Acacia angustissima (Mill.) Ktze. - Prairie Acacia

Acacia angustissima plant

Family - Fabaceae

Stems - Multiple from a woody crown, to 1m tall, red in strong sunlight, carinate, stout, erect, branching, herbaceous to suffrutescent, with pustulate-based hairs (the base of the hairs giving the stems a scabrous feel).

Acacia angustissima stem

Leaves - Alternate, petiolate, stipulate, bipinnate (even), to +15cm long. Petioles to 3cm long, lacking glands. Expanded portion at the base of the petiole to 3mm long. Petioles and rachis with shallow adaxial groove, pubescent as the stem. Stipules thin and dry, ferruginous, ~3mm long, ~.5mm wide, with ciliate margins, other wise glabrous. Pinnae opposite to sub-opposite, +/-11 per side. Petiolules of pinnae -1mm long, antrorse pubescent. Leaf divisions with many pinnules (+/-50 per margin). Pinnules -4mm long, -1.5mm broad, acute, entire, oblique at the base, dull green above, light green below with a dark green midrib, with a few sparse hairs at the margins.

Acacia angustissima leaf

Acacia angustissima pinnulesPinnules.

Inflorescence - Pedunculate, axillary globose clusters mostly in the apical half of the stem. Peduncles light green, sparse antrorse pubescent, carinate, +/-2cm long. Pedicels light green, to 1mm long. Flowers +/-25 per head.

Acacia angustissima inflorescence

Flowers - Petals 5, green, erect, united at the very base, glabrous, acute, to -3mm long, 1mm broad, oblong elliptic. Calyx greatly reduced, .7mm long with very shallow lobes. Stamens very numerous (+100), well exserted from the perianth. Filaments white, glabrous, 5-8mm long, capillary. Anthers pale yellow, .1mm broad. Ovary superior, green, 1-1.2mm long in flower, .5mm in diameter, cylindric, subtended by a yellow ring-like nectary. Stipe .5-.7mm long. Style 1, as long as the filaments, light green, 3-4mm long. Fruits not seen.

Acacia angustissima flower

Flowering - June - October.

Habitat - Rocky glades, open hillsides, exposed ledges along bluffs.

Origin - Native to U.S., Central and South America, Australia, Africa.

Other info. - This species can be found in the southwest corner of Missouri. The plant reaches the northeastern edge of its range in Missouri. It is much more common in the southwestern U.S.
The plant can be identified by its bipinnate leaves and globose clusters of flowers. Another species, Desmanthus illinoensis (Michx.) MacM., is similar but has glands on its leaf petioles and fewer stamens per flower (5). This latter species is weedy in Missouri and can be found throughout the state.

Photographs taken at Busiek State Forest, MO., 6-17-05.


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