Systematics of the Southern African Genus Geissorhiza (Iridaceae - Ixioideae)

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Geissorhiza is a large genus of small, corm bearing perennials endemic in the Cape Province of South Africa and is restricted to the winter rainfall area of the south and west coasts and immediate interior. The genus is defined by its asymmetric corms with hard, often woody tunics, herbaceous bracts, and long style with short recurved branches. It is separated by style morphology from the closely related Hesperantha, in which the style is included in the perianth tube and the style branches are long and spreading, and, in addition, the anthers of Hesperantha are introrse at anthesis. Eighty-one species are recognized in this study, 29 described here for the first time. This represents an increase of some 25 over the 52 species of true Geissorhiza admitted by Foster (1941) in his revision of the genus and the eight species placed by Lewis (1941) in Engysiphon, a genus here reduced to synonymy. A novel subgeneric classification is proposed in which two subgenera, Geissorhiza (34 spp.) and Weihea (47 spp.) are recognized, each including several sections. Geissorhiza is seen as a late Tertiary genus that probably differentiated in upland southeastern Africa when much of southern Africa including the Cape Region was still heavily forested. The continuing deterioration of the climate in the Pliocene and Pleistocene that culminated in the establishment of a Mediterranean climate in the southwestern Cape probably gave the impetus to the extensive radiation in the mountains and western coastal belt in this area, where most species, including all of the more specialized taxa, occur. Primitive taxa are concentrated along the relatively well-watered southern Cape coast, where little speciation has occurred and the primitive taxa appear to have differentiated slowly by a process of phyletic evolution. The geography of Geissorhiza is analyzed in detail and several important centers of endemism, corresponding closely with Weimarck’s phytogeographical subdivisions are identified. The major trends in the evolution of Geissorhiza include: the development of imbricate corm tunics from the basic concentric type; the reduction or extreme elongation of the perianth tube; the development of floral zygomorphy; and the elaboration of the leaf lamina, which may become several ribbed and grooved (section Geissorhiza), or may have raised and winged ciliate margins and a winged midrib (section Ciliata) or thickened margins and midrib (especially sections Angustifolia and Ixiopsis). Cytological studies have brought the number of species in which the chromosome number is known to 41, over half the genus. Basic chromosome number is x = 13, and while most species are diploid, polyploidy has been recorded in a few species including the triploid vegetative apomict, G. bolusii. The conservation status of Geissorhiza is reviewed and several seriously threatened species are identified including almost all those endemic to the coastal plain between Cape Town and Piketberg, an intensively farmed area rich in species.

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