The perennial species are found mainly in summer-rainfall areas such as the KwaZulu-NatalDrakensberg; 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.[3] 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 bees of the genus Rediviva (e.g. R. longimanus) that appear to have coevolved with the plants, as they have unusually long forelegs for collecting the oil.[4][5]
In gardens, Diasciacultivars (mostly hybrids) have become extremely popular as colourful, floriferous, easily grown bedding plants in recent years.[6]
Etymology
Surprisingly, the generic name (from the Greekdi = 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.[7]
Description
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)[8] 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.
Species
Around 60–70 species are currently recognised in the genus Diascia:[9]
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).[10] 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.[4] 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.[11]
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.[4]
Garden uses
'Coral Belle', a cultivar used for summer bedding
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.[6]
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).[12][13]
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