Flower development of different genders in the morphologically andromonoecious but functionally monoecious plant Acer elegantulum Fang et P. L. Chiu

Acer elegantulum is morphologically andromonoecious, and hermaphrodite flowers are protogynous. 25.73% of flowers studied were hermaphrodite, and these did not differ in petal length, petal width or flower diameter from staminate flowers, though the lengths of anthers, filaments, styles and ovaries differed significantly. Staminate flowers are functionally male; their staminate flowers developed, dehisced and shed normally, but their ovules and ovaries degenerated gradually at the megasporogenesis stage, and the mature ovaries contained only aborted ovules. Surprisingly, however, despite developing normally until tapetum formation, the anthers of hermaphrodite flowers failed to dehisce when mature, despite developing normally through tapetum formation and dissolution; these flowers’ pistil primordia developed normally, finally forming eight-nucleic embryo sacs. The morphologically hermaphrodite flowers of A. elegantulum are therefore functionally female, and the species is functionally monoecious. We discuss the evolution of staminate and hermaphrodite flowers and suggest that both evolved from ancestrally hermaphrodite flowers, by complete abortion of the pistil or incomplete and late-stage stamen abortion.