The reproductive biology of Anemone canadensis (Ranunculaceae): breeding system and facilitation of sexual selection

Our data show that Anemone canadensis L. is xenogamous and has adaptations that should increase the opportunity for sexual selection and minimize interference between female and male sexual function. The flowers of plants within insect exclosures and hand self-pollinated flowers set very few fruits compared to those that were open-pollinated and hand crosspollinated. Examination of styles with uv-fluorescence microscopy showed that self pollen tubes grew into the transmission tissue but rarely entered the ovary, i.e., there is a gametophytic self-incompatibility system. A number of traits should enhance sexual selection. First, pollen grains arrived on the stigmas beginning on day 1, germinated, but the pollen tubes grew very slowly until day 4 or 5 when the transmission tissue matured. This allowed large numbers of pollen grains to reach the stigmas, presumably from a number of pollen parents. Second, basipetal maturation of the transmission tissue, which underlies the elongate stigmatic surfaces, allows pollen tubes originating near the tip of the style to initiate rapid growth prior to those lower in the style. Thus, rapidly growing pollen tubes from the top of the style were able to catch up with those lower in the style. In essence, all the pollen tubes in a style have an equal opportunity to reach the ovule. Large stigmatic pollen loads and high fruit set suggest there is minimal interference between male and female sexual function. This may be a consequence of the spatial separation of the pistils and stamens, which increases during the life of a flower. Synchrony of function among the pistils of a flower in conjunction with large and/or multipaternal pollen loads may reduce the variance in competition among developing embryos and result in fruits of equivalent size