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Name

Vachellia biaciculata (S. Watson) Seigler & Ebinger, Phytologia 87:  145.  2005.
syn.  Acacia  biaciculata S. Watson, Proc. Amer. Acad. Arts  21:  452.  1886.

Synonymy and types

Basionym:  Acacia biaciculata S. Watson, Proc. Amer. Acad. Arts 21: 452.  1886.  Acaciopsis biaciculata (S. Watson) Britton & Rose, N. Amer. Fl.  23: 96.  1928. - TYPE:  MEXICO.  CHIHUAHUA:  on sandy plains near Chihuahua, 20 Aug 1885, C. G. Pringle 662 (holotype:  US; isotypes:  F, NY, VT).

Formal description

Prostrate shrub, branches to 1 m long, arising from a short, thick caudex.  Twigs light brown to gray, not flexuous, appressed pubescent to tomentose.  Short shoots absent.  Leaves alternate, 2-18 mm long.  Stipular spines light brown, usually becoming dull white with age, symmetrical, terete, straight, aciculate, to 15 (25) x 0.6 mm near the base, glabrous to sometimes lightly pubescent at the base.  Petiole adaxially shallowly grooved, 2.2-3.8 mm long, lightly puberulent to tomentose; petiolar gland solitary, located just below the lower pinna pair, sessile, circular to elliptic, 0.4-0.7 mm across, doughnut-shaped, glabrous.  Rachis adaxially shallowly grooved, 0-14 mm long, lightly puberulent to tomentose, usually lacking a gland between the pinna pairs.  Pinnae 1 to 3 (4) pairs per leaf, 6-11 mm long, 3-7 mm between pinna pairs.  Petiolules 0.5-1.1 mm long.  Leaflets 8 to 12 pairs per pinna, opposite to subopposite, 0.6-1.0 mm between leaflets, elliptic to oblong or oval, 1.9-3.2 x 0.9-1.4 mm, glabrous or nearly so, lateral veins not obvious, only one vein from the base, base oblique, margins usually ciliate, apex obtuse.  Inflorescence a densely flowered globose head, 7.0-8.8 mm across, solitary or in small clusters of 2 to 3 in the leaf axils.  Peduncles 10-30 x 0.4-0.6 mm, puberulent.  Involucre bracts 5 to 8, scattered near the middle of the peduncle, puberulent, early deciduous.  Floral bracts spatulate, 0.8-1.0 mm long, glabrous to lightly puberulent, deciduous.  Flowers sessile, pale yellow; calyx 5-lobed, 0.7-1.3 mm long, glabrous or nearly so; corolla 5-lobed, 1.8-2.3 mm long, glabrous or nearly so; stamen filaments 3-4 mm long, distinct; ovary glabrous, sessile or on a stipe to 0.2 mm long.  Legumes brown to reddish-brown, usually strongly curved, flattened, usually not constricted between the seeds, linear, 30-55 x 3.5-5.0 mm, chartaceous, weakly reticulately striate, appressed pubescent to tomentose, eglandular, indehiscent; a chartaceous pericarpic strip lining each valve; stipe to 5 mm long; apex acuminate and usually beaked.  Seeds uniseriate, no pulp, mottled light and dark brown, oblong to oval, strongly flattened, 5.0-6.3 x 3.0-4.2 mm, smooth; pleurogram oval to U-shaped, 0.5-0.9 mm across.  Flowers in May to August. Chromosome number:  Not determined.

Distribution

Arid, sandy plains between 1,300 and 2,300 m elevation in central Chihuahua, Mexico.  Collections from Jalisco and Querétaro are thought to represent recent introductions or waifs (Clarke et al. 1990).

Additional info

This species has a restricted range, being found on sandy plains in central Chihuahua.  It is not commonly collected, as indicated by the small number of herbarium specimens encountered.  Also, many collections were from near the turn of the century; recent collections were mostly from roadsides.  It is probable that much of the original habitat has been destroyed or altered, and the species is now relatively rare.  The spread of Vachellia species outside of their natural ranges is often associated with the transportation of livestock that ingest and pass the seeds. This is probably the reason for the collections of this species from Jalisco and Querétaro.

Vachellia biaciculata has no clear affinities to other groups of the genus Acacia.  The chartaceous pericarpic strip lining the fruit valves, the involucral bracts scattered near the middle of the peduncle, and the oblong, mottled seeds with a small pleurogram indicate a relationship to the V. constricta group. The characteristics that set this species apart from the rest of this group include the prostrate growth habit; the absence of short shoots at each node of the stem; aciculate spines; pubescent, short, falcate fruits; and the relatively large petiolar gland (usually exceeding 0.5 mm across). 

All specimens of Vachellia biaciculata examined were quite similar in most of their characteristics.  Some variation was found in the degree and extent of pubescence, most individuals being densely puberulent, though nearly glabrous individuals are sometimes found.  Also, variation occurred in the number of pinna pairs per leaf, varying from 2 to 4 on the same specimens, and, though the fruits are usually not constricted between the seeds, a few had strongly, but irregularly constricted fruits.  More than 40% of the specimens examined were cyanogenic, giving a weak reaction after linamarase was added (Clarke et al. 1990).

Flowering time

May - August.

Representative specimens

MEXICO: 

Chihuahua: 

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