Seed micromorphology of Orchidaceae in the Gulf of Guinea (West Tropical Africa)

Abstract

Qualitative micromorphological characters of seeds were analysed using Scanning Electron Microscopy for 95 species of 34 genera, belonging to the subfamilies Vanilloideae, Orchidoideae and Epidendroideae (Orchidaceae) from the Gulf of Guinea countries. Three genera (Dinklageella, Platylepis and Rhipidoglossum) and 69 species were studied for the first time. Vanilla shows a pattern of seed coat distinct to the rest of the studied taxa. Orchidoideae and Epidendroideae show variation in several features: seed shape, orientation of testa cells, morphology of anticlinal walls, cell corners, shape and ornamentation of the periclinal walls and presence of waxes. Two main patterns have been observed: one with parallel medial cells along the longitudinal axis, thin and straight anticlinal walls, with no raised and no thickened cell corners, wide periclinal walls and lack of waxes, for the terrestrial orchids of the subfamily Orchidoideae, the tribes Tropidieae, Nervilieae, Collabieae, Cymbidieae and the subtribe Malaxidinae (Epidendroideae); the other one showed twisted medial cells, thickened anticlinal zone with prominent ridges, raised and thickened cell corners, narrow-to-not visible periclinal walls and presence of waxes, in epiphytic orchids of Vandeae, Podochileae and Dendrobiinae (Epidendroideae). In the epiphytic genera Ansellia and Graphorkis (Cymbidieae) and in Liparis epiphytica (Malaxideae), thickened anticlinal walls and presence of waxes also occurred but parallel medial cells along the longitudinal axis and wide periclinal walls have been observed. This study confirms the diagnostic value of the qualitative features of the seed coat in Orchidaceae and suggests a probable relation with adaptations to habitat for seed dispersal.