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(Last Modified On 9/7/2016)
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Description
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Deciduous perennial geophytes. Corm globose to ovoid, asymmetric or bell-shaped, usually with a basal ridge from which the roots emerge, axial in origin; tunics woody, or rarely membranous to fibrous, concentric or imbricate, then notched below. Stem aerial, simple or branched, terete in section, smooth or minutely hairy or scabrid. Leaves few to several, unifacial, usually with a definite main vein, blades plane to terete or I- or H-shaped in section with margins and or midrib raised and winged, occasionally hairy or sticky; margins with subepidermal sclerenchyma bundle and thin-walled epidermal cells. Inflorescence a spike, flowers usually spirally arranged, rarely solitary on branches; bracts green and soft-textured to membranous, inner smaller than outer and forked at apex. Flowers star-like or funnel–salver-shaped, sometimes with tube elongate, variously coloured, often blue to violet, also pink, yellow, cream, purple, red or bicoloured, mostly actinomorphic, zygomorphic in some species, faintly scented or unscented, with nectar from septal nectaries; perianth tube short to long, funnel-shaped or cylindric; tepals subequal, inner usually shorter than outer, cupped or spreading from base. Stamens symmetrically arranged or unilateral and arching downward (declinate); filaments filiform, equal or unequal with one shorter than other two, exserted, rarely included and inserted well inside tube; anthers erect or ascending, rarely included; pollen. Style filiform, eccentric or unilateral and held below stamens, usually exserted, the branches usually slender and recurved, rarely broadly expanded above. Capsules globose to oblong or cylindric, cartilaginous to bony. Seeds ± globose, flattened at chalazal end, sometimes lightly wrinkled, matte, surface cells domed or areolate. Pollen grains mostly monosulcate-operculate, usually with 2-banded operculum or with more complex apertures, exine perforate-scabrate (rarely grains with complex, multiple apertures and lacking operculum). Basic chromosome number x = 13.
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Etymology
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from the Greek geisson, tile, rhizon, root, for the tiled appearance of the overlapping, segmented corm tunics of subg. Geissorhiza (but not of the other subgenus, Weihea).
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General Notes
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Species ± 103: divided among two subgenera; restricted to the winter-rainfall zone of South Africa, extending from northern Namaqualand in Northern Cape through the western Karoo and southwestern Western Cape and east to Grahamstown in Eastern Cape.
A large genus, Geissorhiza shows relatively limited floral variation, most species having moderate-sized, radially symmetric flowers with a short perianth tube, a long style dividing opposite the anthers but eccentric in position, and slightly asymmetric corms with woody, rarely fibrous or papery tunics. The corm structure is the basis for recognition of two subgenera. Both subgenera have globose to ovoid, asymmetric corms with a basal ridge from which the roots emerge, but in subg. Weihea the tunics are concentric, fragmenting into vertically inot sections drawn inot poinys at the top and bottom. The tunics of subg. Geissorhiza are imbricate, older tucis being displaced upward by new tunics and spitting into sections at the base, the corms then appearing to be coveed with overlapping tiles.
Leaf morphology is surprisingly diverse. A plane surface is the ancestral condition, but the margins and main veins are often thickened rendering the leaf either oval or terete in section as the thickened areas form ridges occluding most of the surface. Such terete leaves with four longitudinal grooved characterize the G. juncea group of subg. Weihea. In several species the margins are raised into wing-like flanges hald at right angles to the surface, this rendering the leaf I-shaped in section. In the most extreme examples, the marginal wings are nearly as wide and the leaf surface over which they arch, thus rendering the leaf H-shaped in section. Surprisingly, such leaves have evolved in both subgenera, notably in sect. Engysiphon of subg. Weihea and the G. heterostyla group of subg. Geissorhiza. Leaves or leaf sheaths are sticky in many species, resulting the accumulation of sand grains but the source of the sticky substance nor its adaptive value has not been investigated.
In an unusual development for Iridaceae, the stamens are unequal in several species of subg. Geissorhiza, in which the upper filament is shorter than the lower two but the anthers are or equal length. The character is most frequent in sections Ciliatae and Planifoliae of the subgenus. G. melaanthera is unique in the genus in having anthers of unequal length and an unusual purple-black colour. Flowers are typically upright and radially symmetric (barring the eccentric style) but nodding, zygomorphic flowers with decumbent, unilateral stamens and style characterize several species of both subgenera, feature poorly preserved in pressed, herbarium specimens.
Molecular studies in Geissorhiza have so far been limited to single species comparisons with other genera of Iridaceae and confirm its immediate relationship with Hesperantha, an alliance first proposed on the basis of similar assymetric corms, woody corm tunics and shared basic chromosome number, x = 13.
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