''Psilotum'' is a genus of

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except ...

-like

vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

s. It is one of two

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...

in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Psilotaceae Psilotaceae is a family of ferns (class Polypodiopsida) consisting of two genera, '' Psilotum'' and '' Tmesipteris'' with about a dozen species. It is the only family in the order Psilotales. Description Once thought to be descendants of early v ...

commonly known as whisk ferns, the other being '' Tmesipteris''. Plants in these two genera were once thought to be descended from the earliest surviving

s, but more recent phylogenies place them as basal ferns, as a sister group to Ophioglossales. They lack true

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

s and leaves are very reduced, 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 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 rhizomes. The stems have many branches with paired enations, which look like small leaves but have no

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. Th ...

. Above these enations there are synangia formed by the fusion of three

sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...

and which produce the

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

s. When mature, the synangia release yellow to whitish spores which develop into a

gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...

less than long. The gametophyte lives underground as a saprophyte, sometimes in a mycorrhizal association. When the gametophyte is mature, it is monoicous, producing both egg and sperm cells. The sperm cells swim using several

flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

and when they reach an egg cell, unite with it to form the young sporophyte. A mature sporophyte may grow to a height of or more but has no apparent leaves. The stem has a core of thick-walled

protostele In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost bo ...

in its centre surrounded by an

endodermis The endodermis is the central, innermost layer of cortex in land plants. It is a cylinder of compact living cells, the radial walls of which are impregnated with hydrophobic substances ( Casparian strip) to restrict apoplastic flow of water to ...

which regulates the flow of water and nutrients. The surface of the stem is covered with

stoma In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bo ...

ta which allow gas exchange with the surroundings. The gametophyte of ''Psilotum'' is unusual in that it branches dichotomously, lives underground and possesses vascular tissue. The nutrition of the gametophyte appears to be myco-heterotrophic, assisted by

endophytic An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to dat ...

fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...

. 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

Taxonomy and naming

The genus ''Psilotum'' was first formally described in 1801 by

Olof Swartz Olof Peter Swartz (21 September 1760 – 19 September 1818) was a Swedish botanist and taxonomist. He is best known for his taxonomic work and studies into pteridophytes. Biography Olof Swartz attended the University of Uppsala where he ...

and the description was published in ''Journal für die Botanik (Schrader)''. The name of the genus is from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

word ''psilos'' meaning "bare", "smooth" or "bald" referring to the lack of the usual plant organs, and the seeming lack of leaves.

Species and distribution

There are two species, '' Psilotum nudum'' and '' Psilotum complanatum'', with a hybrid between them known, ''Psilotum'' × ''intermedium'' W. H. Wagner. The distribution of ''Psilotum'' is tropical and subtropical, in the

New World The term ''New World'' is often used to mean the majority of Earth's Western Hemisphere, specifically the Americas."America." ''The Oxford Companion to the English Language'' (). McArthur, Tom, ed., 1992. New York: Oxford University Press, p. ...

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an ...

, and the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, with a few isolated populations in south-west Europe. The highest latitudes known are in

South Carolina )'' Animis opibusque parati'' ( for, , Latin, Prepared in mind and resources, links=no) , anthem = "Carolina";" South Carolina On My Mind" , Former = Province of South Carolina , seat = Columbia , LargestCity = Charleston , LargestMetro = G ...

Cádiz Cádiz (, , ) is a city and port in southwestern Spain. It is the capital of the Province of Cádiz, one of eight that make up the autonomous community of Andalusia. Cádiz, one of the oldest continuously inhabited cities in Western Europe, ...

province in

Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = '' Plus ultra'' ( Latin)(English: "Further Beyond") , national_anthem = (English: "Royal March") , ...

, 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 The rhyniophytes are a group of extinct early vascular plants that are considered to be similar to the genus '' Rhynia'', found in the Early Devonian (around ). Sources vary in the name and rank used for this group, some treating it as the clas ...

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 eusporangia and a lack of roots. 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

exarch An exarch (; from Ancient Greek ἔξαρχος ''exarchos'', meaning “leader”) was the holder of any of various historical offices, some of them being political or military and others being ecclesiastical. In the late Roman Empire and ea ...

, while it is centrarch in rhyniophytes and trimerophytes. The sporangia of ''Psilotum'' are trilocular synangia resulting from the fusion of three adjacent sporangia, and these are borne laterally on the axes. In the rhyniophytes and trimerophytes the sporangia were single and in a terminal position on branches. Molecular evidence strongly confirms that ''Psilotum'' is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns.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

References

{{Taxonbar, from=Q139732 Psilotaceae Fern genera Taxa named by Olof Swartz