Wikstroemia is a genus of approximately 70–93 species of flowering shrubs, subshrubs, and occasional small trees in the family Thymelaeaceae, the mezereon family, placed in the order Malvales. The genus was described by Endlicher in 1833. Most species are woody, evergreen or deciduous, with opposite or alternate leaves. The inflorescences are typically terminal or subterminal — spicate, racemose, umbelliform, or capitate — sometimes arranged in compound panicles. Flowers are bisexual (or unisexual in Hawaiian species), 4- or 5-merous, with a tubular to cylindrical calyx in yellow, green, white, purplish, or red; true petals are absent, replaced by a calyx whose lobes spread at anthesis. The stamens are twice as many as the calyx lobes and arranged in two series. A hypogynal disk of 2–4 membranous scales is characteristic and is the primary feature distinguishing Wikstroemia from the closely allied genus Daphne, though the boundary between the two genera is contested. The fruit is a succulent berry or rather dry drupe with fleshy cotyledons.
The genus ranges across tropical and subtropical Asia (concentrated in China), Japan, Southeast Asia, the Pacific Islands, and Australia, with a notable secondary radiation in the Hawaiian Islands where species are known collectively as ʻākia. One of the best-known species, Wikstroemia indica, widespread from India through Southeast Asia to the Pacific, is counted among the 50 fundamental herbs of traditional Chinese medicine. Several Hawaiian endemics — such as Wikstroemia villosa (hairy false ohelo of Maui) — have restricted island ranges. Wikstroemia genkwa, native to China and Korea, is cultivated for ornamental and medicinal use.
Etymology
The genus name Wikstroemia honours Johan Emanuel Wikström (1789–1856), a Swedish botanist and student of Thunberg who contributed to the taxonomy of the Thymelaeaceae. The genus was formally described by Stephan Endlicher in his 1833 Prodromus Florae Norfolkicae.
Distribution
Wikstroemia is distributed primarily across tropical and subtropical East Asia and Southeast Asia, with high diversity in China, and extends through Japan, the Philippines, Malesia, Papuasia, Australia, Norfolk Island, and the Pacific Islands including the Marquesas. A significant endemic radiation occurred in the Hawaiian Islands, where multiple species occur on individual islands (Kauaʻi, Oʻahu, Molokaʻi, Lānaʻi, Maui, and Hawaiʻi). Wikstroemia indica has the broadest range, from India eastward through Southeast Asia to Australia and the Pacific.
Taxonomy Notes
The boundary between Wikstroemia and Daphne is contested. The Flora of China (Vol. 13) notes that leaf arrangement, inflorescence type, and flower color are all paralleled within both genera and lack diagnostic value; the hypogynal disk structure is the primary distinguishing feature, though its interpretation is difficult in several species. Halda (2001) united both genera under Daphne. A molecular phylogenetic review of generic limits in Thymelaeaceae remains desirable. Several species formerly placed in Wikstroemia have been transferred to unrelated families: W. bodinieri is a synonym of an Apocynaceae species, and W. hemsleyana belongs to Alstonia (Apocynaceae).
Cultural Uses
Wikstroemia indica — known in Chinese as 了哥王 (liǎo gē wáng) — is listed among the 50 fundamental herbs of traditional Chinese medicine, where it has been used for its medicinal properties. In Hawaii, species of Wikstroemia (ʻākia) were used by Native Hawaiians; the toxic berries and bark were employed as fish poison.
