In 1999 the monotypic genus Amborella was identified as the earliest branching extant angiosperm, based on multiple gene analyses by several research groups. Amborella flowers are small (c. 4 mm diameter), functionally unisexual but with a bisexual organization (in female flowers one or two sterile stamens are present outside of the gynoecium), with spiral phyllotaxis, with c. 8-13 inconspicuous tepals (more in male than in female flowers), with 12-22 stamens in male flowers, and mostly 5 carpels in female flowers. The flower base forms a flat cup, which tears irregularly when the flower opens. The stamens have a short filament and a triangular anther with four bulging pollen sacs forming two thecae that open by a longitudinal slit (not by valves), and a short connective protrusion. The carpels are pronouncedly ascidiate, and they have a secretory stigma with irregular pluricellular protuberances. The stigmas are contiguous in the early phase of anthesis, forming an extragynoecial compitum. Each carpel has a single, median, pendant, orthotropous, bitegmic, crassinucellar ovule. The ventral slit is not postgenitally fused but closed by secretion; however, the inner surfaces are contiguous and form a narrow slit. For an evolutionary evaluation it will be important to compare the floral structure of Amborella with that of other extant early-branching angiosperm lineages that recently have been identified.

Key words: Amborella, basal angiosperms, flower structure, magnoliids