Psilotum is a genus of fern

Fern

A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...

-like vascular plant

Vascular plant

Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...

s, one of two genera in the family Psilotaceae

Psilotaceae

Psilotaceae is a family of fern-like plants consisting of two genera, Psilotum and Tmesipteris. The two genera are very different and in the past Tmesipteris has been placed in its own family, Tmesipteridaceae, but most classifications continue to treat it in Psilotaceae...

, order Psilotales, and class Psilotopsida (the other being Tmesipteris

Tmesipteris

Tmesipteris the "hanging fork fern", is a genus of fern-like vascular plants, one of two genera in the family Psilotaceae, order Psilotales, and class Psilotopsida ....

). The name of the genus is from Greek psilos = naked, because it lacks organs, such as leaves, normally found in other ferns.

Species and distribution

There are two species, Psilotum nudum and Psilotum complanatum, with a hybrid between them known, Psilotum × intermedium W. H. Wagner

Warren H. Wagner

Warren H. Wagner Jr. , known as Herb Wagner, from his middle name, "Herbert," was an eminent American botanist who lived in Michigan...

.

The distribution of Psilotum is tropical and subtropical, in the New World

New World

The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...

, Asia

Asia

Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

, and the Pacific, with a few isolated populations in SW Europe. The highest latitudes known are in South Carolina

South Carolina

South Carolina is a state in the Deep South of the United States that borders Georgia to the south, North Carolina to the north, and the Atlantic Ocean to the east. Originally part of the Province of Carolina, the Province of South Carolina was one of the 13 colonies that declared independence...

, Cádiz

Cádiz

Cadiz is a city and port in southwestern Spain. It is the capital of the homonymous province, one of eight which make up the autonomous community of Andalusia....

 province in Spain

Spain

Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

, and southern Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 for P. nudum. In the U.S., one species is found from Florida to Texas, the other in Hawaii.

Relation to ferns

Psilotum superficially resembles some rhyniophytes in having apparently dichotomous branching, terminal sporangia and a lack of roots. However, careful study of the morphology and anatomy suggested that whisk ferns were not closely related to rhyniophytes.

Molecular evidence strongly confirms that Psilotum is a fern. Psilophytes are sister to ophioglossoid ferns.

Gametophyte

The gametophyte

Gametophyte

A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

 of Psilotum nudum L. Beauv. (P. triquetrum Swartz) 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

Endophyte

An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life without causing apparent disease. Endophytes are ubiquitous and have been found in all the species of plants studied to date; however, most of these endophyte/plant relationships...

 fungi.