Floral organogenesis in Hydrangea aspera (Hydrangeaceae)
马桑绣球(绣球科)的花器官发生和发育

To provide data of floral development for the subsection Asperae, section Hydrangea of the genus Hydrangea, and to have a further insight into the floral morphological evolution of this genus, the organogenesis of fertile flowers in Hydrangea aspera was examined under the scanning electron microscope (SEM). The species has small fertile flowers, arranged in a corymbose cyme. The flowers follow the regular centripetal mode in development. The sepal primordia arise in a two-fifths helix on the periphery of the floral apex. Petals are initiated nearly simultaneously, alternating with sepals. The floral apex appears concave after petal initiation. The species has diplostemony. The whorl of antesepalous stamens arises just as the petal primordia begin to flatten. The carpel primordia are initiated almost at the same time as the initiation of the antepetalous stamens. The thecae of the anthers are thicker than the connectives. Styles and stigmas are differentiated as the incipient carpels extend upward, while the inferior ovary is formed by incipient carpels inserting into receptacle. Septa are continuous in the base of the mature ovary, but discontinuous in the mid and upper regions of ovary, i.e., the ovary is imperfectly 2-loculed. The gynoecium has two free styles and papillate stigmas. Floral developmental characters are compared among Hydrangea aspera and the six species previously reported by Hufford, and three patterns of floral development are revealed in Hydrangea. H. aspera differs from H. anomala subsp. petiolaris and H. macrophylla in the pattern of floral development, but shares an identical development pattern with the other four species. The results show that it might be more reasonable to treat Calyptranthe as a independent section under Hydrangea than to treat it as a subsection of sect. Hydrangea.