Acacia Willd. (1805), in the family Leguminosae subfamily Mimosoideae, contains around 1200 species mainly in the tropics and subtropics in Australia and Africa.
Description Trees to 3m, or shrubs, often with spiny twigs. The leaves are alternate, evergreen or deciduous, pinnate, or reduced to variously shaped phyllodes. The flowers are small, in spherical or elongated bunches, spikes, or few-flowered heads on a long axis, fluffy with long stamens. Sepals 4 or 5, joined at the base to form a cup; petals 4 or 5, very small, equal. Stamens numerous, not joined at the base, longer than the petals. Ovary superior, with 1 carpel containing several ovules; style 1. Pollination is by insects. The fruits are legumes, often interestingly contorted, with several seeds.
Key Recognition Features The fluffy flowerheads are typical of Acacia and all members of the subfamily Mimosoideae.
Evolution and Relationships The pod or legume is typical of all the family Leguminosae, which is often considered to be related to, and more advanced than, the Rosaceae, and parallel to the Myrtaceae and related families. Both Myrtaceae and Leguminosae are particularly diverse in Australia.
Ecology and Geography Acacia species are most common in seasonally dry areas, particularly in Australia and tropical Africa. The temperate species from the colder parts of Australia and Tasmania are commonly planted in Europe, California, South Africa, and other mild climates. Without their natural insect predators, they have often become serious pests in these areas.
Comment Many species of Acacia are grown as ornamentals, especially A. dealbata Link, which is grown in France and sold by florists as mimosa. Acacia farnesiana (L.) Willd. from Dominica is grown in Europe for its scent. Many species, and especially A. saligna (Labill.) H.L. Wendl., are valuable for stabilising sandy soils in dry areas. The most cold-tolerant species are probably A. pravissima F. Muell. from New South Wales and Victoria, and the hardier forms of A. dealbata from New South Wales, Victoria, and Tasmania.