Psilotum is a genus of fern-like vascular plants. It is one of two genera in the familyPsilotaceae 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.
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 endophyticfungi.[6]
Psilotum nudum rhizomes
Clump of Psilotum nudum plants
Psilotum complanatum plant
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]
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]
^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. ISBN9781741755718.
^"Psilotum". [[Royal botanic Gardens Sydney}]]: plantnet. Retrieved 24 December 2016.
^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.
^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.
^Swartz, Olof (1801). Schrader, Heinrich Adolph (ed.). "Genera et species Filicum". Journal für die Botanik (Schrader). 2: 109. Retrieved 24 December 2016.
^Brown, Roland Wilbur (1956). The Composition of Scientific Words. Washington, D.C.: Smithsonian Institution Press. p. 123.
^de Lange, Peter James. "Psilotum nudum". New Zealand Plant Conservation Network. Retrieved 24 December 2016.
^Gifford, Ernest; Adriance Foster (1989). Morphology and Evolution of Vascular Plants, Third Edition. New York: W.H. Freeman and Company. ISBN0716719460.
^ abBell, P.R.; Hemsley, P.R. (1992). Green plants, their origin and diversity (second ed.). Cambridge, U.K.: Cambridge University Press. ISBN0521646731.
^Stewart, W.N.; Rothwell, G.W. (1993). Palaeobotany and the evolution of plants (second ed.). Cambridge, U.K.: Cambridge university press. ISBN0521382947.
^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
