Floral Dimorphism in Psychotria boninensis Nakai (Rubiaceae) Endemic to the Bonin (Ogasawara) Islands

Abstract

Floral and reproductive characters were examined to determine mode of dimorphism in the flowers of Psychotria boninensis Nakai, a species endemic to the Bonin (Ogasawara) Islands, Japan. The flowers reciprocally differed in stigma and anther height, and were composed of long- and short-styled morphs. Pollen grains from the short-styled morph were larger than those from the long-styled morph in size, but no major difference was found between the two morphs in exine sculpture. The short-styled flowers produced less pollen grain than the long-styled flowers, as in some other typically distylous species. The pollen grains produced by the two morphs were positively stained with Cotton Blue solution, and ovaries of the two morphs produced fruits and seeds in field. These results suggest that the flowers of P. boninensis are morphologically distylous, as in most of the typically distylous species, and that the long- and short-styled flowers are functionally bisexual. This is the first record of distyly among the plants endemic to the Bonin Islands.