Floral Ontogeny and Phylogeny in Malvaceae.

Floral ontogeny in representatives of Malvaceae was compared using scanning electron microscopy. Many aspects of floral development have been resolved by this research, including the presence of a petal-stamen ring meristem in malvaceous taxa. There are several ontogenetic pathways to the formation of this ring meristem; in addition, common petal-stamen mounds are usually present during some stage of development. Stamen proliferation is achieved by partitioning of the ring meristem and furcation of common stamen primordia. Time of initiation overlaps between the corolla and the androecium, and between the androecium and the gynoecium. Helical initiation occurs in the outer organ whorl or whorls and simultaneous initiation occurs in the inner organ whorls or the sets of organs composing these whorls. Ring meristems are sometimes present in bractlet, sepal, and carpel whorls. This research details the complex development leading to the multistaminate androecial columns and the development of the syncarpous gynoecia. Several unusual features are elucidated such as common bractlet primordia, displacement of bractlets, and suppression of organs. Features that distinguish bractlets, from floral bracts and from sepals are discussed. The idea that bifurcation and partitioning events are the likely causes of the bisporangiate condition is refuted. Although the exact mechanism behind the formation of sterile teeth was not ascertained, several factors are identified as sometimes contributing, and some are ruled out as causal. In addition, examination of selected taxa in the sister families Bombacaceae and Sterculiaceae reveals the development of other floral structures such as androgynophores, more than one stamen whorl in the androecium, apocarpous gynoecia, and multistamen meristematic pads. The ontogenetic data has been used to characterize the tribes and distinguish among certain genera in Malvaceae as well as to delineate the family. A cladistic analysis utilizing ontogenetic floral data and morphological data shows that Malvavisceae, Gossypieae, and Malveae are monophyletic, but Hibisceae is paraphyletic. Several evolutionary trends in character traits are revealed and some clarification is provided about phylogenetic relationships within the Malvaceae.