Comparative Morphology and Evolution of Podostemaceae Flower

Abstract

Podostemaceae are aquatic angiosperms adapted to amphibious habitats in rivers. The flowers are produced when submerged in fast currents and unfold after exposure to the air. The floral morphology varies and provides diagnostic characters for recognizing three subfamilies. The flowers of subfamily Podostemoideae are characterized by a sac-like spathella enveloping flower buds and filiform ‘tepals’ that alternate with or are lateral to the stamens. In Tristichoideae and Weddellinoideae, the flowers are devoid of a spathella and the tepals are blade-like. To characterize these enigmatic floral organs, morphological, developmental, phylogenetic and genetic data were evaluated and floral development was examined in a few species of Podostemoideae and Weddellinoideae. All developmental, morphological and genetic data indicate that the spathella is a transformed tepal and what has been referred to as tepals are staminodes. Accordingly, the stalk above the spathella is an androgynophore. It uplifts the ‘tepals’ (= staminodes), stamens and pistils, while the ruptured spathella remains at the base. The spathella of Podostemoideae separates the protective and reproductive phases more distinctly than do the tepals in the other subfamilies. This flowering mode may be adapted to the habitat.