Psilotum

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Psilotum
Psilotum.jpg
Closeup of Psilotum nudum
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Plantae
Clade: Tracheophytes
Division: Polypodiophyta
Class: Polypodiopsida
Order: Psilotales
Family: Psilotaceae
Genus: Psilotum
Sw.
Type species
Psilotum nudum
(L.) Beauvois
Species
Synonyms
  • HoffmanniaWilldenow 1789
  • BernhardiaWilldenow 1802
  • TristecaPalisot De Beauvisage ex de Mirbel 1802

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 earliest surviving vascular plants, but more recent phylogenies place them as basal ferns, as a sister group to Ophioglossales. They lack true roots and leaves are very reduced, [1] 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 with three lobes rather than two.

Contents

Description and life cycle

Whisk ferns in the genus Psilotum lack true roots but are anchored by creeping rhizomes. The stems have many branches with paired enations, which look like small leaves but have no vascular tissue. Above these enations there are synangia formed by the fusion of three sporangia and which produce the spores. When mature, the synangia release yellow to whitish spores which develop into a gametophyte less than 2 mm (0.08 in) long. The gametophyte lives underground as a mycoheterotroph, tapping into mycorrhizal networks to access carbon and other nutrients. When the gametophyte is mature, it is monoicous, producing both egg and sperm cells. The sperm cells swim using several flagella and when they reach an egg cell, unite with it to form the young sporophyte. A mature sporophyte may grow to a height of 30 cm (10 in) or more but has no apparent [1] leaves. The stem has a core of thick-walled protostele in its centre surrounded by an endodermis which regulates the flow of water and nutrients. The surface of the stem is covered with stomata which allow gas exchange with the surroundings. [2] [3] [4]

The gametophyte of Psilotum is unusual in that it branches dichotomously, lives underground and possesses vascular tissue. [5] The nutrition of the gametophyte appears to be myco-heterotrophic, assisted by endophytic fungi. [6]

Taxonomy and naming

The genus Psilotum was first formally described in 1801 by Olof Swartz and the description was published in Journal für die Botanik (Schrader). [7] [8] The name of the genus is from the Ancient Greek word psilos meaning "bare", "smooth" or "bald" [9] referring to the lack of the usual plant organs, [10] and the seeming lack of leaves. [1]

Species and distribution

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

The distribution of Psilotum is tropical and subtropical, in the New World, Asia, and the Pacific, with a few isolated populations in south-west Europe. The highest latitudes known are in South Carolina, Cádiz province in Spain, [11] and southern Japan 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 rhyniophytes 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. [12] [1] 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, while it is centrarch in rhyniophytes and trimerophytes. [13] The sporangia of Psilotum are trilocular synangia resulting from the fusion of three adjacent sporangia, [13] and these are borne laterally on the axes. In the rhyniophytes and trimerophytes the sporangia were single and in a terminal position on branches. [14]

Molecular evidence strongly confirms that Psilotum is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns. [15]

Related Research Articles

<span class="mw-page-title-main">Fern</span> Class of vascular plants

The ferns are a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissues that conduct water and nutrients and in having life cycles in which the branched sporophyte is the dominant phase.

<span class="mw-page-title-main">Lycophyte</span> Broadly circumscribed group of spore bearing plants

The lycophytes, when broadly circumscribed, are a group of vascular plants that include the clubmosses. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest lineages of extant (living) vascular plants; the group contains extinct plants that have been dated from the Silurian. Lycophytes were some of the dominating plant species of the Carboniferous period, and included the tree-like Lepidodendrales, some of which grew over 40 metres (130 ft) in height, although extant lycophytes are relatively small plants.

In plant anatomy and evolution a microphyll is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of a microphyll, the leaf vein emerges from the protostele without leaving a leaf gap. Leaf gaps are small areas above the node of some leaves where there is no vascular tissue, as it has all been diverted to the leaf. Megaphylls, in contrast, have multiple veins within the leaf and leaf gaps above them in the stem.

<span class="mw-page-title-main">Pteridophyte</span> Group of plants that reproduce by spores

A pteridophyte is a vascular plant that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden.

<span class="mw-page-title-main">Psilotaceae</span> Family of ferns

Psilotaceae is a family of ferns consisting of two genera, Psilotum and Tmesipteris with about a dozen species. It is the only family in the order Psilotales.

<i>Cooksonia</i> Group of vascular land plants (extinct)

Cooksonia is an extinct group of primitive land plants, treated as a genus, although probably not monophyletic. The earliest Cooksonia date from the middle of the Silurian ; the group continued to be an important component of the flora until the end of the Early Devonian, a total time span of 433 to 393 million years ago. While Cooksonia fossils are distributed globally, most type specimens come from Britain, where they were first discovered in 1937. Cooksonia includes the oldest known plant to have a stem with vascular tissue and is thus a transitional form between the primitive non-vascular bryophytes and the vascular plants.

<span class="mw-page-title-main">Rhyniophyte</span> Extinct group of plants

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. Sources vary in the name and rank used for this group, some treating it as the class Rhyniopsida, others as the subdivision Rhyniophytina or the division Rhyniophyta. The first definition of the group, under the name Rhyniophytina, was by Banks, since when there have been many redefinitions, including by Banks himself. "As a result, the Rhyniophytina have slowly dissolved into a heterogeneous collection of plants ... the group contains only one species on which all authors agree: the type species Rhynia gwynne-vaughanii". When defined very broadly, the group consists of plants with dichotomously branched, naked aerial axes ("stems") with terminal spore-bearing structures (sporangia). The rhyniophytes are considered to be stem group tracheophytes.

<i>Psilotum nudum</i> Species of fern in the family Psilotaceae

Psilotum nudum, the whisk fern, is a fernlike plant. Like the other species in the order Psilotales, it lacks roots.

<i>Psilotum complanatum</i> Species of fern in the family Psilotaceae

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 plant is simple, lacking leaves and roots, and having hanging stems with dichotomous branching, which lack developed leaves but have minute scales. The stems and branches have protostele, with a triangular-shaped core of xylem. The scales are arranged in two rows along the flat stems and branches. The stems are broadly triangular in cross section and 5 mm wide. The branches are 1.5 to 2 mm wide. P. complanatum grows 10 to 75 cm long and stems branch in pairs a number of times up their length and are covered with brownish colored hair-like rhizoids. Small stalks end with simple sporangia from the axils of minute bifid, rounded sporophylls. Bean shaped, monolete spores are produced. The spores germinate best in a dark environment when ammonium is present. The gametophyte is non-photosynthetic, living in association with a fungus for its nutritional needs. Plants grow from a subterranean rhizome which anchors the plant in place and absorbs nutrients by means of filament like rhizoids.

<span class="mw-page-title-main">Polysporangiophyte</span> Spore-bearing plants with branched sporophytes

Polysporangiophytes, also called polysporangiates or formally Polysporangiophyta, are plants in which the spore-bearing generation (sporophyte) has branching stems (axes) that bear sporangia. The name literally means 'many sporangia plant'. The clade includes all land plants (embryophytes) except for the bryophytes whose sporophytes are normally unbranched, even if a few exceptional cases occur. While the definition is independent of the presence of vascular tissue, all living polysporangiophytes also have vascular tissue, i.e., are vascular plants or tracheophytes. Extinct polysporangiophytes are known that have no vascular tissue and so are not tracheophytes.

<i>Aglaophyton</i> Extinct (Devonian) prevascular land plant

Aglaophyton major was the sporophyte generation of a diplohaplontic, pre-vascular, axial, free-sporing land plant of the Lower Devonian. It had anatomical features intermediate between those of the bryophytes and vascular plants or tracheophytes.

<i>Horneophyton</i> Extinct genus of early plants

Horneophyton is an extinct early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is a member of the class Horneophytopsida. Horneophyton is among the most abundant fossil organisms found in the Rhynie chert, a Devonian Lagerstätte in Aberdeenshire, UK. A single species, Horneophyton lignieri, is known. Its probable female gametophyte is the form taxon Langiophyton mackiei.

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. The class was established in 1917, under the name Psilophyta, with only three genera for a group of fossil plants from the Upper Silurian and Devonian periods which lack true roots and leaves, but have a vascular system within a branching cylindrical stem. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida. This classification is no longer in use.

Hicklingia is a genus of extinct plants of the Middle Devonian. Compressed specimens were first described in 1923 from the Old Red Sandstone of Scotland. Initially the genus was placed in the "rhyniophytes", but this group is defined as having terminal sporangia, and later work showed that the sporangia of Hicklingia were lateral rather than strictly terminal, so that it is now regarded as having affinities with the zosterophylls.

Huia is a genus of extinct vascular plants of the Early Devonian. The genus was first described in 1985 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China.

<i>Nothia aphylla</i> Extinct species of spore-bearing plant

Nothia was a genus of Early Devonian vascular plants whose fossils were found in the Rhynie chert in Scotland. It had branching horizontal underground stems (rhizomes) and leafless aerial stems (axes) bearing lateral and terminal spore-forming organs (sporangia). Its aerial stems were covered with small 'bumps' (emergences), each bearing a stoma. It is one of the best described early land plants. Its classification remains uncertain, although it has been treated as a zosterophyll. There is one species, Nothia aphylla.

Junggaria was a genus of rhyniophyte-like land plants known from fossils found in China in Upper Silurian strata. It bore leafless dichotomously or pseudomonopodially branching axes, some of which ended in spore-forming organs or sporangia of complex shape. The genus Cooksonella, found in Kazakhstan from deposits of a similar age, is considered to be an illegitimate synonym.

<i>Asterotheca</i> Genus of plants

Asterotheca is a genus of seedless, spore-bearing, vascularized ferns dating from the Carboniferous of the Paleozoic to the Triassic of the Mesozoic.

<i>Tmesipteris obliqua</i> Species of fern in the family Psilotaceae

Tmesipteris obliqua, more commonly known as the long fork-fern or common fork-fern, is a weeping, epiphytic fern ally with narrow unbranched leafy stems. T. obliqua is a member of the genus Tmesipteris, commonly known as hanging fork-ferns. Tmesipteris is one of two genera in the order Psilotales, the other genus being Psilotum. T. obliqua is endemic to eastern Australia.

Tmesipteris horomaka, commonly known as the Banks Peninsula fork fern, is a fern ally endemic to New Zealand.

References

  1. 1 2 3 4 Friedman, William E.; Moore, Richard C.; Purugganan, Michael D. (2004). "The evolution of plant development". American Journal of Botany . 91 (10). Botanical Society of America (Wiley): 1726–1741. doi: 10.3732/ajb.91.10.1726 . ISSN   0002-9122. PMID   21652320.
  2. ↑ Fairley, Alan; Moore, Philip (2010). Native plants of the Sydney region : from Newcastle to Nowra and west to the Dividing Range (3rd ed.). Crows Nest, N.S.W.: Allen & Unwin. p. 16. ISBN   9781741755718.
  3. ↑ "Introduction to the Psilotales". University of California Museum of Paleontology. Retrieved 24 December 2016.
  4. ↑ "Psilotum". [[Royal botanic Gardens Sydney}]]: plantnet. Retrieved 24 December 2016.
  5. ↑ Holloway, John E. (1939). "The Gametophyte, Embryo, and Young Rhizome of Psilotum triquetrum (Swartz)". Annals of Botany. 3 (2): 313–336. doi:10.1093/oxfordjournals.aob.a085063.
  6. ↑ Manton, Irene (1942). "A Note on the Cytology of Psilotum with Special Reference to Vascular Prothalli from Rangitoto Island". Annals of Botany. 6 (2): 283–292. doi:10.1093/oxfordjournals.aob.a088408.
  7. ↑ "Psilotum". APNI. Retrieved 24 December 2016.
  8. ↑ Swartz, Olof (1801). Schrader, Heinrich Adolph (ed.). "Genera et species Filicum". Journal für die Botanik (Schrader). 2: 109. Retrieved 24 December 2016.
  9. ↑ Brown, Roland Wilbur (1956). The Composition of Scientific Words. Washington, D.C.: Smithsonian Institution Press. p. 123.
  10. ↑ de Lange, Peter James. "Psilotum nudum". New Zealand Plant Conservation Network. Retrieved 24 December 2016.
  11. ↑ "Psilotum nudum" (PDF). Atlas y Libro Rojo de la Flora Vascular Amenazada de España .
  12. ↑ Gifford, Ernest; Adriance Foster (1989). Morphology and Evolution of Vascular Plants, Third Edition. New York: W.H. Freeman and Company. ISBN   0716719460.
  13. 1 2 Bell, P.R.; Hemsley, P.R. (1992). Green plants, their origin and diversity (second ed.). Cambridge, U.K.: Cambridge University Press. ISBN   0521646731.
  14. ↑ Stewart, W.N.; Rothwell, G.W. (1993). Palaeobotany and the evolution of plants (second ed.). Cambridge, U.K.: Cambridge university press. ISBN   0521382947.
  15. ↑ 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
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