Shrubs or trees [rarely vines], usually unarmed, stipular spines present on A. paradoxa. Stems erect to ascending or pendulous, glabrous or pubescent; twigs not flexuous or slightly so, terete to angled or ridged, short shoots usually absent. Leaves alternate (except fascicled or whorled in A. verticillata), even-bipinnate or phyllodic, leaves often modified (in age) to polymorphic phyllodes (enlarged, flattened petiole without leaflets), usually glandular on margins and/or apex, saplings often with even-pinnate juvenile leaves often not present at maturity; stipules usually present, usually early deciduous, rarely woody, spinose; petiolate, usually with 1 globose gland; pinnae [1 or] 2–31[–50] pairs, mostly opposite; leaflets 8–70 pairs per pinna, opposite, sessile or subsessile, blade margins entire, surfaces glabrous or pubescent. Peduncles usually not elongated in fruit, glabrous or pubescent. Inflorescences 20–200+-flowered, terminal or axillary, globose heads or cylindrical spikes, heads solitary, fascicled, or clustered, or arranged in pseudoracemes or pseudopanicles; bracts present. Flowers mimosoid; calyx cup-shaped, lobes 4 or 5, triangular, glabrous [pubescent]; corolla yellow to cream, cup-shaped, lobes 4 or 5, triangular, membranous, glabrous [pubescent]; stamens 20–150, rarely connate basally, usually exserted, mostly yellow to gold or creamy white; anthers dorsifixed, mostly eglandular; ovary sessile or short-stipitate; style and stigma filiform. Fruits legumes, erect to pendulous, stipitate, stipe usually relatively short, mostly flat, straight to falcate, linear to oblong, apex sometimes beaked, usually dehiscent along sutures, dry, papery to leathery, glabrous or pubescent. Seeds usually 6–10, usually flattened, ellipsoid to ovoid, uniseriate, usually with pulpy, bright-colored aril, forming a cap or encircling seed. x = 13.

Distribution

Introduced; Indian Ocean Islands, Pacific Islands (Kei Islands, New Guinea), Australia, introduced also in South America, introduced elsewhere in tropical and subtropical regions.

Discussion

Species ca. 1300 (15 in the flora).

Acacia species are indigenous mostly in the Southern Hemisphere, with more than 950 in Australia. None is native to the New World.

To preserve as much current usage as possible, at the 17th International Botanical Congress in 2005, the type of the genus Acacia was changed from Acacia scorpioides (Linnaeus) W. Wright to the Australian species Acacia penninervis Sieber ex de Candolle (B. R. Maslin 2008; J. McNeill and N. J. Turland 2010). Presently, there is considerable evidence that the broadly defined genus Acacia is not a natural or monophyletic group (Gill. K. Brown et al. 2008). Therefore, plants of the former Acacia subg. Phyllodineae remain in the genus Acacia (Maslin et al. 2003; Maslin 2008), and other species of the traditional Acacia are transferred to the genera Acaciella, Mariosousa, Parasenegalia Seigler & Ebinger, Pseudosenegalia Seigler & Ebinger, Senegalia, and Vachellia.

Members of Acacia enumerated here are introduced, exotic species proven to be adventive in the United States by vouchered collections; most are restricted to Arizona, southern California, and Florida. In addition, many Australian Acacia species are cultivated in botanical gardens and plant introduction centers and as ornamentals in the nursery trade. Although not clearly established to be adventive, other Acacia species may occasionally be found outside of cultivation; D. Isely (1973) mentioned a few of these, including A. podalyriifolia A. Cunningham ex G. Don, reported from a non-cultivated stand on Santa Catalina Island, California, and more recently from Orange County. Naturalized individuals of A. salicina Lindley and A. stenophylla A. Cunningham ex Bentham are known from Maricopa County, Arizona (ASU). Acacia iteaphylla F. Mueller ex Bentham is naturalized on the University of California-Riverside campus and is possibly invasive (Sanders & Morgan 21588, UCR).

Selected References

None.

Lower Taxa

Acacia auriculiformis, Acacia baileyana, Acacia cultriformis, Acacia cyclops, Acacia dealbata, Acacia decurrens, Acacia longifolia, Acacia mearnsii, Acacia melanoxylon, Acacia paradoxa, Acacia pycnantha, Acacia redolens, Acacia retinodes, Acacia saligna, Acacia verticillata

Key

1	Leaves compound.	> 2
2	Leaves with 2–5 pinna pairs; petioles 0–2 mm.	Acacia baileyana
2	Leaves with (3–)6–31 pinna pairs; petioles 5–33 mm.	> 3
3	Distance between pinna pairs 7–20 mm; leaflet blades 5–15 mm.	Acacia decurrens
3	Distance between pinna pairs 2–7 mm; leaflet blades 1.5–5 mm.	> 4
4	Rachis glands between most pinna pairs.	Acacia dealbata
4	Rachis glands scattered, at pinna pair nodes and internodes.	Acacia mearnsii
1	Leaves phyllodic.	> 5
5	Phyllodes 5–30 mm.	> 6
6	Stipular spines present.	Acacia paradoxa
6	Stipular spines absent.	> 7
7	Phyllodes inequilateral, ± triangular, 5–15 mm wide, alternate.	Acacia cultriformis
7	Phyllodes acicular, linear, 0.5–1.5 mm wide, fascicled or whorled.	Acacia verticillata
5	Phyllodes (20–)30–250 mm.	> 8
8	Phyllodes with pinnate venation.	> 9
9	Pulvinus 4–8 mm; phyllodes 10–35 mm wide.	Acacia pycnantha
9	Pulvinus 1–3.5 mm; phyllodes 3–14(–25) mm wide.	> 10
10	Globose heads 5–7 diam.; phyllode gland not obvious, not disciform.	Acacia retinodes
10	Globose heads 8–12 mm diam.; phyllode gland obvious, disciform.	Acacia saligna
8	Phyllodes with parallel venation.	> 11
11	Inflorescences cylindrical spikes.	> 12
12	Phyllodes falcate; flowers 5-merous.	Acacia auriculiformis
12	Phyllodes not falcate; flowers 4-merous.	Acacia longifolia
11	Inflorescences globose heads.	> 13
13	Twigs fragrant (vanilla-scented) when crushed, resin-ribbed.	Acacia redolens
13	Twigs not fragrant when crushed, not resinous.	> 14
14	Inflorescences pseudoracemes of 2 heads; minor phyllode veins weakly reticulate.	Acacia cyclops
14	Inflorescences pseudoracemes of 2–8 heads; minor phyllode veins prominently reticulate.	Acacia melanoxylon