Editing Isoetes (section)

== 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" />