Didelta is a small genus of two species of succulent shrubs in the daisy family (Asteraceae), native to arid regions of southern Africa. Plants typically grow to 1–2 metres in height and are known in English as "salad thistle" and in Afrikaans as slaaibos.
As members of the Asteraceae, Didelta species produce composite flowerheads — what appears to be a single flower is actually a dense cluster of many individual florets. Each head is subtended by two whorls of free involucral bracts: the 3–5 outer bracts are protruding, triangular, and leaf-like; the inner bracts are lance-shaped and ascending, and may bear spines. The centre of each head is occupied by bisexual, yolk-yellow disc florets, surrounded by a single whorl of infertile yellow ray florets, each tipped with four teeth.
One of the genus's most distinctive features is its fruit-dispersal mechanism. The receptacle (the common base of all florets) swells as the fruitlets develop, eventually becoming woody. At maturity it breaks apart into segments, each corresponding to one of the outer involucral bracts. These woody segments detach from the plant and act as dispersal units, with the ribbed, flask-shaped fruitlets germinating within their protective encasement. A pappus of winged, basally fused scales aids dispersal.
The two species are distinguishable by several features. Didelta spinosa lacks milk sap, has hairless oval leaves with spines that often clasp the stem at the base, and its woody dispersal segments are spineless. Didelta carnosa produces milk sap, has elliptic to linear leaves that are variably felty-hairy, and its dispersal segments are spiny; two varieties are recognised — var. carnosa (hairless or becoming so) and var. tomentosa (persistently densely hairy on the leaf undersides).
Etymology
The name Didelta was established by the French botanist Charles L'Héritier in 1786. The genus name is derived from Greek, reflecting the distinctive double (di-) delta-shaped (triangular) outer involucral bracts that characterise the flowerheads. An earlier name, Breteuillia Buc'hoz (1785), was subsequently suppressed in favour of Didelta.
Distribution
Didelta is restricted to the arid and semi-arid zones of southern Africa. Both species — D. spinosa and D. carnosa — occur in southern Namibia and the western parts of South Africa's Northern Cape and Western Cape provinces.
Taxonomy Notes
The two species were first described by Carl Linnaeus the Younger in 1781 as Polymnia carnosa and Polymnia spinosa. They were subsequently moved through several genera — Choristea (Aiton, 1789), Favonia (Gaertner, 1791), and Distegia (Klatt, 1886) — before the name Didelta L'Hér. (1786) was accepted as the valid genus name. The earlier name Breteuillia Buc'hoz (1785) was suppressed. GBIF records the publication as L'Hér. (1785) Stirp. Nov. 55. T. 28.
