Comprising ca. 28,000, species the Orchidaceae constitute one of the most species-richplant families. Orchids differ from other monocotyledons i.a., in the formation of so-calledpollinaria, which are entities consisting of pollen grains aggregated into compact polliniaand accessory structures, a viscidium and mostly also a pollinium stalk. The viscidiumreleases an adhesive material that attaches the pollinarium to a pollinator. Pollinaria are partof a complex pollination apparatus that enables the orchids to colonize niches in which onlya few individuals of the respective pollinator occur infrequently. Because the aggregatedpollen grains are removed from the flower at once, the development of a mechanical barrierensuring that only suitable pollinators are able to access the flowers and more importantlyto remove the pollen are important selective traits. In this paper we describe the functionalmorphology of the pollination apparatus in two orchid species, Oncidium wentworthianumand O. otogaya, by experimentally mimicking the pollination process. Furthermore, weanalyzed the mechanical resistance of this apparatus by means of force measurementsand showed that it most probably constitutes a hierarchical two-stage barrier. The firststage consists of the presence of the anther cap that not only protects the pollinia, but alsoserves to prevent premature removal of young and unripe pollinaria from the flower. Assoon as the pollinaria are ripe, the anther cap sheds and the second stage of themechanical barrier takes effect, a severable bond between pollinarium and rostellum.This bond can be overcome by a potential pollinator, applying a load of at least 10.8 mN (O.otogaya) or 12.6 mN (O. wentworthianum), respectively, on the viscidium which at thesame time disengages the pollinarium from its anchorage. The adhesive material producedby the viscidium creates sufficient adhesive contact between pollinarium and pollinator.Potential pollinators, such as Centris spp. or Trigona spp. bees, should be well able toexert such forces by pushing their head/forebody into the orchid flowers. Thus, whether apollinator is able to detach the pollinarium depends on both how forcefully it can push andhow strongly it can pull the orchid pollination apparatus.
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