Amborella

Amborella is a sprawling shrub or small tree up to high. It bears alternate, simple evergreen leaves without stipules. Each flower is subtended by bracts. The bracts transition into a perianth of undifferentiated tepals. The tepals typically are arranged in a spiral, but sometimes are whorled at the periphery. Carpellate flowers are roughly in diameter, with 7 or 8 tepals. There are 1 to 3 (or rarely 0) well-differentiated staminodes and a spiral of 4 to 8 free (apocarpous) carpels. Carpels bear green ovaries; they 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, amounting to staminodes. The stamens bear triangular anthers on short broad filaments. An anther consists of four pollen sacs, two on each side, with a small sterile central connective. The anthers have connective tips with small bumps and may be covered with secretions. These features suggest that, as with other basal angiosperms, there is a high degree of developmental plasticity. Typically, 1 to 3 carpels per flower develop into fruit. 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. Amborella has a mixed pollination system, relying on both insect pollinators and wind. ==Taxonomy==

History The Cronquist system, of 1981, classified the family: : Order Laurales :: Subclass Magnoliidae ::: Class Magnoliopsida [=dicotyledons] :::: Division Magnoliophyta [=angiosperms] The Thorne system (1992) classified it: : Order Magnoliales :: Superorder Magnolianae ::: Subclass Magnoliideae [=dicotyledons] :::: Class Magnoliopsida [=angiosperms] The Dahlgren system, of 1980, classified it: : Order Laurales :: Superorder Magnolianae ::: Subclass Magnoliideae [=dicotyledons], :::: Class Magnoliopsida [=angiosperms]. Modern classification Amborella is the only genus in the family Amborellaceae. The APG II system recognized this family, but left it unplaced at order rank due to uncertainty about its relationship to the family Nymphaeaceae. In the more recent APG systems, APG III and APG IV, the Amborellaceae comprise the monotypic order Amborellales at the base of the angiosperm phylogeny. Genomic and evolutionary considerations Amborella is of great interest to plant systematists because molecular phylogenetic analyses consistently place it as the sister group to all other angiosperms. Further, the female gametophyte of Amborella contains eight cells, one cell more than the seven-celled Polygonum-type female gametophyte found in the majority of angiosperms. Amborella, being an understory plant in the wild, is commonly in intimate contact with shade- and moisture-dependent organisms such as algae, lichens and mosses. In those circumstances, some horizontal gene transfer between Amborella and such associated species is not surprising in principle, but the scale of such transfer has caused considerable surprise. Sequencing the Amborella mitochondrial genome revealed that for every gene of its own origin, it contains about six versions from the genomes of an assortment of the plants and algae growing with or upon it. The evolutionary and physiological significance of this is not as yet clear, nor in particular is it clear whether the horizontal gene transfer has anything to do with the apparent stability and conservatism of the species. ==Conservation==

The islands of New Caledonia are a biodiversity hot-spot, preserving many early diverging lineages of plants, of which Amborella is but one. This preservation has been ascribed to climate stability during and since the Tertiary (), stability that has permitted the continued survival of tropical forests on New Caledonia. In contrast, drought conditions dominated the Australian climate towards the end of the Tertiary. Current threats to biodiversity in New Caledonia include fires, mining, agriculture, invasion by introduced species, urbanization and global warming. The importance of conserving Amborella has been dramatically stated by Pillon: "The disappearance of Amborella trichopoda would imply the disappearance of a genus, a family and an entire order, as well as the only witness to at least 140 million years of evolutionary history." Conservation strategies targeted on relict species are recommended, both preserving a diversity of habitats in New Caledonia and ex situ conservation in cultivation. The IUCN conservation status is Least Concern (LC). ==References==