangia on the sides of the stems at the bases of the leaves. Although living species are small, during theCarboniferous, extinct tree-like forms (Lepidodendrales) formed huge forests that dominated the landscape and contributed tocoal deposits.
The nomenclature and classification of plants with microphylls varies substantially among authors. A consensus classification for extant (living) species was produced in 2016 by thePteridophyte Phylogeny Group (PPG I), which places them all in the class Lycopodiopsida, which includes the classesIsoetopsida andSelaginellopsida used in other systems. (SeeTable 2.) Alternative classification systems have used ranks from division (phylum) to subclass. In the PPG I system, the class is divided into three orders,Lycopodiales,Isoetales andSelaginellales.
Club-mosses (Lycopodiales) are homosporous, but the generaSelaginella (spikemosses) andIsoetes (quillworts) are heterosporous, with female spores larger than the male.[2] As a result of fertilisation, the female gametophyte produces sporophytes. A few species ofSelaginella such asS. apoda andS. rupestris are alsoviviparous; the gametophyte develops on the mother plant, and only when the sporophyte's primary shoot and root is developed enough for independence is the new plant dropped to the ground.[3] Many club-mossgametophytes aremycoheterotrophic and long-lived, residing underground for several years before emerging from the ground and progressing to thesporophyte stage.[4]
Lycopodiaceae and spikemosses (Selaginella) are the only vascular plants with biflagellate sperm, an ancestral trait in land plants otherwise only seen inbryophytes. The only exceptions areIsoetes andPhylloglossum, which independently has evolved multiflagellated sperm cells with approximately 20 flagella[5][6] (sperm flagella in other vascular plants can count at least thousand at most, but the number is generally much lower, and flagella are completely absent in seed plants except for Ginkgo and cycads).[7] Because only two flagella puts a size limit on the genome, we find the largest known genomes in the clade inIsoetes, as multiflagellated sperm is not exposed for the same selection pressure as biflagellate sperm in regard of size.[8]
The extant lycophytes arevascular plants (tracheophytes) withmicrophyllous leaves, distinguishing them from theeuphyllophytes (plants withmegaphyllous leaves). The sister group of the extant lycophytes and their closest extinct relatives are generally believed to be thezosterophylls, aparaphyletic orplesion group. Ignoring some smaller extinct taxa, the evolutionary relationships are as shown below.[9][10][11]
| tracheophytes |
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| (vascular plants) |
As of 2019[update], there was broad agreement, supported by both molecular and morphological evidence, that the extant lycophytes fell into three groups, treated as orders in PPG I, and that these, both together and individually, aremonophyletic, being related as shown in the cladogram below:[11]
| extant lycophytes | |
The rank and name used for the taxon holding the extant lycophytes (and their closest extinct relatives) varies widely. Table 1 below shows some of the highest ranks that have been used. Systems may use taxa at a rank lower than the highest given in the table with the same circumscription; for example, a system that uses Lycopodiophyta as the highest ranked taxon may place all of its members in a single subclass.
| Highest rank | Name | Example sources |
|---|---|---|
| Division (phylum) | Lycophyta | Taylor et al. (2009),[12] Mauseth (2014)[10] |
| Division (phylum) | Lycopodiophyta | Niklas (2016)[13] |
| Subdivision (subphylum) | Lycopodiophytina | Ruggiero et al. (2015)[14] |
| Class | Lycopsida | Kenrick & Crane (1997)[9][15] |
| Class | Lycopodiopsida | PPG I (2016)[11] |
| Subclass | Lycopodiidae | Chase & Reveal (2009)[16] |
Some systems use a higher rank for a more broadly defined taxon of lycophytes that includes some extinct groups more distantly related to extant lycophytes, such as thezosterophylls. For example, Kenrick & Crane (1997) use the subdivision Lycophytina for this purpose, with all extant lycophytes falling within the class Lycopsida.[9] Other sources exclude the zosterophylls from any "lycophyte" taxon.[12]
In thePteridophyte Phylogeny Group classification of 2016 (PPG I), the three orders are placed in a single class, Lycopodiopsida, holding all extant lycophyte species. Older systems have used either three classes, one for each order, or two classes, recognizing the closer relationship between Isoetales and Selaginellales. In these cases, a higher ranked taxon is needed to contain the classes (see Table 1). As Table 2 shows, the names "Lycopodiopsida" and "Isoetopsida" are both ambiguous.
| Order | 3 classes e.g.IUCN Red List, 2004[17] | 2 classes e.g. Yatsentyuk et al. (2001)[18] | 1 class PPG I[11] |
|---|---|---|---|
| Lycopodiales | Lycopodiopsida | Lycopodiopsida | Lycopodiopsida |
| Isoetales | Isoetopsida | Isoetopsida | |
| Selaginellales | Sellaginellopsida |
The PPG I system divides up the extant lycophytes as shown below.
Some extinct groups, such aszosterophylls, fall outside the limits of the taxon as defined by the classifications in Table 1 above. However, other extinct groups fall within somecircumscriptions of this taxon. Taylor et al. (2009) and Mauseth (2014) include a number of extinct orders in their division (phylum) Lycophyta, although they differ on the placement of some genera.[12][10] The orders included by Taylor et al. are:[12]
Mauseth uses the order †Asteroxylales, placingBaragwanathia in the Protolepidodendrales.[10]
The relationship between some of these extinct groups and the extant ones was investigated by Kenrick and Crane in 1997. When the genera they used are assigned to orders, their suggested relationship is:[19]
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The Lycopodiopsida are distinguished from other vascular plants by the possession of microphylls and by their sporangia, which are lateral as opposed to terminal and which open (dehisce) transversely rather than longitudinally. In some groups, the sporangia are borne on sporophylls that are clustered into strobili. Phylogenetic analysis shows the group branching off at the base of the evolution of vascular plants and they have a long evolutionary history.Fossils are abundant worldwide, especially incoal deposits. Fossils that can be ascribed to the Lycopodiopsida first appear in theSilurian period, along with a number of other vascular plants. The SilurianBaragwanathia longifolia is one of the earliest identifiable species.Lycopodolica is another Silurian genus which appears to be an early member of this group.[20] The group evolved roots independently from the rest of the vascular plants.[21][22]
From theDevonian onwards, some species grew large and tree-like. Devonian fossil lycopsids fromSvalbard, growing in equatorial regions, raise the possibility that they drew down enough carbon dioxide to change the Earth's climate significantly.[23] During theCarboniferous,tree-like plants (such asLepidodendron,Sigillaria, and other extinct genera of the orderLepidodendrales) formed huge forests that dominated the landscape. Unlike modern trees, leaves grew out of the entire surface of the trunk and branches, but fell off as the plant grew, leaving only a small cluster of leaves at the top. The lycopsids had distinctive features such asLepidodendron lycophytes, which were marked with diamond-shaped scars where they once had leaves. Quillworts (order Isoetales) andSelaginella are considered their closest extant relatives and share some unusual features with thesefossil lycopods, including the development of both bark,cambium andwood, a modified shoot system acting as roots, bipolar andsecondary growth, and an upright stance.[3][24] The remains ofLepidodendron lycopods formed many fossilcoal deposits. InFossil Grove, Victoria Park, Glasgow, Scotland, fossilized lycophytes can be found insandstone.
The Lycopodiopsida had their maximum diversity in thePennsylvanian (Upper Carboniferous), particularly tree-likeLepidodendron andSigillaria that dominated tropical wetlands. The complex ecology of these tropical rainforestscollapsed during the Middle Pennsylvanian due to a change in climate.[25] InEuramerica, tree-like species apparently became extinct in the Late Pennsylvanian, as a result of a transition to a much drier climate, giving way toconifers,ferns andhorsetails. InCathaysia (now South China), tree-like species survived into thePermian. Nevertheless, lycopodiopsids are rare in theLopingian (latest Permian), but regained dominance in theInduan (earliest Triassic), particularlyPleuromeia. After the worldwidePermian–Triassic extinction event, members of this group pioneered the repopulation of habitats as opportunistic plants. The heterogeneity of the terrestrial plant communities increased markedly during the Middle Triassic when plant groups like horsetails, ferns,pteridosperms,cycads,ginkgos and conifers resurfaced and diversified quickly.[26]
Lycophytes form associations with microbes such as fungi and bacteria, includingarbuscular mycorrhizal andendophytic associations.
Arbuscular mycorrhizal associations have been characterized in all stages of the lycophyte lifecycle:mycoheterotrophic gametophyte, photosynthetic surface-dwelling gametophyte, young sporophyte, and mature sporophyte.[4] Arbuscular mycorrhizae have been found inSelaginella spp. roots and vesicles.[27]
During the mycoheterotrophic gametophyte lifecycle stage, lycophytes gain all of their carbon from subterraneanglomalean fungi. In other plant taxa, glomalean networks transfer carbon from neighboring plants to mycoheterotrophic gametophytes. Something similar could be occurring inHuperzia hypogeae gametophytes which associate with the same glomalean phenotypes as nearbyHuperzia hypogeae sporophytes.[4]
Fungal endophytes have been found in many species of lycophyte, however the function of these endophytes in host plant biology is not known. Endophytes of other plant taxa perform roles such as improving plant competitive fitness, conferring biotic and abiotic stress tolerance, promoting plant growth through phytohormone production or production of limiting nutrients.[28] However, some endophytic fungi in lycophytes do produce medically relevant compounds.Shiraia sp Slf14 is an endophytic fungus present inHuperzia serrata that producesHuperzine A, a biomedical compound which has been approved as a drug in China and a dietary supplement in the U.S. to treat Alzheimer's Disease.[29] This fungal endophyte can be cultivated much more easily and on a much larger scale thanH. serrata itself which could increase the availability of Huperzine A as a medicine.
The spores of lycopods are highly flammable and so have been used infireworks.[30]Lycopodium powder, the dried spores of the common clubmoss, was used in Victorian theater to produce flame-effects. A blown cloud of spores burned rapidly and brightly, but with little heat. (It was considered safe by the standards of the time.)[citation needed]
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