Vibratory pollen collection and flower form: bumble-bees on Actinidia, Symphytum, Borago and Polygonatum

Bumble-bees (Bombus spp) vibrated ('buzzed') flowers of Actinidia, Symphytum, Borago and Polygonatum when collecting pollen. In Actinidia anthers, fresh pollen becomes available repeatedly as surface pollen dries, becoming mobile and powdery. When individual anthers were vibrated at known frequencies, pollen surged out at a critical frequency and at its harmonics. In Symphytum, Polygonatum and Borago, the resonating chamber of the vibrational pollen release mechanism may be defined by the introrse, connivent anthers. In a 24-h study of Symphytum, vibration was most frequent, and bee visits peaked, at times of high temperature and vapour pressure deficit. Our investigation indicates that the operation of the mechanism requires the pollen to be dry and powdery and we suggest that some features of the shooting star flower form, notably the reflexed corolla lobes and the exposed, often dark staminal cone, can be interpreted as adaptations related to the floral microclimate.