Comparative floral ontogeny of Hydrobryum loeicum, Hy. tardhuangense and Hanseniella heterophylla (Podostemaceae)

Abstract

In order to elucidate the floral ontogeny of certain Thai species of the subfamily Podostemoideae, the flowering shoots of Hanseniella heterophy/la, Hydrobryum loeicum and Hy. tardhuangense were observed using scanning electron microscope. In all species studied. the flowering shoots protrude from the cavities where the tufted leaves arise on the crustose root. Mostly, the two outer bracts are leaf-like interpreting that the flowering shoots are converted from the vegetative shoots. In Hanseniel/a, bracts in the early developmental stage are similar shapes but different sizes arranging in 4 rows. In the mid-stage, the ventral bracts differentiate into normally 2 lobes, occasionally 3 lobes whereas the dorsal bracts develop into long deltoid shape. In Hydrobryum, the arrangement of bracts is distichous. The spathellas in all species initiate from tissue surrounding the floral meristem. Prior to the fusion of ventral and dorsal apical margins, the spathella apices become a triangle space in Hanseniella, narrow long space in Hy. loeicum and two-lipped in Hy. tardhuangense; and the floral organs are initiated inside the spathella. The two tepal primordia, the primary androecial primordium and the gynoecial primordium are initiated simultaneously. The two stamen primordia are differentiated from the primary androcial primordium when the gynoecium changes into a cylindrical shape and two carpel primordia are initiated. These dramatic ontogenetic changes and heterochronic shifts in the initiation of floral organs are highly specialized and very distinct floral development which have not found in any families of flowering plants.