acacia

''Acacia'', commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australasia. The genus name is New Latin, borrowed from the Greek (), a term used by Dioscorides for a preparation extracted from the leaves and fruit pods of '' Vachellia nilotica'', the original type of the genus. In his ''Pinax'' (1623), Gaspard Bauhin mentioned the Greek from Dioscorides as the origin of the Latin name.

In the early 2000s it had become evident that the genus as it stood was not monophyletic and that several divergent lineages needed to be placed in separate genera. It turned out that one lineage comprising over 900 species mainly native to Australia, New Guinea, and Indonesia was not closely related to the much smaller group of African lineage that contained ''A. nilotica''—the type species. This meant that the Australasian lineage (by far the most prolific in number of species) would need to be renamed. Botanist Leslie Pedley named this group ''Racosperma'', which received little acclaim in the botanical community.
Australian botanists proposed a less disruptive solution setting a different type species for ''Acacia'' ('' A. penninervis'') and allowing this largest number of species to remain in ''Acacia'', resulting in the two Pan-Tropical lineages being renamed '' Vachellia'' and '' Senegalia'', and the two endemic American lineages renamed '' Acaciella'' and '' Mariosousa''. Although many botanists still disagreed that this was necessary, this solution was eventually officially adopted at the Melbourne International Botanical Congress in 2011.

Acacia remains a widely used common name across genera.

A number of species have been introduced to various parts of the world, and two million hectares of commercial plantations have been established. The heterogeneous group varies considerably in habit, from mat-like subshrubs to canopy trees in a forest.

Description

Several species in the genus bear vertically oriented phyllodes, which are green, broadened leaf petioles that function like leaf blades, an adaptation to hot climates and droughts. Some phyllodinous species have a colourful aril on the seed. A few species have cladodes rather than leaves.

Taxonomy

The genus was first validly named in 1754 by Philip Miller. In 1913 Nathaniel Lord Britton and Addison Brown selected ''Mimosa scorpioides'' (≡ ''Acacia scorpioides'' () = '' Acacia nilotica'' () ), a species from Africa, as the lectotype of the name. The genus as recognized in 1986 contained 1352 species. That year however, Pedley published a paper in which he questioned the monophyletic nature of the genus, and proposed a split into three genera: ''Acacia'' sensu stricto (161 species), ''Senegalia'' (231 species) and ''Racosperma'' (960 species), the last name first proposed in 1829 by Carl Friedrich Philipp von Martius as the name of a section in ''Acacia'', but raised to generic rank in 1835. In 2003, Pedley published a paper with 834 new combinations in ''Racosperma'' for species, most of which were formerly placed in ''Acacia''. All but 10 of these species are native to Australasia, where it constitutes the largest plant genus.

In 2003, Anthony Orchard and Bruce Maslin filed a proposal to conserve the name ''Acacia'' with a different type in order to retain the Australasian group of species in the genus ''Acacia''. Following a controversial decision to choose a new type for ''Acacia'' in 2005, the Australian component of ''Acacia s.l.'' now retains the name ''Acacia''. At the 2011 International Botanical Congress held in Melbourne, the decision to use the name ''Acacia'', rather than the proposed ''Racosperma'' for this genus, was upheld. Other '' Acacia s.l.'' taxa continue to be called ''Acacia'' by those who choose to consider the entire group as one genus.

Australian species of the genus ''Paraserianthes'' '' s.l.'' are deemed its closest relatives, particularly '' P. lophantha''. The nearest relatives of ''Acacia'' and ''Paraserianthes s.l.'' in turn include the Australian and South East Asian genera '' Archidendron'', '' Archidendropsis'', ''Pararchidendron'' and '' Wallaceodendron'', all of the tribe Ingeae.

Etymology

The origin of "wattle" may be an Old Teutonic word meaning "to weave". From around 700 CE, ' was used in Old English to refer to the interwoven branches and sticks which formed fences, walls and roofs. Since about 1810 it refers to the Australian legumes that provide these branches.

Species

One species of ''Acacia'' ('' sensu stricto'') is native to Madagascar, one to Reunion island, 12 to Asia, and the remaining species (over 900) are native to Australasia and the Pacific Islands. These species were all given combinations by Pedley when he erected the genus ''Racosperma'', hence ''Acacia pulchella'', for example, became ''Racosperma pulchellum''.
However, these were not upheld with the retypification of ''Acacia''.

Phylogeny

Acacias in Australia probably evolved their fire resistance about 20 million years ago when fossilised charcoal deposits show a large increase, indicating that fire was a factor even then. With no major mountain ranges or rivers to prevent their spread, the wattles began to spread all over the continent as it dried and fires became more common. They began to form dry, open forests with species of the genera '' Allocasuarina'', ''Eucalyptus'' and '' Callitris'' (cypress-pines).

The southernmost species in the genus are '' Acacia dealbata'' (silver wattle), '' Acacia longifolia'' (coast wattle or Sydney golden wattle), '' Acacia mearnsii'' (black wattle), and '' Acacia melanoxylon'' (blackwood), reaching 43°30' S in Tasmania, Australia.

Fossil record

An ''Acacia''-like long fossil seed pod has been described from the Eocene of the Paris Basin. ''Acacia''-like fossil pods under the name ''Leguminocarpon'' are known from late Oligocene deposits at different sites in Hungary. Seed pod fossils of †''Acacia parschlugiana'' and †''Acacia cyclosperma'' are known from Tertiary deposits in Switzerland. †''Acacia colchica'' has been described from the Miocene of West Georgia. Pliocene

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