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{{Short description|Genus of ferns in the family Psilotaceae}}
{{Italic title}}
{{Automatic taxobox
| name = ''Psilotum''
| image = Psilotum.jpg
| image_upright = 1.1
| image_caption = Closeup of ''Psilotum nudum''
| taxon = Psilotum
| authority = [[Olof Swartz|Sw.]]
|type_species = ''[[Psilotum nudum]]''
|type_species_authority = ([[Carl Linnaeus|L.]]) Beauvois
| subdivision_ranks = [[Species]]
| subdivision =
* ''[[Psilotum complanatum|P. complanatum]]'' [[Olof Swartz|Sw.]]
* ''P. ×intermedium'' Wagner
* ''[[Psilotum nudum|P. nudum]]'' ([[Carl Linnaeus|L.]]) Beauvois
| synonyms=
* ''Hoffmannia'' Willdenow 1789
* ''Bernhardia'' Willdenow 1802
* ''Tristeca'' Palisot De Beauvisage ex de Mirbel 1802
}}

'''''Psilotum''''' is a genus of [[fern]]-like [[vascular plant]]s. It is one of two [[Genus|genera]] in the [[Family (biology)|family]] [[Psilotaceae]] commonly known as '''whisk ferns''', the other being ''[[Tmesipteris]]''. Plants in these two genera were once thought to be descended from the earliest surviving [[vascular plant]]s, but more recent phylogenies place them as basal ferns, as a sister group to [[Ophioglossales]]. They lack true [[root]]s and [[Leaf|leaves]] are very reduced,<ref name="Friedman-et-al-2004">{{cite journal | last1=Friedman | first1=William E. | last2=Moore | first2=Richard C. | last3=Purugganan | first3=Michael D. | title=The evolution of plant development | journal=[[American Journal of Botany]] | publisher=[[Botanical Society of America]] ([[Wiley (publisher)|Wiley]]) | volume=91 | issue=10 | year=2004 | issn=0002-9122 | doi=10.3732/ajb.91.10.1726 | pages=1726–1741| pmid=21652320 | doi-access=free }}</ref> the stems being the organs containing photosynthetic and conducting tissue. There are only two species in ''Psilotum'' and a hybrid between the two. They differ from those in ''Tmesipteris'' in having stems with many branches and a [[Sporangium#Synangium|synangium]] with three lobes rather than two.

==Description and life cycle==
Whisk ferns in the genus ''Psilotum'' lack true roots but are anchored by creeping [[rhizome]]s. The stems have many branches with paired [[enation]]s, which look like small leaves but have no [[vascular tissue]]. Above these enations there are synangia formed by the fusion of three [[Sporangium|sporangia]] and which produce the [[spore]]s. When mature, the synangia release yellow to whitish spores which develop into a [[gametophyte]] less than {{convert|2|mm|in|sigfig=1|abbr=on}} long. The gametophyte lives underground as a mycoheterotroph, tapping into mycorrhizal networks to access carbon and other nutrients. When the gametophyte is mature, it is [[monoicous]], producing both egg and sperm cells. The sperm cells swim using several [[Flagellum|flagella]] and when they reach an egg cell, unite with it to form the young [[sporophyte]]. A mature sporophyte may grow to a height of {{convert|30|cm|in|sigfig=1|abbr=on}} or more but has no apparent<ref name="Friedman-et-al-2004" /> leaves. The stem has a core of thick-walled [[Stele (biology)#Protostele|protostele]] in its centre surrounded by an [[endodermis]] which regulates the flow of water and nutrients. The surface of the stem is covered with [[stoma]]ta which allow gas exchange with the surroundings.<ref name="Fairley">{{cite book|last1=Fairley|first1=Alan|last2=Moore|first2=Philip|title=Native plants of the Sydney region : from Newcastle to Nowra and west to the Dividing Range|date=2010|publisher=Allen & Unwin|location=Crows Nest, N.S.W.|isbn=9781741755718|page=16|edition=3rd}}</ref><ref name="UCMP">{{cite web|title=Introduction to the Psilotales|url=http://www.ucmp.berkeley.edu/plants/pterophyta/psilotales.html|publisher=University of California Museum of Paleontology|access-date=24 December 2016}}</ref><ref name="RBGS">{{cite web|title=''Psilotum''|url=http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&showsyn=&dist=&constat=&lvl=gn&name=Psilotum|publisher=[[Royal botanic Gardens Sydney}]]: plantnet|access-date=24 December 2016}}</ref>

The gametophyte of ''Psilotum'' is unusual in that it branches dichotomously, lives underground and possesses vascular tissue.<ref name="Holloway">{{cite journal|last1=Holloway|first1=John E.|title=The Gametophyte, Embryo, and Young Rhizome of ''Psilotum triquetrum'' (Swartz)|journal=Annals of Botany|date=1939|volume=3|issue=2|pages=313–336|doi=10.1093/oxfordjournals.aob.a085063}}</ref> The nutrition of the gametophyte appears to be [[myco-heterotrophs|myco-heterotrophic]], assisted by [[endophyte|endophytic]] [[fungi]].<ref name="Manton">{{cite journal|last1=Manton|first1=Irene|title=A Note on the Cytology of Psilotum with Special Reference to Vascular Prothalli from Rangitoto Island|journal=Annals of Botany|date=1942|volume=6|issue=2|pages=283–292|doi=10.1093/oxfordjournals.aob.a088408}}</ref><gallery>
File:Psilotum nudum Rhizome.jpg|''Psilotum nudum'' rhizomes
File:Psilotum nudum Sydney Opera House.JPG|Clump of ''Psilotum nudum'' plants
File:Psilotum complanatum - Lyman Plant House, Smith College - DSC04275.JPG|''Psilotum complanatum'' plant
</gallery>

==Taxonomy and naming==
The genus ''Psilotum'' was first formally described in 1801 by [[Olof Swartz]] and the description was published in ''Journal für die Botanik (Schrader)''.<ref name=APNI>{{cite web|title=''Psilotum''|url=https://id.biodiversity.org.au/instance/apni/508374|publisher=APNI|access-date=24 December 2016}}</ref><ref name="Sw.">{{cite journal|last1=Swartz|first1=Olof|editor-last=Schrader|editor-first=Heinrich Adolph|title=Genera et species Filicum|journal=Journal für die Botanik (Schrader)|date=1801|volume=2|page=109|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433011408949;view=1up;seq=123|access-date=24 December 2016}}</ref> The name of the genus is from the [[Ancient Greek]] word ''psilos'' meaning "bare", "smooth" or "bald"<ref name="RWB">{{cite book|author-link1=Roland W. Brown|last1=Brown|first1=Roland Wilbur|title=The Composition of Scientific Words|date=1956|publisher=Smithsonian Institution Press|location=Washington, D.C.|page =123}}</ref> referring to the lack of the usual plant organs,<ref name="NZPCN">{{cite web|last1=de Lange|first1=Peter James|title=''Psilotum nudum''|url=http://www.nzpcn.org.nz/flora_details.aspx?ID=2229|publisher=New Zealand Plant Conservation Network|access-date=24 December 2016}}</ref> and the seeming lack of leaves.<ref name="Friedman-et-al-2004" />

The Smith et al. classification of 2006, based on molecular phylogeny, placed ''Psilotum'' in [[Psilotaceae]].<ref name=Smith-2006>{{cite journal |last1=Smith | first1=Alan R. |last2=Pryer |first2=Kathleen M. |author-link2=Kathleen M. Pryer |last3=Schuettpelz |first3=Eric |last4=Korall |first4=Petra |last5=Schneider |first5=Harald |last6=Wolf |first6=Paul G. |year=2006 |title=A classification for extant ferns |journal=[[Taxon (journal)|Taxon]] |volume=55 |issue=3 |pages=705–731 |url=https://www.idigbio.org/wiki/images/9/95/Smith_et_al_2006.pdf |doi=10.2307/25065646 |jstor=25065646 }}</ref> Subsequent classifications have maintained this placement.<ref name=Christenhusz-2011>{{cite journal |last1=Christenhusz |first1=Maarten J. M. |author-link1=Maarten J. M. Christenhusz |last2=Zhang |first2=Xian-Chun |last3=Schneider |first3=Harald |year=2011 |title=A linear sequence of extant families and genera of lycophytes and ferns |journal=[[Phytotaxa]]|volume=19|pages=7–54 |doi=10.11646/phytotaxa.19.1.2 |url=http://www.mapress.com/phytotaxa/content/2011/f/pt00019p054.pdf}}</ref><ref name=ChriChas14>{{Cite journal |last1=Christenhusz |first1=Maarten J.M. |author-link1=Maarten J. M. Christenhusz |last2=Chase |first2=Mark W. |author-link2=Mark Wayne Chase |date=2014 |title=Trends and concepts in fern classification |journal=[[Annals of Botany]] |volume=113 |issue=9 |pages=571–594 |doi=10.1093/aob/mct299 |pmid=24532607 |pmc=3936591}}</ref><ref name=PPGI>{{citation |mode=cs1|author=PPG I |title=A community-derived classification for extant lycophytes and ferns |year=2016 |journal=[[Journal of Systematics and Evolution]] |volume=54 |issue=6 |pages=563–603 |doi=10.1111/jse.12229|s2cid=39980610 |doi-access=free }}</ref>

==Species and distribution==
There are two species, ''[[Psilotum nudum]]'' and ''[[Psilotum complanatum]]'', with a hybrid between them known, ''Psilotum'' × ''intermedium'' [[Warren H. Wagner|W. H. Wagner]].

The distribution of ''Psilotum'' is tropical and subtropical, in the [[New World]], [[Asia]], and the [[Pacific]], with a few isolated populations in south-west Europe. The highest latitudes known are in [[South Carolina]], [[Cádiz]] province in [[Spain]],<ref name="Libro Rojo">{{cite web | url = http://www.mma.es/portal/secciones/biodiversidad/inventarios/inb/flora_vascular/pdf/1066.pdf | work = [[Atlas y Libro Rojo de la Flora Vascular Amenazada de España]] | title = Psilotum nudum}}</ref> and southern [[Japan]] for ''P. nudum''. In the U.S., ''P. nudum'' is found from Florida to Texas, and ''P. complanatum'' in Hawaii.

==Relation to ferns==
''Psilotum'' superficially resembles certain extinct early vascular plants, such as the [[rhyniophyte]]s and the trimerophyte genus ''Psilophyton''. The unusual features of ''Psilotum'' that suggest an affinity with early vascular plants include dichotomously branching sporophytes, aerial stems arising from horizontal rhizomes, a simple vascular cylinder, homosporous and terminal [[eusporangiate fern|eusporangia]] and a lack of roots.<ref name=GiffordFoster>{{cite book |last1=Gifford |first1=Ernest |title=Morphology and Evolution of Vascular Plants, Third Edition|year=1989|publisher=W.H. Freeman and Company|location=New York|isbn=0716719460|author2=Adriance Foster}}</ref><ref name="Friedman-et-al-2004" /> Unfortunately, no fossils of psilophytes are known to exist. A careful study of the morphology and anatomy suggests that whisk ferns are not closely related to rhyniophytes, and that the ancestral features present in living psilophytes represent a reduction from a more typical modern fern plant. Significant differences between ''Psilotum'' and the rhyniophytes and trimerophytes are that the development of its vascular strand is [[xylem#Development|exarch]], while it is [[xylem#Development|centrarch]] in rhyniophytes and trimerophytes.<ref name=Bell>{{cite book|last1=Bell|first1=P.R.|last2=Hemsley|first2=P.R.|year=1992|title=Green plants, their origin and diversity|edition=second|publisher=Cambridge University Press|location=Cambridge, U.K.|isbn=0521646731}}</ref> The sporangia of ''Psilotum'' are [[locule|trilocular]] synangia resulting from the fusion of three adjacent sporangia,<ref name=Bell/> and these are borne laterally on the axes. In the rhyniophytes and trimerophytes the sporangia were single and in a terminal position on branches.<ref name=Stewart>{{cite book|last1=Stewart|first1=W.N.|last2=Rothwell|first2=G.W.|year=1993|edition=second|title=Palaeobotany and the evolution of plants|publisher=Cambridge university press|location=Cambridge, U.K.|isbn=0521382947}}</ref>

Molecular evidence strongly confirms that ''Psilotum'' is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns.<ref name=QiuandPalmer1999>Qiu, Y-L and Palmer, J (1999) "Phylogeny of early land plants: insights from genes and genomes." Trends in Plant Science 4(1), 26-30</ref>

==References==
{{Reflist|30em}}

==External links==
{{Wikibooks|position=right|Dichotomous Key|Psilotaceae}}

{{Taxonbar|from=Q139732}}
{{Authority control}}
[[Category:Psilotaceae]]
[[Category:Fern genera]]
[[Category:Taxa named by Olof Swartz]]
[[Category:Taxa described in 1802]]

Psilotum - Revision history

imported>Choess: /* Taxonomy and naming */ fix url