Reproductive structures of the basal angiosperm Amborella trichopoda (Amborellaceae)

Amborella, the putatively basalmost extant angiosperm, is poorly known in its reproductive structures. This study offers new results. Inflorescences are botryoids or panicles with accessory flowers in the lower bracts. Flowers are small. Floral phyllotaxis is spiral throughout. The flower base forms a shallow cup that tears into irregular lobes when the flower opens and expands so that the inner floral organs are presented on a flat plate. The flowers are functionally unisexual but they have a bisexual organization, as the female flowers regularly have one or two staminodes outside of the gynoecium. Female flowers have fewer organs than male ones and are slightly smaller. Floral organ number is variable. Male flowers have nine to 11 tepals and 12 to 21 stamens; female flowers have seven to eight tepals, usually one to two staminodes, and usually five carpels. The stamens have a short filament, a triangular introrse anther with two bulging disporangiate thecae that dehisce longitudinally, and a small (secretory?) connective protrusion. The staminodes of the female flowers superficially look like stamens. However, the connective protrusion is more or less lacking; sporogenous tissue is differentiated but meiosis does not take place. The carpels are pronouncedly ascidiate and shortly stipitate and have a wet capitate stigma with multicellular protrusions. The surfaces of the inner canal between the stigma and the ovary are appressed to each other but apparently not postgenitally fused, though they are lined with secretion. The single ventral median ovule is bitegmic, crassinucellar, pendant, and orthotropous. The micropyle is formed by the inner integument. The floral vascular system is simple; each organ has a single strand. The carpels develop into drupelets. The pericarp has a thin fleshy outer layer and a pitted sclerified inner layer. The drupelets are scentless and almost tasteless. A tentative list of plesiomorphic floral features is given.