Floral Organogenesis in Plowmanianthus (Commelinaceae)

Abstract

Floral organogenesis in three species of Plowmanianthus (subtribe Dichorisandrinae; tribe Tradescantieae) is described. Development begins as in other commelinaceous monosymmetric or monosymmetric‐derived flowers for which comparable data are available, with the precocious development of the upper (outer) sepal. Later, six stamen primordia emerge from the floral apex, but the lower three are soon aborted such that there are only three stamens at anthesis. Development of the lower petal is precocious. That is, although all three petals are at first similar in size and appearance following their simultaneous or nearly simultaneous initiation, the lower petal expands much more rapidly than either of the inner petals and initiates its fringe of moniliform hairs first. Early and middle phases of floral ontogeny in Plowmanianthus most closely resemble those of the closely related Cochliostema, especially in regards to suppression of the lower stamens and the laterally appressed, introrsely oriented, strongly curved anther thecae of the upper stamens. The staminal features unique to Cochliostema, such as union and curvature of the filaments, spiraling of the thecae, and development of filament‐borne structures enveloping the anthers, occur relatively late in development in this genus and are lacking in Plowmanianthus. Floral ontogenetic studies are still needed for other Dichorisandrinae, such as Geogenanthus, Siderasis, and additional species of the relatively large genus Dichorisandra, before an adequate understanding can be attained of how ontogenetic processes have evolved and shaped the remarkable floral diversity in this subtribe.