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Acacia karroo - spines

Acacia karroo - leaves and spines

Acacia karroo - habit

Acacia karroo - enlarged spines

Name

Acacia karroo Hayne

Formal description

Shrub or small tree 12 m tall.  Bark reddish brown to dark brown to black, rough to smooth.  Twigs dark reddish brown to dark gray, not flexuous, glabrous.  Short shoots commonly present above the stipular spines, to 2 mm long, covered with acuminate stipules and old leaf bases, sometimes absent.  Leaves alternate, sometimes a few clustered together at a node, 20-60 mm long.  Stipular spines light brown to light gray, symmetrical, terete, straight, stout, to 100 x 3.5 mm near the base (sometimes fusiform-inflated and up to 10 mm wide and 200 mm long), glabrous.  Petiole adaxially grooved, 5-20 mm long, glabrous; petiolar gland solitary, usually located near the middle of the petiole to just below the first pinna pair, sessile to short stalked, circular to elongated, 0.8-3.5 mm long, apex flattened, glabrous, sometimes absent.  Rachis adaxially grooved, 0-60 mm long, glabrous, a sessile, circular gland 0.2-1.0 mm across between some or all of the pinna pairs.  Pinnae 1 to 7 pairs per leaf, 20-55 mm long, 4-15 mm between pinna pairs.  Petiolules 1.3-4.5 mm long.  Leaflets (8)11 to 26 pairs per pinna, opposite, 1.1-2.9 mm between leaflets, oblong to obovate, 4.0-9.0 x 1.2-2.5 mm, glabrous, lateral veins not obvious, only one vein from the base, base oblique, margins not ciliate, apex broadly acute to obtuse.  Inflorescence a densely flowered globose head, 7-12 mm across, solitary or in clusters of 2 to 10 at the nodes, or in elongated leafless, terminal, racemose clusters on an elongated axis.  Peduncles 10-30 x 0.4-0.8 mm, glabrous.  Involucre 4- to 5- lobed or unlobed, usually located near the middle of the peduncle, glabrous, persistent, sometimes a few flowers in the axis.  Floral bracts spatulate, 1.2-1.8 mm long, puberulent, deciduous.  Flowers sessile, bright yellow; calyx 5-lobed, 1.3-1.9 mm long, glabrous or nearly so; corolla 5-lobed, 2.0-3.2 mm long, glabrous or nearly so; stamen filaments 4.0-6.0 mm long distinct ovary glabrous, on a stipe to 0.2 mm long.  Legumes dark brown to black, curved, flattened, slightly constricted between the seeds, oblong, 40-155 x 5-9 mm, coriaceous, reticulately striate, glabrous, eglandular, dehiscent, a chartaceous pericarpic strip lining each valve; stipe to 10 mm long; apex acuminate and usually beaked.  Seeds uniseriate, no pulp, light brown to light greenish brown, oblong, flattened, 6.3-9.4 x 3.5-5.5 mm, smooth; pleurogram U-shaped, 2.5-3.5 mm across. Chromosome number: Not determined.

Distribution

Rarely planted and sometimes escapes in disturbed habitats in the New World tropics and subtropics.  A common species of southern Africa that occupies a diverse range of habitats.

Additional info

Acacia karroo Hayne is a highly variable, polymorphic taxon in which formal intraspecific categories are not easily recognized (Ross 1979).  This species differs from all native South American acacias by having the involucral bracts near the middle of the peduncle and legumes that have an pericarpic strip inside each valve, characteristics found in members of the Vachellia rigidula species group of North America.  The description presented here is based largely on the New World specimens examined.

Representative specimens

CUBA: 

UNITED STATES: 

California: 

Orange Co.: 

BOLIVIA: 

Cochabamba: 

CHILE: 

Santiago: 

Tarapaca: 

PERU: 

Lima: 

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