Relationships and floral biology of Bidens cosmoides (Asteraceae)

Abstract

Bidens cosmoides, in the monotypic section Degeneria, is a morphologically unique species endemic to Kaua'i. Contrary to previous reports it will hybridize successfully with other Hawaiian species of Bidens, all in section Campylotheca. All Hawaiian species of Bidens are interfertile, suggesting that they are the products of adaptive radiation from a single ancestral introduction rather than two separate introductions and lineages as previously postulated. The elongated styles of B. cosmoides, exserted 20-30 mm beyond the corollas, that present pollen on the style tips, are unique in the genus. Flowers produce more than 30 times as much nectar by volume than do other Hawaiian species of Bidens, but the sugar concentration of the nectar is only half that of other species (30% vs. 60%). These unique floral features appear to represent adaptations to pollination by birds, although pollination of B. cosmoides has not yet been observed in nature.