Anther tripping and pollen dispensing in Berberis thunbergii

Stamens of Berberis thunbergii respond to a tactile stimulus by snapping toward the stigma. When filaments are stimulated ('tripped'), anthers that strike an object (e.g., a glass cover slip or floral visitor) deposit some of the sticky pollen that is held together by viscin threads. When stamens were stimulated experimentally, 53% of the total pollen was deposited on the first trip, and 20% on the second, resulting in a pattern of diminishing proportional removal. Pollen removal by bees resulted in a similar pattern. However, small bees (e.g., Andrenids) tripped fewer stamens per visit than large bees (i.e., Bombus spp.). Our results do not support either the 'numerical' or 'proportional' removal patterns of Harder and Thomson (1989) but do conform to the diminishing proportional removal model proposed by Harder (1990) for plant/pollinator associations where there is a high variance in the number of pollinator visits.