AFRICAN

ORCHIDS

Hederorkis Thouars, Nouv. Bull. Sci. Soc. Philom. Paris 19: 319 (1809).
Heterotypic Synonyms:
Scandederis Thouars, Hist. Orchid.: t. 91 (1822), nom. inval.

Description:
An epiphytic or lithophytic, robust plant. Stem scandent, sympodial rhizo-matous, covered with imbricate sheaths disintegrated into fibres, pseudobulbs rudimentary,. Leaves two, arising from the top of each shoot, elliptic, acute, divergent, thick, shortly articulated to a sheath. Inflorescence lateral simple raceme arising from a node on rhizome or near tip of shoot, few-flowered, longer than leaves. Flowers non-resupinate. Sepals and lateral petals free, somewhat similar, elliptic to oblanceolate, obtuse. No mentum. Lip ecallose, free to base, sparsely hairy on upper surface, obscurely trilobed, the lateral lobes erect, upcurved at the tips, the middle lobe porrect, larger than the lateral ones. Gynostemium elongate, slender, gently arched; column part free, much longer than the anther, narrowly winged, column foot rudimentary; anther incumbent, operculate, subglobose, slightly dorsiventrally compressed; connective narrow, rather thin; pollinia 4, in two pairs, obliquely superposed, unequal in size and form, oblong-ellipsoid, hard, caudiculae sticky, connecting pollinia with tegula; apical clinandrium obscure, collar-like; stigma elliptic, deeply concave; rostellum bent forwards, short, truncate; viscidium single, oblong-obovate, lamellar, delicate; tegula single, oblong, lamellar; rostellum deeply incised after removal of pollinarium.
Etymology:
Its name is derived from two words, Latin hedera (ivy) and Greek orkis (orchid) and refers to the scandent habit of the plant.
Type:
Hederorkis scandens Thouars.
Taxonomic note:
Hederorkis, described in 1809 by du Petit Thouars has been considered to belong to Bulbophyllum (note in Kew Herbarium, according to Cribb in and Hunt. However, Bosser indicated closer relationship of the genus to Polystachya due to the gynostemium structure, and Dressler agreed. Thouars used two set of names in his revolutionary system Rasmussen clarified the nomenclature of H. scandens.
Both species of Hederorkis are plants with similar habit and floral structure; however, Hederorkis seychellensis is a larger, more robust plant with an oblong, fleshy lip adorned by two longitudinal calli in the basal third. The lip of Hederorkis scandens is smaller, deeply constricted in the middle, widest in apical half and free from any callosities. Hederorkis seychellensis is uncommon in Mahé and very rare in Silhouette and probably does not occur in Aldabra. The material collected in Aldabra atoll is sterile and may not represent this species. H. scandens is known from four localities in Mauritius and one from Réunion. The islands on which the species occur are small and well known botanically, but Hederorkis have been reported from very few localities. The range of distribution of both species of Hederorkis coincide with an area of extremely high floral diversity and endemism, the Madagascar and the Indian Ocean Islands hotspot. The hotspot is one of the world’s top conservation priority due to its remarkable biodiversity and extensive deforestation. A potential distribution range for both species is relatively small, the known extent of occurrence of Hederorkis seychellensis is about 170 km2 and about 4560 km2 of Hederorkisscandens. Moreover, the islands are residential and strongly deforested, so the extent of potential habitats for Heredorkis is quite limited.
A key to the species of Hederorkis
Habitat:
The two species have different ecological requirements, which is associated with different weather conditions. The Mascarenes, where H. scandens occurs, are sometimes subjected to very high levels of rainfall, up to 6,000 millimeters per year in La Réunion, in contrast, the Seychelles, where Hederorkis seychellensis grows, are relatively dry region with a lower altitude reaching only 914 millimeters at its highest in Mourne Seychellois National Park, however the climate in La Réunion can be dry on western slopes.
Distribution:
W. Indian Ocean.
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