Description
Amborella is a sprawling shrub or small tree up to eight meters high. It bears alternate or decussate, simple evergreen leaves without stipules. The leaves are two ranked, with distinctly serrated or rippled margins, and about 8 to 10 cm long.
The species is dioecious: each flower produces both stamens and carpels, but only one sex develops fully and is fertile in the flowers of an individual plant. The small, creamy white, inconspicuous flowers are arranged in terminal panicles of 2 to 30 flowers borne in the axils of foliage leaves. Each flower is subtended by bracts and has a perianth of tepals (undifferentiated sepals and petals) arranged in a spiral or possibly whorled at the periphery. These features suggest that, as with other basal angiosperms, there is a high degree of developmental plasticity. The bracts gradually transition into tepals, making it difficult to determine where a flower actually begins.
Carpellate flowers are approximately 3 to 4 mm in diameter, with 7 or 8 tepals. There are 1 to 3 (or rarely none) well differentiated staminodes and a spiral of 4 to 8 free carpels (apocarpous). Carpels have green ovaries and lack a style. They contain a single ovule with the micropyle directed downwards.
Staminate flowers are approximately 4 to 5 mm in diameter, with 6 to 15 tepals. These flowers bear 10 to 21 spirally arranged stamens, which become progressively smaller toward the center. The innermost may be sterile. Filaments are represented by a short broad stalk. Anthers are triangular and consist of four pollen sacs, two on each side, with a small sterile central connective. Stamens have papillate secretory tips. A small pyramidal structure in the center of the flower may represent a vestigial gynoecium.
Typically, 1 to 3 carpels develop into fruit per flower. The fruit is an ovoid red drupe (approximately 5 to 7 mm long and 5 mm wide) borne on a short (1 to 2 mm) stalk. The remains of the stigma can be seen at the tip of the fruit. The skin is papery, surrounding a thin fleshy layer containing a red juice. The inner pericarp is lignified and surrounds the single seed. The embryo is small and surrounded by copious endosperm.
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