Comparative anther and pistil anatomy of three flowering morphs of andromonoecious Acer oblongum (Sapindaceae s.l.) and its adaptive significance

A comparative anatomical study of the reproductive parts of the three floral morphs of Acer oblongum (Sapindaceae) was undertaken to ascertain the functionality of the parts present and their adaptive significance. Individual trees of A. oblongum possess three types of flower morphs. Though morphologically all flowering morphs appear to be bisexual, the male or female function is temporally suppressed to achieve successful cross‐pollination. We studied the role of endothecial thickenings in the selective dehiscence of anthers. Anatomical analysis showed the presence of fibrous thickenings in the endothecium cells of staminate type I flowers. However, minimal opening of anther along the line of dehiscence was observed in hermaphrodite flowers that showed poor differentiation of endothecium. Although an endothecium is formed also in anthers of hermaphrodite flowers, the anthers fail to dehisce because not all endothecial cells have fibrous thickenings, especially towards the stomium. In hermaphrodite flowers, mature pollen grains were formed only after stigma divergence when the receptivity was already lost (after stage 3). Furthermore, pistil anatomy revealed a functional embryo sac only in hermaphrodite flowers. Functionally male flowers, with an undeveloped pistil, serve as the only source of pollen for fertilization.