Acacia is a genus of flowering tropical and subtropical trees and shrubs. The individual flowers of Acacia species are small but usually occur in dense, globular heads or elongated spikes. The flowers are characteristically yellow, although sometimes white, and typically have five sepals and petals, many stamens, and a single pistil. The almost countless stamens give the flower clusters a soft, fuzzy, golden appearance and make the plant an attractive ornamental.
The leaves of acacias are either pinnate (existing as small leaflets along both sides of the leafstalk) or reduced to a leafstalk with no blades. In many of the Australian
and Pacific forms, the leafstalk and leaf axis are flat vertically, the leaflets are partially or completely absent, and the basal portion of the leafstalk develops into thorns.Species
Acacia belongs to the family Fabaceae, in the order Fabales, class Magnoliopsida. There are about 500 species, some 300 of which are found in Australia and on the islands of the Pacific. The rest are widely distributed in the tropics and warmer parts of the temperate zones. They are especially numerous in the bush veldt of South Africa. About 70 species exist in the Americas, nearly half of them native to Mexico. A few inhabit the drier areas of the central and southwestern United States.
Numerous species of Acacia are cultivated in greenhouses, with those in warmer areas being grown outdoors as ornamentals. The kangaroo thorn (Acacia paradoxa) of Australia is grown for Easter displays. This spreading shrub also serves as a hedge plant and as a plant cover for sand dunes. Leafstalks in this species are about 1 inch (2.5 cm) long and 0.25 inch (0.6 cm) wide. The yellow flower heads are about 0.25 inch in diameter.
Species cultivated in California include the Bailey acacia (A. baileyana), a shrub or small tree with exceedingly fine gray-green leaflets; the green, or tan, wattle (A. decurrens), a striking tree native to southeastern Australia and Tasmania; the sweet acacia (A. farnesiana), a shrub with richly scented, deep-yellow flower heads, widely grown in the tropics;
the golden wattle (A. pycnantha), an Australian tree with conspicuous leafstalks and abundant flower heads; and the pyramidal blackwood acacia (A. melanoxylon) of southeastern Australia, used as a street tree in California.Uses
The acacia known commonly as gum arabic (A. senegal), found in the country of Sudan and in the northern Sahara, is the main source of the compound gum arabic, which has been used for over 2,000 years in paints and watercolors. The substance is also used in confections, medicines, calico printing, and dyeing and in the making of silk, paper, and cosmetics.
The bark of many acacias is rich in tannin, a compound with a variety of commercial uses, including in the production of leather. The black wattle (A. mearnsii) and the golden wattle are particularly rich in tannin, which makes up almost half of the bark of these two species. Acacias also furnish good lumber. A. melanoxylon, for example, is used in the manufacture of furniture, cabinets, oars, tools, and gunstocks.
In the deserts of Asia and Africa, goats and camels browse on leaves and young shoots of acacias, while in Australia members of this genus serve as forage for cattle and sheep. In Africa, California, and elsewhere, certain species of Acacia have been used as binders for the shifting sands of the seacoasts. Some of the species provide food and shelter for insects, such as the stinging ant and the lac insect of India.
Certain plants are incorrectly called acacias. These include the plume albizzia (Albizzia distachya); the silk tree (Albizzia julibrissin); the three-thorned acacia, or honey locust (Gleditsia triacanthos); and the false acacia, or black locust (Robinia pseudoacacia).
Reff: Edwin Matzke “Acacia Life” 2010. Columbia University