Ontogeny of the Fertile Flower in Platycrater arguta (Hydrangeaceae)

Abstract

The monotypic genus Platycrater has had varying systematic placements. Recent phylogenetic analyses place Platycrater in the Hydrangeaceae, tribe Hydrangeeae. The ontogeny of fertile flowers of Platycrater arguta is examined using scanning electron microscopy to provide further insights into the floral morphological evolution in the poorly resolved tribe Hydrangeeae. Sepals are initiated successively. Petals are initiated nearly simultaneously. The gynoecium is initiated before the androecium. Stamen initiation in the polystemonous androecium is somewhat irregular but is centrifugal overall, which is distinctive among the otherwise centripetal members of Hydrangeeae. It is similar to that of Deinanthe in having stamens initiated on an extended hypanthium. Gynoecial development in the genus resembles that of Hydrangea and Cardiandra, in which the styles remain separate. The pattern of floral development supports the taxonomic placement of Platycrater in Hydrangeeae rather than in Philadelpheae, in which polystemonous members have common androecium primordia. In addition, the results indicate that Platycrater may be closely related to Hydrangea, Deinanthe, and Cardiandra.