Underground orchids
Rhizanthella gardneri
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Monocots
Order: Asparagales
Family: Orchidaceae
Subfamily: Orchidoideae
Tribe: Diurideae
Subtribe: Rhizanthellinae
R.S.Rogers
Genus: Rhizanthella
R.S.Rogers[1]
Type species
Rhizanthella gardneri
R.S.Rogers (1928)
Synonyms[2]

Cryptanthemis Rupp

Rhizanthella, commonly known as underground orchids,[3] is a genus of flowering plants in the orchid family, Orchidaceae and is endemic to Australia. All are leafless, living underground in symbiosis with mycorrhizal fungi. The inflorescence is a head of flowers held at, or just above the ground but mostly covered by soil or leaf litter and little is known about the mechanism of pollination.

Description

Orchids in the genus Rhizanthella are mostly underground, perennial, sympodial, mycotrophic herbs with fleshy underground stems which produce new shoots at nodes where there are colourless leaf-like cataphylls. There are no roots and new tubers form at the end of short stems. The leaves are reduced to scale-like structures lacking chlorophyll, pressed against and sheathing the stems.[3][4][5][6]

The inflorescence is a head containing many flowers and is held at, or just above ground level but the head is usually covered with leaf litter or soil. The head is surrounded by a large number of overlapping bracts and each flower has an erect, elongated bract at its base. The flowers are non-resupinate, arranged in a spiral, inward-facing, dull coloured and lack a stalk. The sepals and petals form a short, curved hood over the labellum and column, open on one side. The lateral sepals are joined to each other and to the dorsal sepal at their bases. The petals are joined at their bases to the column and are shorter than the sepals. The labellum is different in size, shape and colouration from the other petals and sepals, is thick, fleshy and has no nectar. The column is short with short wings. Flowering time depends on species and is followed by the fruit which is a berry that does not split open (indehiscent) and which contains 50 to 100 seeds.[3][4][5][6]

Underground orchids do not possess chloroplasts but they retain plastid genes, although R. gardneri possesses one of the smallest organelle genome yet described in land plants.[7]

Taxonomy and naming

The first formal description of an underground orchid was by Richard Sanders Rogers who published his description of R. gardneri in the Journal of the Royal Society of Western Australia in 1928.[8][9] The name "Rhizanthella" is a diminutive of Rhizanthes, a parasitic plant in the Family Rafflesiaceae.[10] The name "Rhizanthes" is derived from the Ancient Greek words rhiza meaning "root"[11]: 666  and anthos meaning "flower".[11]: 338 

Species list

Four species are recognised by the World Checklist of Selected Plant Families and a fifth species has been formally described, but not as yet accepted by other authorities:

In 2020, a fifth species, Rhizanthella speciosa, found in New South Wales, was described by Mark Clements and David Jones in the journal Lankesteriana but as at September 2020, the name has not been accepted by the World Checklist of Selected Plant Families.[16]

Distribution and habitat

Rhizanthella gardneri occurs in the south-west of Western Australia where it grows in association with broombush (Melaleuca uncinata).[3] R. johnstonii, also from WA, was split from R. gardneri in 2018.[17][18] Rhizanthella omissa has only been collected once, at an elevation of 1,200 m (4,000 ft) in the Lamington National Park in Queensland.[19] Rhizanthella slateri, formerly known as Cryptanthemis slateri, occurs in the Blue Mountains and similar ranges in New South Wales where it grows in sclerophyll forest.[6] R. speciosa was discovered in 2016 in wet sclerophyll forest in Barrington Tops, which contrasts with the more-open dry forest habitat of R. slateri.[18]

Ecology

The pollination mechanism of Rhizanthella is not known. A single specimen of a small fly from the genus Megaselia, some small wasps and termites are the only observations of insects carrying pollinia of Rhizanthella.[4]

References

  1. ↑ "Rhizanthella". Kew Science - Plants of the World Online. Retrieved 25 September 2020.
  2. ↑ "Rhizanthella". World Checklist of Selected Plant Families (WCSP). Royal Botanic Gardens, Kew. Retrieved 20 July 2016.
  3. 1 2 3 4 Hoffman, Noel; Brown, Andrew (2011). Orchids of South-West Australia (3rd ed.). Gooseberry Hill: Noel Hoffman. pp. 386–389. ISBN 9780646562322.
  4. 1 2 3 Alec M. Pridgeon; Phillip J. Cribb; Mark W. Chase; Finn N. Rasmussen, eds. (2001). Genera Orchidacearum, Volume 2, Orchidoideae (part 1). Oxford, England: Oxford University Press. pp. 186–193. ISBN 0198507100.
  5. 1 2 "Rhizanthella". FloraBase. Western Australian Government Department of Biodiversity, Conservation and Attractions.
  6. 1 2 3 Weston, Peter H. "Genus Rhizanthella". Royal Botanic Garden Sydney: plantnet. Retrieved 20 July 2016.
  7. ↑ Delannoy, Etienne; Fujii, Sota; Colas des Francs-Small, Catherine; Brundrett, Mark; Small, Ian (2 February 2011). "Rampant Gene Loss in the Underground Orchid Rhizanthella gardneri Highlights Evolutionary Constraints on Plastid Genomes". Molecular Biology and Evolution. 28 (7): 2077–2086. doi:10.1093/molbev/msr028. PMC 3112369. PMID 21289370.
  8. ↑ "Rhizanthella gardneri". APNI. Retrieved 20 July 2016.
  9. ↑ Rogers, Richard Sanders (1928). "A New Genus of Australian Orchid". Journal of the Royal Society of Western Australia. 15 (1): 1. Retrieved 26 September 2020.
  10. ↑ Quattrocchi, Umberto (2000). R - Z. Boca Raton, FL: CRC World Dictionary of Pant Names (R-Z). p. 2296. ISBN 0849326788.
  11. 1 2 Brown, Roland Wilbur (1956). The Composition of Scientific Words. Washington, D.C.: Smithsonian Institution Press.
  12. ↑ "Rhizanthella gardneri". Kew Science - Plants of the World Online. Retrieved 25 September 2020.
  13. ↑ "Rhizanthella johnstonii". Kew Science - Plants of the World Online. Retrieved 25 September 2020.
  14. ↑ "Rhizanthella omissa". Kew Science - Plants of the World Online. Retrieved 25 September 2020.
  15. ↑ "Rhizanthella slateri". Kew Science - Plants of the World Online. Retrieved 25 September 2020.
  16. ↑ "Rhizanthella speciosa". APNI. Retrieved 27 September 2020.
  17. ↑ Dixon, Kingsley W.; Christenhusz, Maarten J. M. (2018-01-12). "Flowering in darkness: a new species of subterranean orchid Rhizanthella (Orchidaceae; Orchidoideae; Diurideae) from Western Australia". Phytotaxa. 334 (1): 75–79. doi:10.11646/phytotaxa.334.1.12. ISSN 1179-3163.
  18. 1 2 Clements, Mark A.; Jones, David L. (2020-07-30). "Notes on Australasian Orchids 6: A new species of Rhizanthella (Diurideae, subtribe Prasophyllinae) from Eastern Australia". Lankesteriana: 221–227–221–227. doi:10.15517/lank.v20i2.43271. ISSN 2215-2067.
  19. ↑ "Rhizanthella omissa". Internet Orchid Species Photo Encyclopedia. Retrieved 20 July 2016.

Further reading

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