Psilotaceae is a family of

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

s (class Polypodiopsida) consisting of two genera, ''

Psilotum ''Psilotum'' is a genus of fern-like vascular plants. It is one of two genera in the family Psilotaceae commonly known as whisk ferns, the other being ''Tmesipteris''. Plants in these two genera were once thought to be descended from the earlies ...

'' and ''

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 ...

'' with about a dozen species. It is the only family in the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Psilotales.

Description

Once thought to be descendants of early vascular plants (the Psilophyta of the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

period), Psilotaceae have been shown by molecular phylogenetics to be ferns (

Polypodiopsida The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

), and a sister group of the Ophioglossaceae. The family contains two genera, ''

'' and ''

''. 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 An epiphyte is a plant or plant-like organism that grows on the surface of another plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphyt ...

found in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

, and

New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...

. All members of Psilotaceae are

vascular plant Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...

s without any true roots. Rather, the plants are anchored by an underground system of

rhizome In botany and dendrology, a rhizome ( ) is a modified subterranean plant stem that sends out roots and Shoot (botany), shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from ...

s. The small, stem-like

gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the se ...

s of Psilotaceae are located in this rhizome system, and they aid in a plant's nutrient absorption through the soil. This is primarily achieved through saprotrophic feeding on organic soil matter and

mycorrhiza A mycorrhiza (; , 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's rhizosphere, the plant root system and its surroundings. Mycorrhizae play ...

l interactions. Psilotaceae do not have leaves. Some species have leaf-like structures called enations which have no vascular tissue except for a small bundle at the base. These are almost peg-like, stubby and are generally not considered true leaves, though they likely evolved from them. Members of ''

'' may appear to have leaves, but these are really

phylloclade Phylloclades and cladodes are flattened, photosynthetic shoots, which are usually considered to be modified branches. The two terms are used either differently or interchangeably by different authors. '' Phyllocladus'', a genus of conifer, is na ...

s, or flattened stems. The

sporangia A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...

of Psilotaceae are fused together into small and distinctive yellow balls called synangia (shown in the picture of ''P. nudum'' above). These synangia are located off the stems of the plants. They contain two sporangia each in ''Tmesipteris'' species, and three sporangia each in ''Psilotum'' species. A thick tapetum nourishes the developing

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

s, as is typical of

eusporangiate fern Eusporangiate ferns are vascular spore plants, whose sporangia arise from several epidermal cells and not from a single cell as in leptosporangiate ferns. Typically these ferns have reduced root systems and sporangia that produce large amounts ...

s like Psilotaceae.

Classification

In the molecular phylogenetic classification of Smith et al. in 2006, Psilotales, containing the single family Psilotaceae comprising ''Psilotum'' and ''Tmesipteris'', was placed with the order Ophioglossales in the class Psilotopsida. The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009) which placed all land plants in Equisetopsida, made it a member of subclass

Ophioglossidae Ophioglossidae is one of the four subclasses of Polypodiopsida (ferns). This subclass consists of the ferns commonly known as whisk ferns, grape ferns, adder's-tongues and moonworts. It is equivalent to the class Psilotopsida in previous treat ...

, equivalent to Smith's Psilotopsida. The placement of Psilotales in subclass Ophioglossidae has subsequently been followed in the classifications of Christenhusz and Chase (2014) and PPG I (2016). In the past ''Tmesipteris'' has been placed in its own family, Tmesipteridaceae, but this has not been maintained in the system of Smith et al. and later classifications.

References

External links

Introduction to the Psilotales
{{Authority control Fern families