Amborellaceae

Amborellaceae is a family of flowering plants native to New Caledonia. It consists of only one genus, Amborella, and one species, Amborella trichopoda. It is currently considered by plant systematists as the most basal lineage of angiosperms.

Description
The Amborellaceae are shrubs or small trees that have two-ranked leaves with no stipules and distinctly rippled or wavy margins. They are dioecious, and the flowers are small, grown in terminal cymose inflorescences, with a perianth of undifferentiated sepals and petals arranged in spirals, rather than whorls of the more derived angiosperms.

The Amborella has parts arranged in a spiral, in an indeterminate number (5-8 perianth parts), several stamens without a well-defined stalk or filament, and an indeterminate number of free carpels, making the plant apocarpous. The more derived families of angiosperms, known as the eudicots, usually have two distinct perianth whorls, the calyx and corolla, each with a well-defined number of parts, usually 4 to 5, stamens on the filaments, and compound ovaries with combined carpels, making them syncarpous.

Phylogeny
The family is currently accepted by most plant systematists as the most basal lineage within the angiosperms clade. By 'most basal', scientists mean that the Amborellaceae diverged the earliest than any other group of flowering plants. Comparing the characteristics that all the other angiosperms share with each other, but not with the Amborellaceae, may give scientists clues to the features early flowering plants had, and how these features evolved over time. An early 20th century idea was of the Amborellaceae being "primitive", or less derived, angiosperms was accepted until relatively recently was modeled on a Magnolia blossom with several parts arranged in spirals on an elongated recipracle, rather than having the small number of parts in whorls of more derived flowers. However, studies of a well-preserved possible fossil aquatic angiosperm, Archaefructus, have raised some questions to which characteristics are more ancestral.

In a study created to clarify relationships between the well-sequenced and well-studied model plants, such as the thale cress (Arabidopsis thaliana) and the basal angiosperms such as Amborella, Nuphar of the Nymphaeaceae, Illcium, the monocots, and the more derived angiosperms, known as the eudicots, scientists examined the chloroplast genomes and sequence tags of these organisms, and other seed plants to create the cladogram shown right. Note that in the image, the angiosperms are all the plants not labelled as gymnosperms. This hypothesized relationship of the extant seed plants places the Amborellaceae as a sister taxon to all the other angiosperms, and shows the gymnosperms as a monophyletic sister group to the angiosperms, which supports the theory that the Amborellaceae branched off earliest from all the other living angiosperms. The dashed line between Amborella and Nuphar is meant to indicate uncertainty about the relationship between the Amborellaceae and Nymphaeaceae, and whether or not they form a clade that is a sister to the angiosperms, rather than the Amborellaceae alone being a monophyletic sister group to the angiosperms.

Taxonomy
The APG II system recognized this family, but left it unranked at the order rank due to uncertainty between its relationship to the Nymphaeaceae. More recently, the Angiosperm Phylogeny Website now assigns it to its own order, the Amborellales.

Older systems
The Cronquist system of 1981 assigned the family
 * to the order Laurales,
 * in subclass Magnoliidae,
 * in class Magnoliopsida [=dicotyledons],
 * of division Magnoliophyta [=angiosperms].

The Thorne system of 1992 placed it
 * in the order Magnoliales,
 * in the superorder Magnolianae,
 * in subclass Magnoliideae [=dicotyledons],
 * of class Magnoliopsida [=angiosperms].

The Dahlgren system placed it
 * in the order Laurales, which was assigned to
 * superorder Magnolianae,
 * in subclass Magnoliideae [=dicotyledons],
 * of class Magnoliopsida [=angiosperms].