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Vachellia pacencis - habit

Vachellia pacencis - inflorescence with pistils

Vachellia pacencis - habit

Vachellia pacencis - immature fruit

Vachellia pacencis - inflorescence

Vachellia pacencis - stipular spines

Vachellia pacencis - branch

Vachellia pacencis - leaf

Vachellia pacencis - petiolar gland

Name

Vachellia pacencis (Rudd & Carter) Seigler & Ebinger, Phytologia 87:  164.  2005.
syn.  Acacia pacensis Rudd & Carter, Madroño 30:  177.  1983.

Synonymy and types

Basionym:  Acacia pacensis Rudd & A. M. Carter, Madroño  30: 177.  1983. - TYPE:  MEXICO.  BAJA CALIFORNIA SUR:  La Paz, 1 Nov 1890, T. S. Brandegee 190 (holotype:  UC).

Formal description

Shrub to 2 m tall.  Bark reddish brown, smooth.  Twigs dark reddish to purplish brown, not flexuous, glabrous.  Short shoots commonly present above the stipular spines, to 8 mm long, covered with acuminate stipules and old leaf bases.  Leaves alternate, also commonly clustered on the short shoots, 20-55 mm long.  Stipular spines light reddish brown, becoming light gray with age, symmetrical, terete, straight, stout, to 17(35) x 1.6 mm near the base, glabrous to lightly pubescent toward the base.  Petiole adaxially grooved, 3-9 mm long, usually sparsely puberulent; petiolar gland solitary, located on the upper half of the petiole, stalked, circular, 0.1-0.3 mm across, apex depressed, glabrous, commonly absent.  Rachis adaxially grooved, 15-44 mm long, sparsely puberulent, a stalked, circular, very small gland between the terminal and occasionally other pinna pairs.  Pinnae 3 to 6(9) pairs per leaf, 4-10 mm long, 1-6 mm between pinna pairs.  Petiolules 0.6-1.3 mm long.  Leaflets 4 to 8 pairs per pinna, opposite, 0.4-1.2 mm between leaflets, oblong, 1.5-3.2 x 0.6-1.4 mm, glabrous, lateral veins not obvious, only one vein from the base, base oblique, margins occasionally ciliate, apex obtuse.  Inflorescence a densely flowered globose head, 6-8 mm across, solitary or in clusters of 2 to 4 on the short shoots.  Peduncles 9-12 x 0.4-0.6 mm, puberulent.  Involucre 4- to 5- lobed, located at the base of the globose head, lightly puberulent, persistent.  Floral bracts spatulate, 0.8-1.2 mm long, usually glabrous, deciduous.  Flowers sessile, pale yellow; calyx 5-lobed, 0.9-1.4 mm long, glabrous; corolla 5-lobed, 1.8-2.1 mm long, glabrous; stamen filaments 2.6-3.5 mm long, distinct; ovary glabrous, on a stipe to 0.2 mm long.  Legumes dark reddish-brown, straight to slightly curved, subterete in cross section, constricted and septate between the seeds, oblong, 55-150(190) x 5-7 mm, coriaceous, not striate, puberulent, occasionally with minute, reddish, deciduous glands, indehiscent; stipe to 12 (20) mm long; apex beaked.  Seeds uniseriate, no pulp, reddish-brown to dark purplish-brown, ovoid to nearly circular, slightly flattened, 4.8-7.0 x 3.5-4.5 mm, sometimes pitted along the margins; pleurogram oval to U-shaped, 1.3-1.8 mm across. Flowers in June. Chromosome number:  Not determined.

Distribution

Arid, rocky hillsides at low elevation in Baja California del Sur, Mexico, where it is restricted to Isla Espíritu Santo, Isla Partida del Sur, and the east side of Bahía de La Paz.

Additional info

Endemic to the southern tip of Baja California del Sur, Vachellia pacensis is easily separated from other members of the V. farnesiana group by its very small stalked petiolar and rachis glands (although petiolar glands are commonly absent), the small leaves and leaflets, and the long, pubescent, subterete, and constricted fruits.  Other distinguishing features include the stout, whitish stipular spines that are closely spaced along the stem, the short peduncles that do not exceed 12 mm in length, and the seeds that are pitted along the margins but smooth over the pleurogram.

In spite of its limited distribution, this taxon exhibits some variation.  Leaf size is highly variable, the rachis is usually about 15 mm long may sometimes exceed 45 mm, whereas the pinnae are mostly close to 5 mm long, but sometimes twice as long.  Also, the numerous spines are mostly less than 15 mm long, but are sometimes twice that length.

Vachellia pacensis has commonly been confused with members of the V. constricta group, mostly due to its small leaves with small leaflets and few pinna pairs (Brandegee 1892, Johnston 1924, Rudd and Carter 1983).  Vachellia pacensis, however, is more closely aligned with the V. farnesiana group.  Features such as the involucral bracts at the apex of the peduncle, the indehiscent fruits septate between the seeds, the lack of a pericarpic strip, and the round, slightly flattened, uniformly colored seeds are typical of the V. farnesiana group, contrasting sharply with the involucral bracts near the middle of the peduncle, the dehiscent fruits that are not septate between the seeds, the pericarpic strip, and the oblong, compressed, mottled seeds typical of members of the V. constricta group.  All specimens of V. pacensis tested were strongly cyanogenic (Clarke et al. 1989).

Flowering time

June.

Representative specimens

MEXICO:

Baja California del Sur:

 

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