Floral ontogeny of Sinojackia xylocarpa (Styracaceae), with special reference to the development of the androecium.

Using scanning electron microscopy, we studied the floral ontogeny of Sinojackia xylocarpa. There are 6-7 (-8) sepals. Sepal initiation is staggered; adaxial sepals arise later than abaxial and lateral ones. There are (5-) 6-7 (-8) petals, initiated simultaneously. Petals alternate with the sepals, and occasionally there are two petals instead of one between two sepals. The (10-) 12-14 (-16) stamens are initiated centripetally in two sets (whorls). These floral organ numbers deviate from those of the otherwise mostly pentamerous family Styracaceae. The ovary consists of three (rarely four) locules. In each of the locules, two rows of ovules are differentiated basipetally. Placentation is axile with (5-) 6 (-7) ovules in each locule. Ovules are unitegmic and are ascending with the micropyle directed downwards. Intra-ovarian trichomes are present as in other representatives of Styracaceae and seem to be an apomorphic character of the family as they are absent in the closely related Symplocaceae and Diapensiaceae. Various levels of organ union occur in anthetic S. xylocarpa. The calyx is synsepalous and the ovary syncarpous. Possibly, the basal connation of petals and stamens is postgenital (and not congenital), but this needs further study. The outward curvature of the young anthers of the inner stamen whorl superficially simulates an obdiplostemonous androecium. However, the sequence of stamen initiation shows a diplostemonous pattern. [ABSTRACT FROM AUTHOR]