Acacia

Vachellia is a genus of flowering plants in the legume family, Fabaceae, commonly known as thorn trees or acacias. It belongs to the subfamily Mimosoideae. Its species were considered members of genus Acacia until 2009.^[2][3] Vachellia can be distinguished from other acacias by its capitate inflorescences and spinescent stipules.^[4] Before discovery of the New World, Europeans in the Mediterranean region were familiar with several species of Vachellia, which they knew as sources of medicine, and had names for them that they inherited from the Greeks and Romans.^[5]

The wide-ranging genus occurs in a variety of open, tropical to subtropical habitats, and is locally dominant.^[6] In parts of Africa, Vachellia species are shaped progressively by grazing animals of increasing size and height, such as gazelle, gerenuk, and giraffe. The genus in Africa has thus developed thorns in defence against such herbivory.^[7]

Nomenclature

By 2005, taxonomists had decided that Acacia sensu lato should be split into at least five separate genera. The ICN dictated that under these circumstances, the name of Acacia should remain with the original type, which was Acacia nilotica.^[1] However, that year the General Committee of the IBC decided that Acacia should be given a new type (Acacia verticillatum) so that the ~920 species of Australian acacias would not need to be renamed Racosperma. This decision was opposed by 54.9% or 247 representatives at its 2005 congress, while 45.1% or 203 votes were cast in favor. However, since a 60% vote was required to override the committee, the decision was carried, and a nom. cons. propositum was listed in Appendix III (p. 286).^[8][9] The 2011 congress voted 373 to 172 to uphold the 2005 decision, which means that the name Acacia and a new type follow the majority of the species in Acacia sensu lato, rather than this genus.^[10] However, some members of the botanical community remain unconvinced,^[11] and the use of Acacia in the scientific literature continues to exceed the use of the new generic names.

Description

The members of Vachellia are trees or shrubs, sometimes climbing, and are always armed. Younger plants, especially, are armed with spines which are modified stipules, situated near the leaf bases. Some (cf. V. tortilis, Vachellia hebeclada, V. luederitzii and V. reficiens) are also armed with paired, recurved prickles (in addition to the spines).^[12] The leaves are alternate and bipinnately arranged, and their pinnae are usually opposite. The racemose inflorescences usually grow from the leaf axils. The yellow or creamy white flowers are produced in spherical heads, or seldom in elongate spikes, which is the general rule in the related genus Senegalia. The flowers are typically bisexual with numerous stamens, but unisexual flowers have been noted in V. nilotica (cf. Sinha, 1971).^[13] The calyx and corolla are usually 4 to 5-lobed. Glands are usually present on the rachis and the upper side of the petiole. The seed pod may be straight, curved or curled, and either dehiscent or indehiscent.^[12]

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  V. vernicosa, or "viscid acacia", in habitat, Far West Texas

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  V. vernicosa, detail of inflorescence

Species list

Of the 163 species currently assigned to Vachellia, 52 are native to the Americas, 83 to Africa, Madagascar and the Mascarene Islands, 32 to Asia and 9 to Australia and the Pacific Islands.^[14] Vachellia comprises the following species:^{[15][2][16][17][3][18][19][20][21]}

Incertae sedis

These species are suspected to belong to Vachellia, but have not been formally transferred.^[18]

• Acacia callicoma Meisn.
• Acacia harala Thulin & Gifri
• Acacia hunteri Oliv.
• Acacia johnwoodii Boulos
• Acacia planifrons Koenig ex Wight & Arn.
• Acacia pseudo-eburnea J.R. Drumm. ex Dunn
• Acacia tanjorensis Ragup., Thoth. & Mahad.
• Acacia yemenensis Boulos

Hybrids

• Vachellia × cedilloi (Rico Arce) Seigler & Ebinger
• Vachellia campechiana × pennatula
• Vachellia erioloba × haematoxylon
• Vachellia × gladiata (Saff.) Seigler & Ebinger
• Vachellia kirkii × seyal
• Vachellia macracantha × pennatula
• Vachellia seyal var. fistula × xanthophloea
• Vachellia × standleyi (Saff.) Seigler & Ebinger

References