''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 t ...
-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 al ...
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 nomenclat ...
in the
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
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 va ...
commonly known as whisk ferns, the other being ''
Tmesipteris
''Tmesipteris'', the hanging fork ferns, is a genus of ferns, one of two genera in the family Psilotaceae, order Psilotales (the other being ''Psilotum'').
''Tmesipteris'' is restricted to certain lands in the Southern Pacific, notably Australia, ...
''. Plants in these two genera were once thought to be descended from the earliest surviving
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 al ...
s, but more recent phylogenies place them as basal ferns, as a sister group to
Ophioglossales
Ophioglossaceae, the adder's-tongue family, is a small family of ferns. In the Pteridophyte Phylogeny Group classification of 2016 (PPG I), it is the only family in the order Ophioglossales, which together with the Psilotales is placed in the sub ...
. 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
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
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
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 cy ...
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
In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...
s. The stems have many branches with paired
enation
Enations are scaly leaflike structures, differing from leaves in their lack of vascular tissue. They are created by some leaf diseases and occur normally on ''Psilotum''. Enations are also found on some early plants such as ''Rhynia'', where the ...
s, 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. There ...
. 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 cy ...
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, f ...
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
mycorrhiza
A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
l association. When the gametophyte is mature, it is
monoicous
Monoicy () is a sexual system in haploid plants (mainly bryophytes) where both sperm and eggs are produced on the same gametophyte, in contrast with dioicy, where each gametophyte produces only sperm or eggs but never both.Crandall-Stotler, B.J. ...
, 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 sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase.
Life cycle
The sporophyte develops from the zygote pr ...
. 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 b ...
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 the ...
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 bor ...
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
Myco-heterotrophy (from Greek μύκης , "fungus", ἕτερος ', "another", "different" and τροφή ', "nutrition") is a symbiotic relationship between certain kinds of plants and fungi, in which the plant gets all or part of its food fro ...
, 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 date; h ...
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
.
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 st ...
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 peri ...
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
''Psilotum nudum'', the whisk fern, is a fernlike plant. Like the other species in the order Psilotales, it lacks roots.
Its name, ''Psilotum nudum'', means "bare naked" in Latin, because it lacks (or seems to lack) most of the organs of typica ...
'' and ''
Psilotum complanatum
''Psilotum complanatum'', the flatfork fern, is a rare herbaceous epiphytic fern ally in the genus ''Psilotum''. There is some evidence that it might be a true fern that has lost some typically fern-like characteristics.
Morphologically, the plan ...
'', 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. 3 ...
,
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 area ...
, 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 = ...
,
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")
, i ...
,
and southern
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
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 class R ...
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