Psilotaceae is a family 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 exce ...

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

Description

Once thought to be descendants of early vascular plants (the Psilophyta of the Devonian period), Psilotaceae have been shown by molecular phylogenetics to be ferns (

Polypodiopsida 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 exce ...

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

'' and '' Tmesipteris''. 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 New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

, and New Caledonia. All members of Psilotaceae are

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

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 Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...

feeding on organic soil matter and mycorrhizal 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 '' Tmesipteris'' may appear to have leaves, but these are really phylloclades, or flattened stems. The

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 ...

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 spores, 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 o ...

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 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 su ...

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 treatm ...

, 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
{{Taxonbar, from=Q500230 Fern families