Orchid seed diversity

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The Orchidaceae, with approximately 22,000 species, ranks among the largest plant families. Despite extensive literature, knowledge about orchid seeds remains limited; they are typically characterized as small, wind-dispersed "dust seeds." This monograph analyzes around 1,400 collections of orchid seeds using SEM and other methods over four decades, evaluating about 7,000 micrographs from 1,100 species across 352 genera. The findings reveal remarkable diversity in seed size (ranging from 0.1 mm in Oberonia to 6 mm in Epidendrum) and shape, as well as intricate seed-coat architecture and surface sculpturing. A consistent terminology for seed-coat characteristics is proposed, with subtribal to tribal taxa often well-defined by these traits. Fifteen selected characters were mapped onto robust molecular phylogenetic trees, showing alignment with major clades. Combinations of these characters, categorized into 17 seed types, frequently distinguish tribes such as Cymbidieae and Epidendreae. Unique features, like polyembryony (up to 12 embryos per seed in Thecostele) and specialized seed-coat adaptations for dispersal (e.g., in Galeola), are limited to a few genera. This work includes an atlas with 624 micrographs and provides essential data for identifying major orchid seed groups, along with a terminology for taxonomic use. The character reconstruction based on modern phylogenetic hypotheses supports further systematic studies of t