Disa (plant)


Disa is a genus of flowering plants in the family Orchidaceae. It comprises about 182 species. Most of the species are indigenous to tropical and southern Africa, with a few more in the Arabian Peninsula, Madagascar, and Réunion. Disa bracteata is naturalised in Western Australia, where the local name is "African weed-orchid."
The genus Disa was named by P.J. Bergius in 1767. It was named after Disa, the heroine of a Swedish legend.

Description

The plants grow from a fleshy tuberous root which is a source of maltodextrins which are used as a sugar substitute. Some species attain a height of 90 cm.
The flowers are solitary or arranged in racemes. The petals and the lip are small. The flowers consist essentially of the sepals. The flowers range in color from very light to dark red.

Pollination

Disa exhibits a variety of pollination syndromes. Each species of Disa usually has a single species as pollinator and nearly every available pollinating insect is employed by some species of Disa. Species that adapted to the same pollinator often independently evolved a similar floral morphology which confounded the infrageneric classification of Disa until cladistic analysis was applied to DNA sequences from this genus.
Examples of convergent evolution in Disa pollination include the following:
Disa serves as an example of how speciation can be caused by changes in pollinator availability and evolution.
Some Disa species are pollinated by sunbirds and have pollinaria that stick to the feet of the sunbirds when they perch on the inflorescence.

Phylogeny

The first molecular phylogeny of the genus involved comparison of nuclear ribosomal ITS1, 5.8S rDNA, and ITS2 sequences, and showed that Herschelia and Monadenia were nested within a paraphyletic Disa.
In Genera Orchidacearum volume 2, Disa and Schizodium compose the subtribe Disinae of the tribe Diseae. After that volume was published in 2001, molecular phylogenetic studies showed that Schizodium is nested within Disa. Schizodium comprises only six species, all endemic to South Africa.
In a classification of orchids that was published in 2015, Chase et alii placed Schizodium in synonymy under Disa. They also defined the subtribe Disinae as consisting of Pachites, Disa and Huttonaea. This version of Disinae is probably not monophyletic, but was created as a holding classification, to avoid the unnecessary designation of subtribes before further studies can clarify the relationships of these three genera.

Horticulture

The species Disa uniflora is well known as an ornamental. It is a spectacular red orchid known as "The Pride of Table Mountain." Other commonly cultivated species include Disa aurata, Disa cardinalis, Disa crassicornis, Disa racemosa, Disa sagittalis, and Disa tripetaloides. Some of the species are grown only in African gardens.
Once very rare in cultivation, Disa uniflora is gaining in popularity as a cut flower. However, they are difficult to grow, because of the needed mineral composition of the potting soil. Also, if exposed to excessive moisture, they can be easily killed by rot.

Hybrids

The following species have been used to create more than 400 hybrids : Disa cardinalis, Disa caulescens, Disa racemosa, Disa tripetaloides, Disa uniflora, Disa aurata and Disa venosa.
Species currently recognized:

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