ENDRESS, PETER K.* AND ANTON IGERSHEIM. Institute of Systematic Botany, University of Zurich, Zurich, Switzerland. - Floral structure of Amborella, the earliest branching extant angiosperm.
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