7. Acaciella Britton & Rose (acacia)

About 15 species, North America to South America, most diverse in Mexico.

As traditionally circumscribed, Acacia Mill. is the largest genus in the subfamily Mimosoideae, comprising about 1,350 species widely distributed in the tropics and warm-temperate regions of the world, and divisible into three large subgenera. Beginning in the 1960s, systematists studying variation in overall morphology, pollen morphology, and phytochemistry began making a case for separation of three or more groups from within Acacia as separate genera. These proposals were not accepted widely by the botanical community. With the advent of molecular studies on the mimosoid legumes, it became clear that these three large groups were more closely related to other genera of Mimosoideae than to each other. However, because of the complexity of the relationships involved and the inadequate number of species in this massive group sampled for molecular work to date, details of how these lineages should be classified remain unresolved (Miller and Bayer, 2003). Luckow et al. (2003) and Maslin et al. (2003) reviewed the studies published to that time that provided evidence for the breakup of Acacia, and they advocated the recognition of five to seven total genera, with four of these containing species native to the New World.

With the growing concern that three or more genera would be recognized in place of Acacia in the broad sense, botanists began to discuss which of the major lineages should retain the name Acacia and which should receive other names. This is of more than taxonomic importance, as numerous species in the overall group are cultivated around the world as ornamentals, for timber, and for various other uses. The thought that, regardless of which group remained under the well-known name Acacia, a large number of species would have to be renamed under other, more obscure generic epithets caused lengthy and heated nomenclatural discussions internationally. In the strict sense, the name Acacia was first typified to represent a group of about 160 species of nearly worldwide occurrence. However, Orchard and Maslin (2003) formally proposed that the name Acacia be conserved with the type species of A. penninervis Sieber ex DC., which belongs to a mostly Australian group traditionally known as Acacia subgenus Phyllodineae (DC.) Ser. They argued that because this represents by far the largest discrete assemblage (about 960 species) within the overall group, fixing the name Acacia on it would result in the fewest required transfers of species to other genera when Acacia is dismembered into more natural, smaller genera. After committees under the auspices of the International Association of Plant Taxonomists reviewed the proposal and gave it their tentative blessing, it received lengthy discussion during the nomenclatural session at the most recent International Botanical Congress, held in 2005 in Vienna, Austria. There the delegates voted to formally approve the proposal. The result of this nomenclatural conservation is that the generic name Acacia becomes associated with a group of about 960 mostly Australian species formerly called subgenus Phyllodineae (DC.) Ser., including most of the species widely cultivated in warmer parts of the world.

The studies of Luckow et al. (2003), Maslin et al. (2003), and Miller and Bayer (2003) supported the transfer of the A. angustissima complex, traditionally called Acacia subgenus Aculeiferum Vassal section Filicinae Vassal, to the genus Acaciella. However, aside from confirming that this group is distinct from other American acacias, none of these workers was able to provide strong evidence for a close relationship with any particular group within the mimosoid legumes. Further research will be necessary to resolve this issue. Acaciella is notable for the absence of spines, thorns, prickles, and rachis glands, the presence of dehiscent fruits, seeds lacking an endosperm and an aril, and details of pollen ultrastructure and morphology.