Orchid seed diversity: A scanning electron microscopy survey

The Orchidaceae with some 22 000 species is one of the two largest plant families. Despite of the vast literature on orchids, rather little is known about their seeds, which are generally considered as wind dispersed, small and reduced “dust seeds”. Based on some 1400 collections of orchid seeds analysed by SEM and other methods over the last four decades, about 7000 micrographs of some 1100 species from 352 (out of c. 880) genera were evaluated for this first monograph on orchid seeds. Orchid seeds exhibit an astonishing diversity. This is not only reflected in their sizes (between 0.1 mm in Oberonia and 6 mm in Epidendrum) and shapes, but especially in the complexity of their lightweight seed-coat architecture and hierarchical surface sculpturing. A consistent terminology for characters of the orchid seed coat is proposed. Taxa at subtribal to tribal levels are often well characterized by seed-coat characters. Fifteen selected characters were mapped on well-supported molecular phylogenetic trees and were found to be largely consistent with the major clades. Combinations of characters classified into 17 seed types often delimit tribes (e.g. Cymbidieae, Epidendreae, Vandeae). Highly specialized features like polyembryony (up to 12 embryos per seed in Thecostele) or highly adaptive (with respect to dispersal biology) seed-coat features (e.g. in Galeola, or the sophisticated seed-attachment mechanism of Chiloschista) are restricted to only a few genera. This monograph provides a first atlas (624 micrographs) and data to identify the major groups of orchid seeds and a terminology for taxonomic purposes. The first character reconstruction of seed characters based on modern molecular phylogenetic hypotheses allows an application for further systematic studies of the family.