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Name

Vachellia pennatula (Schlechtendal & Chamisso) Seigler & Ebinger var. pavicephala (Seigler & Ebinger) Seigler & Ebinger. Phytologia 87:164. 2005.
syn. Acacia pennatula (Schltdl. & Cham.) Benth. subsp. parvicephala Seigler and Ebinger, Syst. Bot. 13:12. 1988.

Synonymy and types

Basionym: Acacia pennatula (Schltdl. & Cham.) Benth. subsp. parvicephala Seigler and Ebinger, Syst. Bot. 13:12. 1988. - TYPE: MEXICO. YUCATAN: Chichén Itzá, in second growth, 6 Jun. 1938, C.L.Lundell & Lundell 7418 (holotype: F; isotypes: LL, NY, TEX, US).

Formal description

Tree to 15 m tall. Bark dark brown to dark gray, smooth to shallowly furrowed. Twigs dark reddish brown to dark gray, slightly flexuous, pubescent with matted, white to grayish hairs mostly less than 0.3 mm long, sometimes becoming glabrous with age. Short shoots absent. Leaves alternate, 60-200 mm long. Stipular spines reddish brown, symmetrical, terete, straight, stout, to 25(40) x 1-3 mm near the base, densely pubescent at the base and sometimes throughout with erect, yellow-green hairs. Petiole adaxially grooved, 3-8 mm long, pubescent with matted, white to grayish hairs mostly less than 0.3 mm long, sometimes becoming glabrous with age; petiolar gland solitary, located on the upper half of the petiole, sessile, circular to slightly elongated, 1.0-2.5 mm long, apex flattened with the margins sometimes raised, glabrous. Rachis adaxially grooved, 60-200 mm long, densely pubescent, a sessile, circular gland, 0.4-0.7 mm across, between the upper 1 to 3(5) pinna pairs. Pinnae 17 to 48 pairs per leaf, 15-38 mm long, 2-5 mm between pinna pairs. Petiolules 0.4-1.1 mm long. Leaflets 24 to 60 pairs per pinna, opposite, 0.3-0.6 mm between leaflets, linear, 0.8-3.0 x 0.4-0.7 mm, glabrous to lightly pubescent, lateral veins not obvious, only one vein from the base, base oblique, margins ciliate, apex broadly acute to obtuse. Inflorescence a densely flowered globose head 5-8 mm across, solitary or in clusters of 2 to 5(10) in the leaf axil. Peduncles 7-12 mm long, densely pubescent with matted, white to grayish  hairs, sometimes becoming glabrous with age. Involucre 4- to 6-lobed, located at the base of the globose head, puberulent, persistent. Floral bracts spatulate, 1.2-2.0 mm long, apex puberulent, deciduous. Flowers sessile, yellow; calyx 5-lobed, 0.4-0.8 mm long, the lobes puberulent; corolla 5-lobed, 1.4-1.9 mm long, the lobes puberulent; stamen filaments 3.0-4.5 mm long, distinct; ovary glabrous to lightly pubescent, on a stipe to 0.2 mm long. Legume dark brown to black, straight, not inflated, flattened to slightly elliptic in cross section, not constricted between the seeds, oblong, 50-90 x 12-16 mm, coriaceous, reticulately striate, glabrous, glandular, indehiscent; stipe mostly absent; apex rounded. Seeds uniseriate, imbedded in a light brown pulp, light brown, ovoid to ellipsoid, slightly flattened, 6.5-8.4 x 4.2-6.0 mm, smooth; pleurogram U-shaped, 2-4 mm across. Flowers from March to May. Chromosome number: Not determined.

Distribution

In arid, mostly disturbed habitats from sea level to 1500 m in southern Mexico, and south into Honduras.

Additional info

Vachellia pennatula var. parvicephala has a very restricted geographic range in southern Mexico and Central America, and many of the collections are at relatively low elevations, some from near sea level. Flowering material is easily separated from var. pennatula as the inflorescences are much smaller (5-8 mm across), the flowers and floral bracts are smaller, and the peduncles are very short, less than 12 mm long. Mature fruiting material also differs, but overlaps in its characteristics with var. pennatula. Generally fruits of var. parvicephala are smaller than those of var. pennatula, being less than 90 mm long, less than 16 mm wide, and 6-10 mm thick.  Vachellia pennatula var. pennatula fruits, in contract, are usually longer, mostly 16-30 mm wide, and 7-12 mm thick. Sterile material is more difficult to distinguish as the thick yellow-green pubescence of var. pennatula become less obvious and thinner in older material.

More than 40 specimens of Vachellia pennatula var. parvicephala were tested for the presence cyanide and more than 50% gave a positive reaction (Seigler and Ebinger 1988).  The cyanogenic glycoside present is proacacipetalin, the same compound found in V. macracantha.

Based on the description given by Standley (1930a, 1930b), it is probable that he intended his name Acacia milleriana to apply to this small inflorescence form of A. pennatula. Standley, however, based his name on Mimosa campechiana Miller (1768), which is a synonym of A. cochliacantha (Rudd 1966). The valid name for this taxon therefore is var. parvicephala.

Flowering time

March - May.

Representative specimens

HONDURAS:

Comayagua:

Francisco Morazán:

Morazán:

Yoro:

MEXICO:

Campeche:

Chiapas:

Quintana Roo:

Yucatán:

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