Psilotaceae
Encyclopedia
Psilotaceae is a family of fern-like plants (in order
Psilotales) 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. The relationships of Psilotaceae have been unclear, in part because the plants lack roots or true leaves, but recent molecular systematic studies suggest a relationship to the fern
family Ophioglossaceae
.
The first genus, Psilotum
, consists of small shrubby plants of the dry tropics commonly known as "Whisk ferns". The other genus, Tmesipteris
, is an epiphyte
found in Australia
, New Zealand
, and New Caledonia
. There has long been controversy about the relationships of the Psilotaceae, with some claiming that they are fern
s (Pteridophyta), and others maintaining that they are descendants of the first vascular plants (the Psilophyta
of the Devonian
period). Recent evidence from DNA
demonstrates a much closer relationship to the ferns, and that they are closely related to the Ophioglossales
, in particular.
All Psilotaceae share a few characteristics. They are vascular plant
s and they lack leaves, having instead small outgrowths called enations. The enations are not considered true leaves because there is only a vascular bundle just underneath them, but not inside, as in leaves. Psilotales also do not have true root
s. They are anchored by rhizoid
s. Absorption is aided by symbiotic fungi
called mycorrhizae.
Three sporangia
are united into a synangium, which is considered to be a very reduced series of branches. There is a thick tapetum to nourish the developing spore
s, as is typical of eusporangiate plants. The gametophyte
looks like a small piece of subterranean stem, but produces antheridia
and archegonia
.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Psilotales) consisting of two genera, Psilotum
Psilotum
Psilotum is a genus of fern-like vascular plants, one of two genera in the family Psilotaceae, order Psilotales, and class Psilotopsida...
and 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 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. The relationships of Psilotaceae have been unclear, in part because the plants lack roots or true leaves, but recent molecular systematic studies suggest a relationship to the 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...
family Ophioglossaceae
Ophioglossaceae
Ophioglossaceae, the Adder's tongue family, is a family of ferns, currently thought to be most closely related to Psilotaceae, the two together comprising the class Psilotopsida as the sibling group to the rest of the ferns. The Ophioglossaceae is one of two groups of ferns traditionally known as...
.
The first genus, Psilotum
Psilotum
Psilotum is a genus of fern-like vascular plants, one of two genera in the family Psilotaceae, order Psilotales, and class Psilotopsida...
, consists of small shrubby plants of the dry tropics commonly known as "Whisk ferns". The other genus, 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 ....
, is an epiphyte
Epiphyte
An epiphyte is a plant that grows upon another plant non-parasitically or sometimes upon some other object , derives its moisture and nutrients from the air and rain and sometimes from debris accumulating around it, and is found in the temperate zone and in the...
found in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, and New Caledonia
New Caledonia
New Caledonia is a special collectivity of France located in the southwest Pacific Ocean, east of Australia and about from Metropolitan France. The archipelago, part of the Melanesia subregion, includes the main island of Grande Terre, the Loyalty Islands, the Belep archipelago, the Isle of...
. There has long been controversy about the relationships of the Psilotaceae, with some claiming that they are 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...
s (Pteridophyta), and others maintaining that they are descendants of the first vascular plants (the Psilophyta
Psilophytopsida
Psilophytopsida is a now obsolete class containing one order, Psilophytales, which was previously used to classify a number of extinct plants which are now placed elsewhere...
of the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period). Recent evidence from DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
demonstrates a much closer relationship to the ferns, and that they are closely related to the Ophioglossales
Ophioglossales
Ophioglossales are a small group of pteridophyte plants. Traditionally they are included in the division Pteridophyta, the ferns, originally as a family and later as the order Ophioglossales...
, in particular.
All Psilotaceae share a few characteristics. They are 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 and they lack leaves, having instead small outgrowths called enations. The enations are not considered true leaves because there is only a vascular bundle just underneath them, but not inside, as in leaves. Psilotales also do not have true root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s. They are anchored by rhizoid
Rhizoid
Rhizoids are thread-like growths from the base or bottom of a plant, found mainly in lower groups such as algae, fungi, bryophytes and pteridophytes, that function like roots of higher plants ....
s. Absorption is aided by symbiotic fungi
Fungus
A fungus is a member of a large 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, Fungi, which is separate from plants, animals, and bacteria...
called mycorrhizae.
Three sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
are united into a synangium, which is considered to be a very reduced series of branches. There is a thick tapetum to nourish the developing spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s, as is typical of eusporangiate plants. 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....
looks like a small piece of subterranean stem, but produces antheridia
Antheridium
An antheridium or antherida is a haploid structure or organ producing and containing male gametes . It is present in the gametophyte phase of lower plants like mosses and ferns, and also in the primitive vascular psilotophytes...
and archegonia
Archegonium
An archegonium , from the ancient Greek ἀρχή and γόνος , is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gamete. The archegonium has a long neck canal and a swollen base...
.