Microtis, commonly known as onion orchids or mignonette orchids is a genus of about 20 species of plants in the orchid family, Orchidaceae. Onion orchids are terrestrialherbs with a single leaf at the base of the plant. They are similar to orchids in the genus Prasophyllum in that they have an onion-like leaf. The flowers are small but often scented and attractive to their insect pollinators. They are widespread in Asia, Australia and some Pacific islands.
Description
Orchids in the genus Microtis are terrestrial, perennial, deciduous, sympodial herbs with a few inconspicuous, fine roots and an egg-shaped to almost spherical tuber. The tuber often produces two tubers on the end of long, root-like stolons. There is a single, linear, cylindrical, onion-like leaf at the base of the plant. The leaf resembles that of the closely related genus Prasophyllum except that is entirely green (usually red at the base in Prasophyllum) and exudes clear mucilage when damaged.[2][3][4][5][6][7]
The inflorescence is a raceme with a few to many resupinate green flowers spirally arranged on a flowering stem. Each flower has a short stalk with a small bract near its base. The broad dorsalsepal is sharp-pointed, dished on the lower side and forms a horizontal hood over the column. The lateral sepals are similar to, but much narrower than the dorsal sepal. The petals are smaller than the sepals, thin and are spread below or under the dorsal sepal. As is usual in orchids, one petal is highly modified as the central labellum. The labellum is egg-shaped to oblong and hangs or is curved against the ovary. The sexual parts of the flower are fused to the column, which is shaped like half a cylinder. Flowering time depends on the climatic region where the species is found and the fruit that follows flowering is a non-fleshy, dehiscentcapsule containing up to 500 seeds.[2][3][5][6][8]
The leaves of onion orchids are susceptible to fungal disease and most leaves appear to have some damage. The rust Uromyces microtidis has been identified as a pathogen.[3]
Pollination
Microtis flowers are insect pollinated. For some species, the insect is a small wasp from a species of Ichneumonidae or Braconidae. Microtis are rarely pollinated by ants because the mouthparts of ants usually have antibiotic secretions which damage pollen grains. However, several species of Microtis, including M. parviflora are pollinated by wingless worker ants from the genera Iridomyrmex (Family Dolichoderinae), Meranops (Family Myrmeciinae) and Rhytidoponera (Family Ponerinae), having been attracted by nectar secreted from the base of the labellum.[4][12]
Autogamy
Autogamy has been observed in several species of onion orchid. In some cases, the pollen grains fall onto the stigma and germinate but in others, including in some populations of M. parvifolia, if the flowers have not been cross-pollinated, the stigma grows upwards until it contacts the pollinia, so that seed is always produced.[4]
Species
The taxonomy of Microtis has been confused ever since Robert Brown first described the genus. He did not include Ophrys unifoliaG.Forst. (1786)[13] and Epipactis porrifoliaSw. (1800)[14] and these names were used by later authors, although now recognised as Microtis media. The small size of the flowers and their tendency to change when dried as herbarium species led to further confusion.[3]
Microtis alba R.Br. (1810) – white mignonette orchid (SW Australia)
Microtis unifolia (G.Forst.) Rchb.f. (1871) – common onion orchid (widespread from China, Japan, Philippines, Indonesia, Australia, New Zealand)
Use in horticulture
Most Microtis are very easy to grow, and readily volunteer themselves in other pots. Onion orchids are often found in gardens around Melbourne from wind-born seed.[4]
^ abHoffman, Noel; Brown, Andrew (2011). Orchids of South-West Australia (3rd ed.). Gooseberry Hill: Noel Hoffman. pp. 333–349. ISBN9780646562322.
^ abcdefBates, Robert J. (1984). "The genus Microtis R.Br. (Orchidaceae): A taxonomic revision with notes on biology". Journal of the Adelaide Botanic Garden. 7 (1): 45–89.
^ abcdPridgeon (ed.), Alec M.; Cribb (ed.), Phillip J.; Chase (ed.), Mark W.; Rasmussen (ed.), Finn (2001). Genera Orchidacearum, Volume 2, Orchidoideae (part 1). Oxford, England: Oxford University Press. pp. 181–186. ISBN0198507100. {{cite book}}: |last1= has generic name (help)
^ abRowe, Ross. "Genus Microtis". Royal Botanic Gardens Sydney: plantnet. Retrieved 9 July 2016.
^ ab"Microtis". Royal Botanic Gardens Victoria:Vicflora. Retrieved 9 July 2016.
^"Microtis". State Herbarium of South Australia:efloraSA. Retrieved 9 July 2016.
^Backer, C.A. (1936). Verklarend woordenboek der wetenschappelijke namen van de in Nederland en Nederlandsch-Indië in het wild groeiende en in tuinen en parken gekweekte varens en hoogere planten (Edition Nicoline van der Sijs).
Jones D.L. (1996). "Microtis angusii, a new species of Orchidaceae from Australia". The Orchadian. 12 (1): 10–12.
R. Peakall, A. J. Beattie (1989). "Pollination of the Orchid Microtis parviflora R. Br. by Flightless Worker Ants". Functional Ecology. 3 (5). British Ecological Society: 515–522. doi:10.2307/2389565. JSTOR2389565.