Editing Isoetes

{{Use dmy dates|date=October 2023}}
{{Short description|Genus of vascular plants in the family Isoetaceae}}{{Automatic taxobox
| fossil_range = {{fossil range|Jurassic|Recent|earliest=Late Permian}}
| image = Isoetes tegetiformans.jpg
| image_caption = ''[[Isoetes tegetiformans]]'' with U.S. penny for scale
| taxon = Isoetes
| authority = [[Carl Linnaeus|L.]]
| subdivision_ranks = Species
| subdivision = See text
}}
'''''Isoetes''''', commonly known as the '''quillworts''', is a genus of [[Lycopodiopsida|lycopod]]. It is the only living genus in the family [[Isoetaceae]] and order [[Isoetales]]. {{As of|2016}}, there were about 200 recognized species,<ref name="TroiaPereira2016">{{cite journal|last1=Troia|first1=Angelo|last2=Pereira|first2=Jovani B.|last3=Kim|first3=Changkyun|last4=Taylor|first4=W. Carl|title=The genus Isoetes (Isoetaceae): a provisional checklist of the accepted and unresolved taxa|journal=Phytotaxa|volume=277|issue=2|year=2016|pages=101|issn=1179-3163|doi=10.11646/phytotaxa.277.2.1}}</ref> with a [[cosmopolitan distribution]] mostly in aquatic habitats but with the individual species often scarce to rare. Species virtually identical to modern quillworts have existed since the [[Jurassic]] epoch,<ref name="Wood2020">{{Cite journal |last1=Wood |first1=Daniel |last2=Besnard |first2=Guillaume |last3=Beerling |first3=David J. |last4=Osborne |first4=Colin P. |last5=Christin |first5=Pascal-Antoine |date=18 June 2020 |title=Phylogenomics indicates the "living fossil" Isoetes diversified in the Cenozoic |journal=PLOS ONE |language=en |volume=15 |issue=6 |pages=e0227525 |bibcode=2020PLoSO..1527525W |doi=10.1371/journal.pone.0227525 |issn=1932-6203 |pmc=7302493 |pmid=32555586 |doi-access=free}}</ref> though the timing of the origin of modern ''Isoetes'' is subject to considerable uncertainty.<ref name="Wikström-2023">{{Cite journal |last1=Wikström |first1=Niklas |last2=Larsén |first2=Eva |last3=Khodabandeh |first3=Anbar |last4=Rydin |first4=Catarina |date=January 2023 |title=No phylogenomic support for a Cenozoic origin of the "living fossil" Isoetes |journal=American Journal of Botany |language=en |volume=110 |issue=1 |pages=e16108 |doi=10.1002/ajb2.16108 |issn=0002-9122 |pmc=10108322 |pmid=36401556}}</ref>

The name of the genus may also be spelled ''Isoëtes''. The [[Diaeresis (diacritic)|diaeresis]] (two dots over the e) indicates that the o and the e are to be pronounced in two distinct syllables. Including this in print is optional; either spelling (''Isoetes'' or ''Isoëtes'') is correct.<ref>[http://www.iapt-taxon.org/nomen/main.php?page=art60 International Code of Nomenclature  for algae, fungi, and plants (Melbourne Code)] see section 60.6: "The diaeresis, indicating that a vowel is to be pronounced separately from the preceding vowel (as in Cephaëlis, Isoëtes), is a phonetic device that is not considered to alter the spelling; as such, its use is optional."</ref>

==Description==
[[File:Isoetes megasporangia L.jpg|thumb|right|upright|Quillwort megasporangia]]
Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several (e.g. ''[[Isoetes butleri|I.&nbsp;butleri]]'', ''[[Isoetes histrix|I.&nbsp;histrix]]'' and ''[[Isoetes nuttallii|I.&nbsp;nuttallii]]'') grow on wet ground that dries out in the summer. The quillworts are spore-producing plants and highly reliant on water dispersion. Quillworts have different ways to spread their spores based on the environment. Quillwort [[leaf|leaves]] are hollow and quill-like, with a minute [[ligule]] at the base of the upper surface.<ref name=Stace>{{cite book |last=Stace |first=C. A. |author-link=Stace, C. A. |year=2010 |title=New Flora of the British Isles |edition=3rd |publisher=Cambridge University Press |location=Cambridge, U.K. |isbn=9780521707725}}</ref>{{rp|7}} arising from a central [[corm]]. The sporangia are sunk deeply in the leaf bases. Each leaf will either have many small spores or fewer large spores. Both types of leaf are found on each plant.<ref>{{Cite book |last=Levyns |first=M.R. |title=A Guide to the Flora of the Cape Peninsula |publisher=Juta & Company, Limited |year=1966 |edition=2nd Revised |oclc=621340}}</ref> Each leaf is narrow, {{convert|2|-|20|cm|sigfig=1}} long (exceptionally up to {{convert|100|cm|sigfig=1|disp=or|abbr=on}}) and {{convert|0.5|-|3.0|mm|2|abbr=on}} wide; they can be either [[evergreen]], winter [[deciduous]], or dry-season deciduous. Only 4% of total biomass, the tips of the leaves, is chlorophyllous.<ref name = "book - form&function">[https://books.google.com/books?id=YJFB4l262WkC&dq=Only+the+tips+of+the+leaves+are+chlorophyllous+%284%25+of+total+biomass%29&pg=PA171 Tropical Alpine Environments: Plant Form and Function]</ref>

The roots broaden to a swollen base up to {{convert|5|mm|1|abbr=on}} wide where they attach in clusters to a bulb-like, underground [[rhizome]] characteristic of most quillwort species, though a few (e.g. ''[[Isoetes tegetiformans|I.&nbsp;tegetiformans]]'') form spreading mats. This swollen base also contains male and female sporangia, protected by a thin, transparent covering (''velum''), which is used diagnostically to help identify quillwort species. They are [[spore|heterosporous]]. Quillwort species are very difficult to distinguish by general appearance. The best way to identify them is by examining their megaspores under a microscope. Moreover, habitat, texture, spore size, and velum provide features that distinguish Isoëtes taxa.<ref>Isoëtes Linnaeus, Sp. Pl. 2: 1100. 1753; Gen. Pl. ed. 5, 486, 1754.</ref> They also possess a vestigial form of secondary growth in the basal portions of its cormlike stem, an indication that they evolved from larger ancestors.<ref>[https://books.google.com/books?id=aKzSBAAAQBAJ&dq=%22The+extant+genus+Isoetes+possesses+a+vestigial+form+of+secondary+growth%22&pg=PA7 The Formation of Wood in Forest Trees: The Second Symposium Held under the Auspices of the Maria Moors Cabot Foundation for Botanical Research]</ref>

==Biochemistry and genetics==
Quillworts use [[crassulacean acid metabolism]] (CAM) for carbon fixation. Some aquatic species do not have [[stoma]]ta and the leaves have a thick cuticle which prevents CO<sub>2</sub> uptake, a task that is performed by their hollow roots instead, which absorb CO<sub>2</sub> from the sediment.<ref>{{cite book |url=https://books.google.com/books?id=KFExDwAAQBAJ&q=%22Isoetes+leaves+lack+stomata+pores%2C+as+do+most+submerged+plants%22&pg=PA108 |title=Ecology of High Altitude Waters]|isbn=9780191056666|last1=Jacobsen|first1=Dean|last2=Dangles|first2=Olivier|date=18 August 2017|publisher=Oxford University Press }}</ref> This has been studied extensively in ''Isoetes andicola''.<ref name="book - form&function" /> [[Crassulacean acid metabolism|CAM]] is normally considered an adaptation to life in arid environments to prevent water loss with the plants opening their stomata at night rather than in the heat of the day. This allows CO<sub>2</sub> to enter and minimises water loss. As mostly submerged aquatic plants, quillworts do not lack water and the use of CAM is considered to avoid competition with other aquatic plants for CO<sub>2</sub> during daytime.<ref name="Haas2021-12">{{cite web |last1=Haas |first1=Michael J. |title=Quillwort genome highlights divergences in aquatic CAM photosynthesis |url=https://globalplantcouncil.org/quillwort-genome-highlights-divergences-in-aquatic-cam-photosynthesis/ |website=The Global Plant Council |date=2 December 2021 |access-date=29 December 2021}}</ref>

The first detailed quillwort genome sequence, of ''[[Isoetes taiwanensis|I. taiwanensis]]'',<ref name="Wickell et al 2021">{{cite journal |last1=Wickell |first1=David |last2=Kuo |first2=Li-Yaung |last3=Yang |first3=Hsiao-Pei |last4=others |first4=and 11 |title=Underwater CAM photosynthesis elucidated by Isoetes genome |journal=Nature Communications |date=2021 |volume=12 |issue=1 |page=6348 |doi=10.1038/s41467-021-26644-7 |pmid=34732722 |pmc=8566536 |bibcode=2021NatCo..12.6348W }}</ref> showed that there were differences from CAM in terrestrial plants. CAM involves the enzyme [[phosphoenolpyruvate carboxylase]] (PEPC) and plants have two forms of the enzyme. One is normally involved in photosynthesis and the other in central metabolism. From the genome sequence, it appears that in quillworts, both forms are involved in photosynthesis. In addition, [[Circadian rhythm|circadian]] expression of key CAM pathway genes peaked at different times of day than in angiosperms.<ref>[https://btiscience.org/explore-bti/news/post/secrets-of-quillwort-photosynthesis-could-boost-crop-efficiency/ Secrets of quillwort photosynthesis could boost crop efficiency]</ref> These fundamental differences in biochemistry suggest that CAM in quillworts is probably another example of [[convergent evolution]] of CAM during the more than 300 million years since the genus diverged from other plants. However, they may also be because of differences between life in water and in the air.<ref name="Wickell et al 2021" /> The genome sequence also provided two insights into its structure. First, genes and [[non-coding DNA|repeated non-coding regions]] were fairly evenly distributed across all the [[chromosomes]]. This is similar to genomes of other non-seed plants, but different from the seed plants ([[angiosperms]]) where there are distinctly more genes at the ends of chromosomes. Secondly, there was also evidence that the whole genome had been duplicated in the ancient past.<ref name="Wickell et al 2021" />

There are species that switch from CAM to C3 photosynthesis when they go from being submerged in water to living terrestrially, and develop stomata on their leaves. Some species (''I.&nbsp;palmeri'', ''I.&nbsp;lechleri'' and ''I.&nbsp;karsteni''), even under aerial conditions, rarely form stomata, and in some cases (''I.&nbsp;triquetra'' and ''I.&nbsp;andina'') appear to have completely lost the ability to produce stomata.<ref>[https://books.google.no/books?id=iv1MEAAAQBAJ&pg=PA192&dq=macrophytes+CAM+C3+stomata+palmeri+andina&hl=no&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwjG3KXtsKuLAxWIFxAIHc5lAKUQ6AF6BAgFEAM#v=onepage&q=macrophytes%20CAM%20C3%20stomata%20palmeri%20andina&f=false Permian Extinctions]</ref>

== Reproduction ==
[[File:Isoetes.reproduction.tif|thumb|Reproductive cycle of ''Isoetes'']]

=== Overview ===
Like all land plants, ''Isoetes'' undergoes an alternation of generations between a diploid sporophyte stage and a sexual haploid gametophyte stage. However, the dominance of one stage over the other has shifted over time. The development of vascular tissue and subsequent diversification of land plants coincides with the increased dominance of the sporophyte and reduction of the gametophyte. ''Isoetes'', as members of the [[Lycopodiopsida]] class, are part of the oldest extant lineage that reflects this shift to a sporophyte dominant lifecycle. In closely related lineages, such as the extinct ''[[Lepidodendron]]'', spores were dispersed by the sporophyte through large collections of sporangia called strobili for wind-based spore dispersal.<ref>{{Cite book|last=Kenrick, Paul.|url=https://www.worldcat.org/oclc/37107157|title=The origin and early diversification of land plants : a cladistic study|date=1997|publisher=Smithsonian Institution Press|others=Crane, Peter R.|isbn=1-56098-730-8|location=Washington, DC|oclc=37107157}}</ref> However, ''Isoetes'' are small heterosporous semi-aquatic plants, with different reproductive needs and challenges than large tree-like land plants.

===Description===
Like the rest of the Lycopodiopsida class, ''Isoetes'' reproduces with spores.<ref name="Taylor-1992">{{Cite journal|last1=Taylor|first1=W. Carl|last2=Hickey|first2=R. James|date=1992|title=Habitat, Evolution, and Speciation in Isoetes|url=https://www.jstor.org/stable/2399755|journal=Annals of the Missouri Botanical Garden|volume=79|issue=3|pages=613|doi=10.2307/2399755|jstor=2399755}}</ref> Among the lycophytes, both ''Isoetes'' and the [[Selaginellaceae]] (spikemosses) are [[Heterospory|heterosporous]], while the remaining lycophyte family [[Lycopodiaceae]] (clubmosses) is [[homosporous]].<ref>{{Cite journal|date=2016|title=A community-derived classification for extant lycophytes and ferns|journal=Journal of Systematics and Evolution|language=en|volume=54|issue=6|pages=563–603|doi=10.1111/jse.12229|s2cid=39980610|issn=1759-6831|doi-access=free}}</ref> As heterosporous plants, fertile Isoetes sporophytes produce megaspores and microspores, which develop in the megasporangia and microsporangia.<ref name="Farmer-1890">{{Cite journal|last=FARMER|first=J. BRETLAND|date=1890|title=On Isoetes lacustris, L.|url=https://www.jstor.org/stable/43234433|journal=Annals of Botany|volume=5|issue=17|pages=37–62|jstor=43234433|issn=0305-7364}}</ref> These spores are highly ornate and are the primary way by which species are identified, although no one functional purpose of the intricate surface patterns is agreed upon.<ref>{{Cite journal|last=Hickey|first=R. James|date=January 1986|title=Isoetes Megaspore Surface Morphology: Nomenclature, Variation, and Systematic Importance|url=http://dx.doi.org/10.2307/1547394|journal=American Fern Journal|volume=76|issue=1|pages=1–16|doi=10.2307/1547394|jstor=1547394|issn=0002-8444}}</ref> The megasporangia occur within the outermost [[Microphylls and megaphylls|microphylls]] (single-veined leaves) of the plant while the microsporangia are found in the innermost microphylls.<ref name="La Motte-1933">{{Cite journal|last=La Motte|first=Charles|title=Morphology of the Megagametophyte and the Embryo Sporophyte Ofisoetes Lithophila|date=April 1933|url=http://doi.wiley.com/10.1002/j.1537-2197.1933.tb08887.x|journal=American Journal of Botany|language=en|volume=20|issue=4|pages=217–233|doi=10.1002/j.1537-2197.1933.tb08887.x}}</ref> This pattern of development is hypothesized to improve the dispersal of the heavier megaspore.<ref name="Taylor-1992" /> These spores then germinate and divide into mega- and micro- gametophytes.<ref name="Farmer-1890" /><ref>{{Cite journal|last1=SCOTT|first1=D. H.|last2=HILL|first2=T. G.|date=1900|title=The Structure of Isoetes Hystrix|url=https://www.jstor.org/stable/43235515|journal=Annals of Botany|volume=14|issue=55|pages=413–454|doi=10.1093/oxfordjournals.aob.a088787|jstor=43235515|issn=0305-7364}}</ref><ref name="La Motte-1937">{{Cite journal|last=LA MOTTE|first=CHARLES|date=1937|title=Morphology and Orientation of the Embryo of Isoetes|url=https://www.jstor.org/stable/42906582|journal=Annals of Botany|volume=1|issue=4|pages=695–715|doi=10.1093/oxfordjournals.aob.a083498|jstor=42906582|issn=0305-7364}}</ref> The microgametophytes have antheridia, which in turn produce sperm.<ref name="La Motte-1937" /> The megagametophytes have archegonia, which produce egg cells.<ref name="La Motte-1937" /> Fertilization takes place when the motile sperm from a microgametophyte locates the archegonia of a megagametophyte and swims inside to fertilize the egg.

Outside of heterospory, a distinguishing feature of ''Isoetes'' (and ''[[Selaginella]]'') from other [[pteridophyte]]s, is that their gametophytes grow inside the spores.<ref name="Farmer-1890" /><ref name="La Motte-1937" /><ref name="La Motte-1933" /> This means that the gametophytes never leave the protection of the spore that disperses them, cracking the [[perispore]] (the outer layer of the spore) just enough to allow the passage of gametes. This is fundamentally different from ferns, where the gametophyte is a photosynthetic plant exposed to the elements of its environment. However, containment creates a separate problem for Isoetes, which is that the gametophytes have no way to acquire energy on their own. ''Isoetes'' sporophytes solve this problem by provisioning starches and other nutrients to the spores as an energy reserve for the eventual gametophytes.<ref name="La Motte-1937" /><ref>{{Cite journal|last1=Abeli|first1=Thomas|last2=Mucciarelli|first2=Marco|date=2010|title=Notes on the Natural History and Reproductive Biology of Isoëtes malinverniana|url=https://www.jstor.org/stable/41237871|journal=American Fern Journal|volume=100|issue=4|pages=235–237|doi=10.1640/0002-8444-100.4.235|jstor=41237871|s2cid=83658338|issn=0002-8444}}</ref> Although not a homologous process, this provisioning is somewhat analogous to other modes of offspring resource investment in seed-plants, such as fruits and seeds. The extent to which resources provisioned to the megaspore also support the growth of the new sporophyte is unknown in ''Isoetes''.

=== Dispersal ===
Spore dispersal occurs primarily in water ([[hydrochory]]) but may also occur via adherence to animals ([[Seed dispersal|zoochory]]) and as a result of ingestion ([[Seed dispersal|endozoochory]]).<ref name="Taylor-1992" /><ref name="Troia-2016">{{Cite journal|last=Troia|first=Angelo|date=16 June 2016|title=Dispersal and colonization in heterosporous lycophytes: palynological and biogeographical notes on the genusIsoetesin the Mediterranean region|url=http://dx.doi.org/10.1080/00837792.2016.1191171|journal=Webbia|volume=71|issue=2|pages=277–281|doi=10.1080/00837792.2016.1191171|bibcode=2016Webbi..71..277T |s2cid=89179370|issn=0083-7792}}</ref> These are among the reasons suggested for the ornamentations of the spore, with some authors demonstrating that certain patterns seem well-adapted for sticking to relevant animals like waterfowl.<ref name="Troia-2016" /> Another critical element of dispersal is the observation that in some species of ''Isoetes'', the outer coat of megaspores have pockets that trap microspores, a condition known as synaptospory.<ref name="Troia-2016" /><ref name="Lellinger-1979">{{Cite journal|last1=Lellinger|first1=David B.|last2=Kramer|first2=K. U.|date=April 1979|title=Synaptospory: A Hypothesis|url=http://dx.doi.org/10.2307/1546895|journal=American Fern Journal|volume=69|issue=2|pages=48|doi=10.2307/1546895|jstor=1546895|issn=0002-8444}}</ref> Typically, heterospory means that colonization and long-dispersal are more difficult due to the fact that a single spore cannot grow a bisexual gametophyte and thus cannot establish a new population from a single spore as can happen in homosporous ferns.<ref>{{Cite journal|last1=Sessa|first1=Emily B.|last2=Testo|first2=Weston L.|last3=Watkins|first3=James E.|date=20 April 2016|title=On the widespread capacity for, and functional significance of, extreme inbreeding in ferns|journal=New Phytologist|volume=211|issue=3|pages=1108–1119|doi=10.1111/nph.13985|pmid=27094807|issn=0028-646X|doi-access=free|bibcode=2016NewPh.211.1108S }}</ref> ''Isoetes'' may mitigate this issue via microspores stuck to megaspores, greatly increasing the possibility of successful fertilization upon dispersal.<ref name="Troia-2016" /><ref name="Lellinger-1979" />

== Taxonomy ==
Compared to other genera, ''Isoetes'' is poorly known. The first critical monograph on their taxonomy, written by [[Norma Etta Pfeiffer]], was published in 1922 and remained a standard reference into the twenty-first century.<ref name="Pfeiffer 1922">{{cite journal |last1=Pfeiffer |first1=Norma E. |date=1922 |title=Monograph of the Isoetaceae |url=https://archive.org/details/jstor-2990000/page/n1/mode/2up |journal=Annals of the Missouri Botanical Garden |volume=9 |issue=2 |pages=79 – |doi=10.2307/2990000 |jstor=2990000 |access-date=29 December 2021}}</ref><ref name="Haas2021-11">{{cite web |last1=Haas |first1=Michael J. |title=Secrets of quillwort photosynthesis could boost crop efficiency |url=https://news.cornell.edu/stories/2021/11/secrets-quillwort-photosynthesis-could-boost-crop-efficiency |access-date=12 March 2023 |website=Cornell Chronicle}}</ref> Even after studies with cytology, scanning electron microscopy, and chromatography, species are difficult to identify and their phylogeny is disputed. Vegetative characteristics commonly used to distinguish other genera, such as leaf length, rigidity, color, or shape are variable and depend on the habitat. Most classification systems for ''Isoetes'' rely on spore characteristics, which make species identification nearly impossible without microscopy.<ref>{{cite book |last1=Cody |first1=William |url=https://archive.org/details/fernsfernallieso0000cody |title=Ferns and Fern Allies of Canada |last2=Britton |first2=Donald |publisher=Agriculture Canada |year=1989 |isbn=9780660131023 |url-access=registration}}</ref> Some [[botanist]]s split the genus, separating two South American species into the genus ''Stylites'', although molecular data place these species among other species of ''Isoetes'', so that ''Stylites'' does not warrant taxonomic recognition.<ref name="LarsénRydin2016">{{cite journal |last1=Larsén |first1=Eva |last2=Rydin |first2=Catarina |year=2016 |title=Disentangling the Phylogeny of Isoetes (Isoetales), Using Nuclear and Plastid Data |journal=International Journal of Plant Sciences |volume=177 |issue=2 |pages=157–174 |doi=10.1086/684179 |issn=1058-5893 |s2cid=85737029}}</ref> 

=== Evolution ===
[[File:Isoetites sp UWBM37862 WaG 1996 img1.png|thumb|right|Undescribed ''[[Isoetites]]'' fossil<br/>[[Klondike Mountain Formation]]]]
The earliest fossil that has been assigned to the genus is {{extinct}}''[[Isoetes beestonii]]'' from the latest Permian<ref name="Retallack2013">{{cite journal |last1=Retallack |first1=G.J. |year=2013 |title=Permian and Triassic greenhouse crises |journal=Gondwana Research |volume=24 |issue=1 |pages=90–103 |doi=10.1016/j.gr.2012.03.003|bibcode=2013GondR..24...90R }}</ref> of New South Wales, Australia, around 252 million years ago.<ref name="Retallack1997">{{Cite journal |last=Retallack |first=G.J. |year=1997 |title=Earliest Triassic origin of ''Isoetes'' and quillwort evolutionary radiation |journal=Journal of Paleontology |volume=71 |issue=3 |pages=500–521 |doi=10.1017/S0022336000039524 |bibcode=1997JPal...71..500R |s2cid=140566050 |issn=0022-3360 |url=https://www.cambridge.org/core/product/identifier/S0022336000039524/type/journal_article |language=en }}</ref> However, the relationships of pre-Jurassic isoetaleans to modern ''Isotetes'' have been regarded as unclear by other authors.<ref name="Wood2020" /> ''Isoetites rolandii'' from the Late Jurassic of North America has been described as the "earliest clear example of a isoetalean lycopsid containing all the major features uniting modern ''Isoetes",'' including the loss of the elongated stem and vegetative leaves. Based on this, it has been stated that "the overall morphology of ''Isoetes'' appears to have persisted virtually unchanged since at least the Jurassic".''<ref name="Wood2020" />'' The timing of the origin of the [[crown group]] is uncertain. Wood ''et al'' (2020) asserted there to be no morphological features that define the major clades within ''Isoetes,'' and no fossils are known that can be definitively assigned to the crown group.''<ref name="Wood2020" />'' While Wood ''et al.'' suggested a young origin dating to the early [[Cenozoic]] based on [[molecular clock]] estimates''<ref name="Wood2020" />'', the results were questioned by Wikström ''et al.'' (2023) who regarded the molecular clock as providing no firm evidence for the origin time of the genus, which could date to the [[Mesozoic]] or even the late [[Paleozoic]], depending on the [[Molecular clock#Calibration|calibration]] method used.<ref name="Wikström-2023" />

===Extant species ===
{{As of|2019|November}}, ''[[Plants of the World Online]]'' accepted the following extant species:<ref name=POWO_328186-2>{{cite web |title=''Isoetes'' {{small|L.}} |work=Plants of the World Online |publisher=Royal Botanic Gardens, Kew|url=https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:328186-2 |access-date=18 November 2019 }}</ref>
{{div col}}
*''[[Isoetes abyssinica|I. abyssinica]]'' {{small|Chiov.}}
*''[[Isoetes acadiensis|I. acadiensis]]'' {{small|Kott}}
*''[[Isoetes aemulans|I. aemulans]]'' {{small|J.P.Roux}}
*''[[Isoetes aequinoctialis|I. aequinoctialis]]'' {{small|Welw. ex A.Br.}}
*''[[Isoetes alcalophila|I. alcalophila]]'' {{small|S.Halloy}}
*''[[Isoetes alpina|I. alpina]]'' {{small|Kirk}}
*''[[Isoetes alstonii|I. alstonii]]'' {{small|C.F.Reed & Verdc.}}
*''[[Isoetes amazonica|I. amazonica]]'' {{small|A.Br.}}
*''[[Isoetes anatolica|I. anatolica]]'' {{small|Prada & Rolleri}}
*''[[Isoetes andicola|I. andicola]]'' {{small|(Amstutz) L.D.Gómez}}
*''[[Isoetes andina|I. andina]]'' {{small|Spruce ex Hook.}}
*''[[Isoetes appalachiana|I. appalachiana]]'' {{small|D.F.Brunt. & D.M.Britton}}
*''[[Isoetes araucaniana|I. araucaniana]]'' {{small|Macluf & Hickey}}
*''[[Isoetes asiatica|I. asiatica]]'' {{small|(Makino) Makino}}
*''[[Isoetes attenuata|I. attenuata]]'' {{small|C.R.Marsden & Chinnock}}
*''[[Isoetes australis|I. australis]]'' {{small|S.Williams}} 
*''[[Isoetes azorica|I. azorica]]'' {{small|Durieu}}
*''[[Isoetes baculata|I. baculata]]'' {{small|Hickey & H.P.Fuchs}}
*''[[Isoetes biafrana|I. biafrana]]'' {{small|Alston}}
*''[[Isoetes bischlerae|I. bischlerae]]'' {{small|H.P.Fuchs}}
*''[[Isoetes bolanderi|I. bolanderi]]'' {{small|Engelm.}} 
*''[[Isoetes boliviensis|I. boliviensis]]'' {{small|U.Weber}}
*''[[Isoetes boomii|I. boomii]]'' {{small|Luebke}} 
*''[[Isoetes boryana|I. boryana]]'' {{small|Durieu}}
*''[[Isoetes boyacensis|I. boyacensis]]'' {{small|H.P.Fuchs}}
*''[[Isoetes bradei|I. bradei]]'' {{small|Herter}}
*''[[Isoetes brasiliensis|I. brasiliensis]]'' {{small|H.P.Fuchs}}
*''[[Isoetes brevicula|I. brevicula]]'' {{small|E.R.L.Johnson}}
*''[[Isoetes butleri|I. butleri]]'' {{small|Engelm.}}
*''[[Isoetes cangae|I. cangae]]'' {{small|J.B.S.Pereira, Salino & Stützel}}
*''[[Isoetes capensis|I. capensis]]'' {{small|}}
*''[[Isoetes caroli|I. caroli]]'' {{small|E.R.L.Johnson}}
*''[[Isoetes caroliniana|I. caroliniana]]'' {{small|(A.A.Eaton) Luebke}} is regarded by ''Plants of the World Online'' as a synonym of ''[[Isoetes valida|I. valida]]'', but other sources treat it as a valid species<ref name=CFLW>{{Citation |mode=cs1 |last1=Hassler |first1=Michael |last2=Schmitt |first2=Bernd |date=November 2019 |contribution=Isoetes caroliniana |title=Checklist of Ferns and Lycophytes of the World |at=8.11 |url=https://worldplants.webarchiv.kit.edu/ferns/ |access-date=18 November 2019 |name-list-style=amp }}</ref>
*''[[Isoetes chubutiana|I. chubutiana]]'' {{small|Hickey, Macluf & W.C.Taylor}}
*''[[Isoetes coromandelina|I. coromandelina]]'' {{small|L.f.}}
*''[[Isoetes creussensis|I. creussensis]]'' {{small|Lazare & S.Riba}}
*''[[Isoetes cristata|I. cristata]]'' {{small|C.R.Marsden & Chinnock}}
*''[[Isoetes cubana|I. cubana]]'' {{small|Engelm.}}
*''[[Isoetes delilei|I. delilei]]'' {{small|(Bory) Rothm.}}
*''[[Isoetes dispora|I. dispora]]'' {{small|Hickey}}
*''[[Isoetes dixitii|I. dixitii]]'' {{small|Shende}}
*''[[Isoetes drummondii|I. drummondii]]'' {{small|A.Braun}}
*''[[Isoetes durieui|I. durieui]]'' {{small|Bory}}
*''[[Isoetes echinospora|I. echinospora]]'' {{small|Durieu}}
*''[[Isoetes ecuadoriensis|I. ecuadoriensis]]'' {{small|Aspl.}}
*''[[Isoetes ekmanii|I. ekmanii]]'' {{small|U.Weber}}
*''[[Isoetes elatior|I. elatior]]'' {{small|A.Braun}}
*''[[Isoetes eludens|I. eludens]]'' {{small|J.P.Roux, Hopper & Rhian J.Sm.}}
*''[[Isoetes engelmannii|I. engelmannii]]'' {{small|A.Braun}}
*''[[Isoetes escondidensis|I. escondidensis]]'' {{small|S.Halloy}}
*''[[Isoetes eshbaughii|I. eshbaughii]]'' {{small|Hickey & H.P.Fuchs}}
*''[[Isoetes flaccida|I. flaccida]]'' {{small|Shuttlew.}}
*''[[Isoetes fluitans|I. fluitans]]'' {{small|M.I.Romero}}
*''[[Isoetes fuliginosa|I. fuliginosa]]'' {{small|R.L.Small & Hickey}}
*''[[Isoetes fuscomarginata|I. fuscomarginata]]'' {{small|H.P.Fuchs}}
*''[[Isoetes gardneriana|I. gardneriana]]'' {{small|Kunze}}
*''[[Isoetes georgiana|I. georgiana]]'' {{small|Luebke}}
*''[[Isoetes giessii|I. giessii]]'' {{small|Launert}}
*''[[Isoetes gigantea|I. gigantea]]'' {{small|U.Weber}}
*''[[Isoetes graniticola|I. graniticola]]'' {{small|D.F.Brunt.}}
*''[[Isoetes gunnii|I. gunnii]]'' {{small|A.Braun}}
*''[[Isoetes gymnocarpa|I. gymnocarpa]]'' {{small|(Gennari) A.Braun}}
*''[[Isoetes habbemensis|I. habbemensis]]'' {{small|Alston}}
*''[[Isoetes hallasanensis|I. hallasanensis]]'' {{small|H.K.Choi, Ch.Kim & J.Jung}} 
*''[[Isoetes haussknechtii|I. haussknechtii]]'' {{small|Troìa & Greuter}}
*''[[Isoetes hawaiiensis|I. hawaiiensis]]'' {{small|W.C.Taylor & W.H.Wagner}}
*''[[Isoetes heldreichii|I. heldreichii]]'' {{small|Wettst.}}
*''[[Isoetes hemivelata|I. hemivelata]]'' {{small|R.L.Small & Hickey}}
*''[[Isoetes herzogii|I. herzogii]]'' {{small|U.Weber}}
*''[[Isoetes hewitsonii|I. hewitsonii]]'' {{small|Hickey}}
*''[[Isoetes hieronymi|I. hieronymi]]'' {{small|U.Weber}}
*''[[Isoetes histrix|I. histrix]]'' {{small|Bory}}
*''[[Isoetes hopei|I. hopei]]'' {{small|J.R.Croft}}
*''[[Isoetes howellii|I. howellii]]'' {{small|Engelm.}}
*''[[Isoetes humilior|I. humilior]]'' {{small|A.Braun}}
*''[[Isoetes hypsophila|I. hypsophila]]'' {{small|Hand.-Mazz.}}
*''[[Isoetes inflata|I. inflata]]'' {{small|E.R.L.Johnson}}
*''[[Isoetes jaegeri|I. jaegeri]]'' {{small|Pitot}}
*''[[Isoetes jamaicensis|I. jamaicensis]]'' {{small|Hickey}}
*''[[Isoetes japonica|I. japonica]]'' {{small|A.Braun}}
*''[[Isoetes jejuensis|I. jejuensis]]'' {{small|H.K.Choi, Ch.Kim & J.Jung}}
*''[[Isoetes junciformis|I. junciformis]]'' {{small|D.F.Brunt. & D.M.Britton}}
*''[[Isoetes karstenii|I. karstenii]]'' {{small|A.Braun}}
*''[[Isoetes killipii|I. killipii]]'' {{small|C.V.Morton}}
*''[[Isoetes kirkii|I. kirkii]]'' {{small|A.Braun}}
*''[[Isoetes labri-draconis|I. labri-draconis]]'' {{small|N.R.Crouch}}
*''[[Isoetes lacustris|I. lacustris]]'' {{small|L.}}
*''[[Isoetes laosiensis|I. laosiensis]]'' {{small|C.Kim & H.K.Choi}}
*''[[Isoetes lechleri|I. lechleri]]'' {{small|Mett.}}
*''[[Isoetes libanotica|I. libanotica]]'' {{small|Musselman, Bolin & R.D.Bray}}
*''[[Isoetes lithophila|I. lithophila]]'' {{small|N.Pfeiff.}}
*''[[Isoetes longissima|I. longissima]]'' {{small|Bory}}
*''[[Isoetes louisianensis|I. louisianensis]]'' {{small|Thieret}}
*''[[Isoetes luetzelburgii|I. luetzelburgii]]'' {{small|U.Weber}}
*''[[Isoetes macrospora|I. macrospora]]'' {{small|}}
*''[[Isoetes malinverniana|I. malinverniana]]'' {{small|Ces. & De Not.}}
*''[[Isoetes maritima|I. maritima]]'' {{small|Underw.}} – maritime quillwort
*''[[Isoetes martii|I. martii]]'' {{small|A.Braun}}
*''[[Isoetes mattaponica|I. mattaponica]]'' {{small|Musselman & W.C.Taylor}}
*''[[Isoetes maxima|I. maxima]]'' {{small|Hickey, Macluf & Link-Pérez}}
*''[[Isoetes melanopoda|I. melanopoda]]'' {{small|J.Gay & Durieu}}  (''I. virginica'' {{small|N.Pfeiff.}})
*''[[Isoetes melanospora|I. melanospora]]'' {{small|Engelm.}} 
*''[[Isoetes melanotheca|I. melanotheca]]'' {{small|Alston}}
*''[[Isoetes mexicana|I. mexicana]]'' {{small|Underw.}} (syn. ''Isoetes montezumae'' {{small|A.A.Eaton}})
*''[[Isoetes microvela|I. microvela]]'' {{small|D.F.Brunt.}}
*''[[Isoetes minima|I. minima]]'' {{small|A.A.Eaton}}
*''[[Isoetes mississippiensis|I. mississippiensis]]'' {{small|S.W.Leonard, ''et al.''}}
*''[[Isoetes mongerensis|I. mongerensis]]'' {{small|E.R.L.Johnson}}
*''[[Isoetes montana|I. montana]]'' {{small|U.Weber}}
*''[[Isoetes mourabaptistae|I. mourabaptistae]]'' {{small|J.B.S.Pereira,''et al.''}}
*''[[Isoetes muelleri|I. muelleri]]'' {{small|A.Braun}}
*''[[Isoetes naipiana|I. naipiana]]'' {{small|P.G.Windisch, Lorscheitt. & Nervo}}
*''[[Isoetes nana|I. nana]]'' {{small|J.B.S.Pereira}}
*''[[Isoetes neoguineensis|I. neoguineensis]]'' {{small|}}
*''[[Isoetes nigritiana|I. nigritiana]]'' {{small|A.Br.}}
*''[[Isoetes nigroreticulata|I. nigroreticulata]]'' {{small|Verdc.}}
*''[[Isoetes novogranadensis|I. novogranadensis]]'' {{small|H.P.Fuchs}}
*''[[Isoetes nuttallii|I. nuttallii]]'' {{small|A.Braun}}
*''[[Isoetes occidentalis|I. occidentalis]]'' {{small|L.F.Hend.}}
*''[[Isoetes olympica|I. olympica]]'' {{small|A.Br.}}
*''[[Isoetes orcuttii|I. orcuttii]]'' {{small|A.A.Eaton}}
*''[[Isoetes organensis|I. organensis]]'' {{small|U.Weber}}
*''[[Isoetes orientalis|I. orientalis]]'' {{small|Hong Liu & Q.F.Wang}}
*''[[Isoetes ovata|I. ovata]]'' {{small|N.Pfeiff.}}
*''[[Isoetes pallida|I. pallida]]'' {{small|Hickey}}
*''[[Isoetes palmeri|I. palmeri]]'' {{small|H.P.Fuchs}}
*''[[Isoetes panamensis|I. panamensis]]'' {{small|Maxon & C.V.Morton}}
*''[[Isoetes parvula|I. parvula]]'' {{small|Hickey}}
*''[[Isoetes pedersenii|I. pedersenii]]'' {{small|H.P.Fuchs ex E.I.Meza & Macluf}}
*''[[Isoetes perralderiana|I. perralderiana]]'' {{small|Durieu & Letourn. ex Milde}}
*''[[Isoetes perrieriana|I. perrieriana]]'' {{small|Iversen}}
*''[[Isoetes philippinensis|I. philippinensis]]'' {{small|Merr. & L.M.Perry}}
*''[[Isoetes phrygia|I. phrygia]]'' {{small|Hausskn.}}
*''[[Isoetes piedmontana|I. piedmontana]]'' {{small|(N.Pfeiff.) C.F.Reed}}
*''[[Isoetes pitotii|I. pitotii]]'' {{small|Alston}}
*''[[Isoetes precocia|I. precocia]]'' {{small|R.L.Small & Hickey}}
*''[[Isoetes pringlei|I. pringlei]]'' {{small|Underw.}}
*''[[Isoetes prototypus|I. prototypus]]'' {{small|D.M.Britton & Goltz}}
*''[[Isoetes pseudojaponica|I. pseudojaponica]]'' {{small|M.Takamiya, Mits.Watan. & K.Ono}}
*''[[Isoetes pusilla|I. pusilla]]'' {{small|C.R.Marsden & Chinnock}}
*''[[Isoetes quiririensis|I. quiririensis]]'' {{small|J.B.S.Pereira & Labiak}}
*''[[Isoetes ramboi|I. ramboi]]'' {{small|Herter}}
*''[[Isoetes riparia|I. riparia]]'' {{small|Engelm. ex A.Braun}} (syn ''I. hyemalis'' {{small|D.F.Brunt.}})
*''[[Isoetes sabatina|I. sabatina]]'' {{small|Troìa & Azzella}}
*''[[Isoetes saccharata|I. saccharata]]'' {{small|Engelm.}}
*''[[Isoetes sahyadrii|I. sahyadrii]]'' {{small|Mahab.}}
*''[[Isoetes saracochensis|I. saracochensis]]'' {{small|Hickey}}
*''[[Isoetes savatieri|I. savatieri]]'' {{small|Franch.}}
*''[[Isoetes schweinfurthii|I. schweinfurthii]]'' {{small|A.Br.}}
*''[[Isoetes sehnemii|I. sehnemii]]'' {{small|H.P.Fuchs}}
*''[[Isoetes septentrionalis|I. septentrionalis]]'' {{small|D.F.Brunt.}}
*''[[Isoetes serracarajensis|I. serracarajensis]]'' {{small|J.B.S.Pereira, Salino & Stützel}}
*''[[Isoetes setacea|I. setacea]]'' {{small|Lam.}}
*''[[Isoetes sinensis|I. sinensis]]'' {{small|T.C.Palmer}} (synonym ''I. coreana'' {{small|Y.H.Chung & H.K.Choi}})
*''[[Isoetes smithii|I. smithii]]'' {{small|H.P.Fuchs}}
*''[[Isoetes spannagelii|I. spannagelii]]'' {{small|H.P.Fuchs}}
*''[[Isoetes spinulospora|I. spinulospora]]'' {{small|C.Jermy & Schelpe}}
*''[[Isoetes stellenbossiensis|I. stellenbossiensis]]'' {{small|A.V.Duthie}}
*''[[Isoetes stephanseniae|I. stephanseniae]]'' {{small|A.V.Duthie}}
*''[[Isoetes stevensii|I. stevensii]]'' {{small|J.R.Croft}}
*''[[Isoetes storkii|I. storkii]]'' {{small|T.C.Palmer}}
*''[[Isoetes taiwanensis|I. taiwanensis]]'' {{small|De Vol}} 
*''[[Isoetes tamaulipana|I. tamaulipana]]'' {{small|Mora-Olivo, A.Mend. & Mart.-Aval.}}
*''[[Isoetes tegetiformans|I. tegetiformans]]'' {{small|Rury}}
*''[[Isoetes tenella|I. tenella]]'' {{small|Léman ex Desv.}}
*''[[Isoetes tennesseensis|I. tennesseensis]]'' {{small|Luebke & Budke}}
*''[[Isoetes tenuifolia|I. tenuifolia]]'' {{small|Jermy}}
*''[[Isoetes tenuissima|I. tenuissima]]'' {{small|Boreau}}
*''[[Isoetes texana|I. texana]]'' {{small|Singhurst, Rushing & W.C.Holmes}}
*''[[Isoetes todaroana|I. todaroana]]'' {{small|Troìa & Raimondo}}
*''[[Isoetes toximontana|I. toximontana]]'' {{small|Musselman & J.P.Roux}}
*''[[Isoetes transvaalensis|I. transvaalensis]]'' {{small|C.Jermy & Schelpe}}
*''[[Isoetes triangula|I. triangula]]'' {{small|U.Weber}}
*''[[Isoetes tripus|I. tripus]]'' {{small|A.Braun}}
*''[[Isoetes truncata|I. truncata]]'' {{small|Clute}}
*''[[Isoetes tuckermanii|I. tuckermanii]]'' {{small|A.Braun ex Engelm.}}
*''[[Isoetes tuerckheimii|I. tuerckheimii]]'' {{small|Brause}}
*''[[Isoetes udupiensis|I. udupiensis]]'' {{small|P.K.Shukla, G.K.Srivast., S.K.Shukla & P.K.Rajagopal}}
*''[[Isoetes ulei|I. ulei]]'' {{small|U.Weber}}
*''[[Isoetes valida|I. valida]]'' {{small|Clute}}
*''[[Isoetes vanensis|I. vanensis]]'' {{small|M.Keskin & G.Zare}}
*''[[Isoetes vermiculata|I. vermiculata]]'' {{small|Hickey}}
*''[[Isoetes viridimontana|I. viridimontana]]'' {{small|M.A.Rosenthal & W.C.Taylor}} 
*''[[Isoetes weberi|I. weberi]]'' {{small|Herter}}
*''[[Isoetes welwitschii|I. welwitschii]]'' {{small|A.Br. ex Kuhn}}
*''[[Isoetes wormaldii|I. wormaldii]]'' {{small|Sim}}
*''[[Isoetes yunguiensis|I. yunguiensis]]'' {{small|Q.F.Wang & W.C.Taylor}}
{{div col end}}
Many species, such as the Louisiana quillwort and the mat-forming quillwort, are [[endangered species]]. Several species of ''Isoetes'' are commonly called '''Merlin's grass''', especially ''[[Isoetes lacustris|I.&nbsp;lacustris]]'', but also the endangered species ''[[Isoetes tegetiformans|I.&nbsp;tegetiformans]]''.

===Hybrids===
{{div col}}
*[[Isoetes × altonharvillii|''I.'' × ''altonharvillii'']] {{small|Musselman}}
*[[Isoetes × brittonii|''I.'' × ''brittonii'']] {{small|D.F.Brunt. & W.C.Taylor}}
*[[Isoetes × bruntonii|''I.'' × ''bruntonii'']] {{small|Knepper & Musselman}}
*[[Isoetes × carltaylorii|''I.'' × ''carltaylorii'']] {{small|Musselman}}
*[[Isoetes × dodgei|''I.'' × ''dodgei'']] {{small|A.A.Eaton}}
*[[Isoetes × eatonii|''I.'' × ''eatonii'']] {{small|R.Dodge}} – Eaton's quillwort
*[[Isoetes × echtuckerii|''I.'' × ''echtuckerii'']] {{small|D.F.Brunt. & D.M.Britton}}
*[[Isoetes × fairbrothersii|''I.'' × ''fairbrothersii'']] {{small|J.D.Montgom. & W.C.Taylor}}
*[[Isoetes × foveolata|''I.'' × ''foveolata'']] {{small|A.A.Eaton}}
*[[Isoetes × gopalkrishnae|''I.'' × ''gopalkrishnae'']] {{small|S.K.Singh, P.K.Shukla & N.K.Dubey}}
*[[Isoetes × harveyi|''I.'' × ''harveyi'']] {{small|A.A.Eaton}} (syn. ''I.'' × ''heterospora'' {{small|Eaton}})
*[[Isoetes × herb-wagneri|''I.'' × ''herb-wagneri'']] {{small|W.C.Taylor}}
*[[Isoetes × hickeyi|''I.'' × ''hickeyi'']] {{small|W.C.Taylor & Luebke}}
*[[Isoetes × jeffreyi|''I.'' × ''jeffreyi'']] {{small|D.M.Britton & D.F.Brunt.}}
*[[Isoetes × marensis|''I.'' × ''marensis'']] {{small|D.M.Britton & D.F.Brunt.}}
*[[Isoetes × michinokuana|''I.'' × ''michinokuana'']] {{small|M.Takamiya, Mits.Watan. & K.Ono}}
*[[Isoetes × novae-angliae|''I.'' × ''novae-angliae'']] {{small|D.F.Brunt. & D.M.Britton}}
*[[Isoetes × paratunica|''I.'' × ''paratunica'']] {{small|D.F.Brunt., Mochalova & A.A.Bobrov}}
*[[Isoetes × pseudotruncata|''I.'' × ''pseudotruncata'']] {{small|D.M.Britton & D.F.Brunt.}}
{{div col end}}

===Fossil species===
*†''[[Isoetes beestonii]]'' {{small|Retallack}} (Permian, Australia)<ref name="Retallack1997"/><ref name="Retallack2013"/>
*†''[[Isoetes bulbiformis]]'' {{small|Drinnan}} (Cretaceous, Australia)<ref name="Pigg2001">{{cite journal |last1=Pigg |first1=K.B. |year=2001 |title=Isoetalean lycopsid evolution: from the Devonian to the present |journal=American Fern Journal |volume=91 |issue=3 |pages=99–114 |doi=10.1640/0002-8444(2001)091[0099:ILEFTD]2.0.CO;2|s2cid=85852292 |url=https://www.biodiversitylibrary.org/part/230740 }}</ref>
*†''[[lsoetes ermayinensis]]'' {{small|Wang}} (Triassic, China)<ref name="Pigg2001"/>
*†''[[Isoetes gramineoides]]'' {{small|Bock}} (Triassic, US)<ref name="Pigg2001"/>
*†''[[Isoetes hillii]]'' {{small|D.M. Britton}} (Miocene, Tasmania)<ref name="Hill1988">{{cite journal |last1=Hill |first1=R.S. |year=1988 |title=Tertiary ''Isoetes'' from Tasmania |journal=Alcheringa |volume=12 |issue=2 |pages=157–162 |doi=10.1080/03115518808619003|bibcode=1988Alch...12..157H }}</ref><ref name="Britton1993">{{cite journal |last1=Britton |first1=D. M. |year=1993 |title=''Isoëtes reticulata'' RS Hill 1987 (Alcheringa 12: 158) is an illegitimate name. |journal=American Fern Journal |volume=83 |issue=4 |pages=128 |doi=10.2307/1547589|jstor=1547589 |url=https://www.biodiversitylibrary.org/part/230632 }}</ref>

==References==
{{Reflist}}

==External links==
{{Commons category}}
{{Wikispecies}}
* [https://web.archive.org/web/20060221182234/http://homepages.caverock.net.nz/~bj/fern/isoetes.htm Checklist of World Ferns, Family Isoetaceae, genus ''Isoetes''; world species list. (143 species)]
* [https://gobotany.nativeplanttrust.org/species/isoetes/echinospora/. Distribution and classification list for world isoetes]

{{Taxonbar|from=Q877973}}
{{Authority control}}

[[Category:Isoetes| ]]
[[Category:Lycophyte genera]]