Acacia sensu lato

English botanist and gardener Philip Miller adopted the name Acacia in 1754. The generic name is derived from ἀκακία (akakia), the name given by early Greek botanist-physician Pedanius Dioscorides (middle to late first century) to the medicinal tree A. nilotica in his book Materia Medica.[4] This name derives from the Ancient Greek word for its characteristic thorns, ἀκίς (akis; "thorn").[5] The species name nilotica was given by Linnaeus from this tree's best-known range along the Nile river. This became the type species of the genus.

The traditional circumscription of Acacia eventually contained approximately 1,300 species. However, evidence began to accumulate that the genus as described was not monophyletic. Queensland botanist Les Pedley proposed the subgenus Phyllodineae be renamed Racosperma and published the binomial names.[6][7] This was taken up in New Zealand but generally not followed in Australia, where botanists declared more study was needed.

Eventually, consensus emerged that Acacia needed to be split as it was not monophyletic. This led to Australian botanists Bruce Maslin and Tony Orchard pushing for the retypification of the genus with an Australian species instead of the original African type species, an exception to traditional rules of priority that required ratification by the International Botanical Congress.[8] That decision has been controversial,[3][9] and debate continued, with some taxonomists (and many other biologists) deciding to continue to use the traditional Acacia sensu lato circumscription of the genus,[8] in defiance of decisions by an International Botanical Congress.[10][11] However, a second International Botanical Congress has now confirmed the decision to apply the name Acacia to the mostly Australian plants, which some had been calling Racosperma, and which had formed the overwhelming majority of Acacia sensu lato.[12][13][14] Debate continues regarding the traditional acacias of Africa, possibly placed in Senegalia and Vachellia, and some of the American species, possibly placed in Acaciella and Mariosousa.

Acacias belong to the subfamily Mimosoideae, the major clades of which may have formed in response to drying trends and fire regimes that accompanied increased seasonality during the late Oligocene to early Miocene (~25 mya).[15] Pedley (1978), following Vassal (1972), viewed Acacia as comprising three large subgenera, but subsequently (1986) raised the rank of these groups to genera Acacia, Senegalia (s.l.) and Racosperma,[6][7] which was underpinned by later genetic studies.

In common parlance, the term "acacia" is occasionally applied to species of the genus Robinia, which also belongs in the pea family. Robinia pseudoacacia, an American species locally known as black locust, is sometimes called "false acacia" in cultivation in the United Kingdom and throughout Europe.

Acacia fasciculifera seedling in the transitional stage between pinnate leaves and phyllodes

The leaves of acacias are compound pinnate in general. In some species, however, more especially in the Australian and Pacific Islands species, the leaflets are suppressed, and the leaf-stalks (petioles) become vertically flattened in order to serve the purpose of leaves. These are known as "phyllodes". The vertical orientation of the phyllodes protects them from intense sunlight since with their edges towards the sky and earth they do not intercept light as fully as horizontally placed leaves. A few species (such as Acacia glaucoptera) lack leaves or phyllodes altogether but instead possess cladodes, modified leaf-like photosynthetic stems functioning as leaves.

The small flowers have five very small petals, almost hidden by the long stamens, and are arranged in dense, globular or cylindrical clusters; they are yellow or cream-colored in most species, whitish in some, or even purple (Acacia purpureopetala) or red (Acacia leprosa 'Scarlet Blaze'). Acacia flowers can be distinguished from those of a large related genus, Albizia, by their stamens, which are not joined at the base. Also, unlike individual Mimosa flowers, those of Acacia have more than ten stamens.[16]

The plants often bear spines, especially those species growing in arid regions. These sometimes represent branches that have become short, hard, and pungent, though they sometimes represent leaf-stipules. Acacia armata is the kangaroo-thorn of Australia, and Acacia erioloba (syn. Acacia eriolobata) is the camelthorn of Africa.

Acacia seeds can be difficult to germinate. Research has found that immersing the seeds in various temperatures (usually around 80 °C (176 °F)) and manual seed coat chipping can improve growth to around 80%.[17]

Acacia collinsii stipules

Swollen stipules of Acacia drepanolobium that serve as ant domatia. An entry hole can be seen at the base of one of the spines of the largest domatia. From the MHNT

In the Central American bullthorn acacias—Acacia sphaerocephala, Acacia cornigera and Acacia collinsii — some of the spiny stipules are large, swollen and hollow. These afford shelter for several species of Pseudomyrmex ants, which feed on extrafloral nectaries on the leaf-stalk and small lipid-rich food-bodies at the tips of the leaflets called Beltian bodies. In return, the ants add protection to the plant against herbivores.[18] Some species of ants will also remove competing plants around the acacia, cutting off the offending plants' leaves with their jaws and ultimately killing them. Other associated ant species appear to do nothing to benefit their hosts.

Similar mutualisms with ants occur on Acacia trees in Africa, such as the whistling thorn acacia. The acacias provide shelter for ants in similar swollen stipules and nectar in extrafloral nectaries for their symbiotic ants, such as Crematogaster mimosae. In turn, the ants protect the plant by attacking large mammalian herbivores and stem-boring beetles that damage the plant.[19]

The predominantly herbivorous spider Bagheera kiplingi, which is found in Central America and Mexico, feeds on nubs at the tips of the acacia leaves, known as Beltian bodies, which contain high concentrations of protein. These nubs are produced by the acacia as part of a symbiotic relationship with certain species of ant, which also eat them.[20]

In Australia, Acacia species are sometimes used as food plants by the larvae of hepialid moths of the genus Aenetus including A. ligniveren. These burrow horizontally into the trunk then vertically down. Other Lepidoptera larvae which have been recorded feeding on Acacia include brown-tail, Endoclita malabaricus and turnip moth. The leaf-mining larvae of some bucculatricid moths also feed on Acacia; Bucculatrix agilis feeds exclusively on Acacia horrida and Bucculatrix flexuosa feeds exclusively on Acacia nilotica.

Acacias contain a number of organic compounds that defend them from pests and grazing animals.[21]

For the same reasons it is favored as an erosion-control plant, with its easy spreading and resilience, some varieties of acacia are potentially invasive species. At least fourteen Acacia species introduced to South Africa are categorized as invasive, due to their naturally aggressive propagation.[39] One of the most globally significant invasive acacias is black wattle A. mearnsii, which is taking over grasslands and abandoned agricultural areas worldwide, especially in moderate coastal and island regions where mild climate promotes its spread. Australian/New Zealand Weed Risk Assessment gives it a "high risk, score of 15" rating and it is considered one of the world's 100 most invasive species.[40] Extensive ecological studies should be performed before further introduction of acacia varieties, as this fast-growing genus, once introduced, spreads quickly and is extremely difficult to eradicate.

The Arbre du Ténéré in Niger was the most isolated tree in the world, about 400 km (249 mi) from any other tree. The tree was knocked down by a truck driver in 1973.[44]

In Nairobi, Kenya, the Thorn Tree Café is named after a Naivasha thorn tree (Acacia xanthophloea)[45] in its centre. Travelers used to pin notes to others to the thorns of the tree. The current tree is the third of the same variety.

Wikispecies has information related to Acacia.

• World Wide Wattle
• Acacia-world
• Wayne's Word on "The Unforgettable Acacias"
• The genus Acacia and Entheogenic Tryptamines, with reference to Australian and related species, by mulga
• A description of Acacia from Pomet's 1709 reference book, History of Druggs Archived 2010-01-11 at the Wayback Machine
• Dr. Duke's Phytochemical and Ethnobotanical Databases
• Flora identification tools from the State Herbarium of South Australia
• Tannins in Some Interrelated Wattles
• List of Acacia Species in the U.S.
• FAO Timber Properties of Various Acacia Species
• FAO Comparison of Various Acacia Species as Forage
• Vet. Path. ResultsAFIP Wednesday Slide Conference – No. 21 February 24, 1999
• Acacia cyanophylla lindl as supplementary feed/for small stock in Libya
• Description of Acacia Morphology
• Nitrogen Fixation in Acacias
• Acacias with Cyagenic Compounds
• Acacia Alarm System
• Acacia in West African plants – A Photo Guide.
• "Acacia" . Encyclopædia Britannica (11th ed.). 1911.