Link & Otto
Link & Otto
Diascia is a genus of around 70 species of herbaceous annual and perennial flowering plants of the family Scrophulariaceae, native to southern Africa, including South Africa, Lesotho and neighbouring areas.
The perennial species are found mainly in summer-rainfall areas such as the KwaZulu-Natal Drakensberg; about 50 species, mostly annuals, are found in the Western Cape and Namaqualand, winter rainfall areas.
Their common name is twinspur, in reference to the two (usually downward-pointing) spurs to be found on the back of the flower. These help to distinguish them from the similar (and closely related) genera Alonsoa and Nemesia. The spurs contain a special oil, which is collected in the wild by certain species of bees that appear to have coevolved with the plants, as they have unusually long forelegs for collecting the oil.
Surprisingly, the generic name (from the Greek di = two and askos = bag, pouch or sack) does not refer to the spurs, but to the two translucent sacs, or pouches, known as 'windows', found in the upper part of the corolla of the original type specimen, Diascia bergiana. Diascia species in the section Racemosae have similar windows, but in some species they merge into one. The windows may help oil-collecting bees to find the correct position within the corolla when gathering oil from the glands within the spurs.
Most diascia species are short-growing, straggling plants, reaching no more than 30–45 centimetres (12–18 in) in height, although Diascia rigescens can reach 60 cm (24 in), and the rather similar D. personata (with which it is often confused) up to 120 cm (47 in) or so. Some Diascia species spread by means of stolons, while others produce multiple lax stems from a single crown. The flowers are borne in loose terminal racemes. The corolla is five-lobed, and normally pink or rose-coloured in the perennial species most commonly seen in cultivation. Dark purplish patches of oil glands may make the flowers of some species appear bicoloured.
Around 60–70 species are currently recognised in the genus Diascia:
- Diascia aliciae Hiern
- Diascia alonzoides Benth.
- Diascia anastrepta Hilliard & B.L.Burtt
- Diascia ausana Dinter
- Diascia austromontana K.E.Steiner
- Diascia barberae Hook.f.
- Diascia batteniana K.E.Steiner
- Diascia bergiana Link & Otto
- Diascia bicolor K.E.Steiner
- Diascia capensis (L.) Britten
- Diascia capsularis Benth.
- Diascia cardiosepala Hiern
- Diascia cordata N.E.Br.
- Diascia cuneata E.Mey. ex Benth.
- Diascia decipiens K.E.Steiner
- Diascia dielsiana Schltr. ex Hiern
- Diascia diffusa Benth.
- Diascia dissecta Hiern
- Diascia dissimulans Hilliard & B.L.Burtt
- Diascia ellaphieae K.E.Steiner
- Diascia elongata Benth.
- Diascia engleri Diels
- Diascia esterhuyseniae K.E.Steiner
- Diascia fetcaniensis Hilliard & B.L.Burtt
- Diascia fragrans K.E.Steiner
- Diascia glandulosa E.Phillips
- Diascia gracilis Schltr.
- Diascia heterandra Benth.
- Diascia hexensis K.E.Steiner
- Diascia humilis K.E.Steiner
- Diascia insignis K.E.Steiner
- Diascia integerrima E.Mey. ex Benth.
- Diascia lewisiae K.E.Steiner
- Diascia lilacina Hilliard & B.L.Burtt
- Diascia longicornis (Thunb.) Druce
- Diascia macrophylla (Thunb.) Spreng.
- Diascia maculata K.E.Steiner
- Diascia megathura Hilliard & B.L.Burtt
- Diascia minutiflora Hiern
- Diascia mollis Hilliard & B.L.Burtt
- Diascia namaquensis Hiern
- Diascia nana Diels
- Diascia nodosa K.E.Steiner
- Diascia nutans Diels
- Diascia pachyceras E.Mey. ex Benth.
- Diascia parviflora Benth.
- Diascia patens (Thunb.) Grant ex Fourc.
- Diascia pentheri Schltr.
- Diascia personata Hilliard & B.L.Burtt
- Diascia purpurea N.E.Br.
- Diascia racemulosa Benth.
- Diascia ramosa Scott-Elliot
- Diascia rigescens E.Mey. ex Benth.
- Diascia rudolphii Hiern
- Diascia runcinata E.Mey. ex Benth.
- Diascia sacculata Benth.
- Diascia scullyi Hiern
- Diascia stachyoides Schltr. ex Hiern
- Diascia stricta Hilliard & B.L.Burtt
- Diascia tanyceras E.Mey. ex Benth.
- Diascia tugelensis Hilliard & B.L.Burtt
- Diascia unilabiata (Thunb.) Benth.
- Diascia veronicoides Schltr.
- Diascia vigilis Hilliard & B.L.Burtt
Coevolution with Rediviva bees
The two spurs found on the back of a Diascia flower (from which it gets the common name twinspur) contain a special oil, which is collected in the wild by at least 8 species of bees of the genus Rediviva. The bees appear to have coevolved with the plants, as the females have developed unusually long, hairy forelegs with which they collect the oil from Diascia spurs to feed their larvae (and sometimes to line their nests with). The spurs vary in average length from 5.3 millimetres (0.21 in) to as much as 13.9 mm (0.55 in), mainly between species (although those of D. capsularis can vary widely between populations); the bees' forelegs vary similarly.  The spurs of Diascia longicornis are about 25 mm (0.98 in) in length, but the existence of a suitably equipped pollinator, Rediviva emdeorum, with forelegs of the same length, was only confirmed in the 1980s. Rediviva longimanus has also been observed pollinating D. longicornis in the Western Cape.
Rediviva neliana, a widespread species, collects from at least 12 species of Diascia, but in general, few different Diascia species grow together in the same locality. As a result, local populations of R. neliana have been found to differ from each other, as each has developed legs that match the spur length of the diascias that are available to them in that locality. This indicates that local populations of R. neliana are coevolving with the flowers on which they depend.
Diascia cultivars have become extremely popular worldwide as bedding plants, suitable for hanging baskets, window boxes and other containers, as well as rockeries and the fronts of herbaceous borders. This explosion of interest is largely thanks to the breeding work done by the late Hector Harrison of Appleby, North Lincolnshire, England. From 1985, he raised hundreds of hybrid seedlings, from which several excellent cultivars have been selected and named. He increased the colour range to include shades of apricot, pink, coral, lilac, red and white. Other nurseries and breeders have continued to build on his pioneering work.
Several species and cultivars have been given the Award of Garden Merit by the British Royal Horticultural Society. The AGM includes a hardiness rating: most have been rated as intermediate between H3 (hardy outside in some regions or particular situations or which, while usually grown outside in summer, needs frost-free protection in winter – e.g. dahlias) and H4 (hardy throughout the British Isles).
- Plantzafrica Diascia integerrima
- "Genera of Scrophulariaceae tribe Hemimerideae". Germplasm Resources Information Network. United States Department of Agriculture. Retrieved 2009-04-10.
- Google Books: Thompson, John N., The Coevolutionary Process
- PlantSystematics.org Diascia (Scrophulariaceae)
- Horticulture Week: Diascia, by Graham Clarke
- Kim E. Steiner (1990). "The Diascia (Scrophulariaceae) window: an orientation cue for oil-collecting bees". Botanical Journal of the Linnean Society 102 (2): 175–195. doi:10.1111/j.1095-8339.1990.tb01874.x.
- Google Books: The European Garden Flora
- "Diascia". The Plant List. Retrieved June 11, 2013.
- Peter W. Price (1997). "Pollination ecology". Insect Ecology (3rd ed.). John Wiley & Sons. p. 248. ISBN 9780471161844.
- Stefan Vogel & Charles D. Michener (1985). "Long bee legs and oil-producing spurs, and a new Rediviva (Hymenoptera, Melittidae; Scrophulariaceae)". Journal of the Kansas Entomological Society 58 (2): 359–364. JSTOR 25084648.
- RHS Online: Award of Garden Merit (AGM)
- RHS Plant Finder 2009-2010. Dorling Kindersley. 2009. p. 242. ISBN 978-1-4053-4176-9.
- "RHS Plant Selector - Diascia barberae 'Blackthorn Apricot'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia barberae 'Fisher's Flora'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia barberae 'Ruby Field'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Dark Eyes'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Elizabeth'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia integerrima". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Joyce's Choice'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Lady Valerie'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Lilac Belle'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Lilac Mist'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia rigescens". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Rupert Lambert'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia 'Twinkle'". Retrieved 18 June 2013.
- "RHS Plant Selector - Diascia vigilis". Retrieved 18 June 2013.
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