Family Amborellaceae

Amborellaceae Pichon.

Habit and leaf form. ‘Arborescent’, or shrubs; without essential oils (or nearly so). Leaves evergreen; alternate; spiral to distichous; simple. Lamina dissected to entire; when dissected, pinnatifid (lobed); pinnately veined. Leaves exstipulate.

Leaf anatomy. Stomata present; anomocytic, or paracytic.

The mesophyll without etherial oil cells.

Stem anatomy. Nodes unilacunar (with one broad trace). Internal phloem absent. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem presumably with tracheids; without vessels. Primary medullary rays narrow. Sieve-tube plastids S-type.

Reproductive type, pollination. Plants dioecious.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’. The ultimate inflorescence unit cymose. Inflorescences axillary. Flowers partially acyclic. The perianth acyclic. Floral receptacle markedly hollowed (more or less, in female flowers), or not markedly hollowed (slightly convex, in male flowers). Free hypanthium present.

Perianth sequentially intergrading from sepals to petals; 5–8; weakly joined (basally); spiralled.

Androecium in male flowers 30–100 (‘more or less numerous’). Androecial members maturing centripetally (?); of the outer cycle adnate (basally, to the tepals); free of one another; 3–5 whorled (‘in several cycles’). Androecium (male flowers) exclusively of fertile stamens. Stamens 30–100 (‘more or less numerous’); more or less laminar. Anthers adnate (with adaxial thecae); non-versatile; dehiscing via longitudinal slits; introrse. Pollen grains aperturate to nonaperturate; if detectably aperturate, obscurely 1 aperturate (or with an irregular, distal, unthickened zone in the exine); sulcate.

Gynoecium 5–8 carpelled; apocarpous; eu-apocarpous (the carpels in a single whorl); superior. Carpel incompletely closed (unsealed at the tip); with a sessile stigma having two expanded flanges; 1 ovuled. Placentation marginal. Ovary stipitate. Ovules anatropous.

Fruit an aggregate. The fruiting carpel indehiscent; drupaceous (stipitate). Seeds endospermic. Embryo well differentiated (minute, basal). Cotyledons 2.

Physiology, biochemistry. Aluminium accumulation demonstrated.

Geography, cytology. Paleotropical. New Caledonia. N = 13 (2n = 26).

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Magnoliiflorae; Laurales. Cronquist’s Subclass Magnoliidae; Laurales. APG (1998) oddment family (but really ‘basal’?). APG 3 (2009) Order: Amborellales.

Species 1. Genera 1; Amborella trichopoda the only species.

The 1999 International Botanical Congress (St. Louis) candidate for ‘the most primitive Angiosperm’!.

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