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Vachellia schottii - stem

Vachellia schottii - habit

Vachellia schottii - pods

Name

Vachellia schottii (Torrey in Emory), Seigler & Ebinger, Phytologia 87:  167.  2005.
syn.  Acacia schottii Torrey in Emory, Rep. U. S. Mex. Bound. Surv.  2:  62.  1858.

Synonymy and types

Acacia schottii Torr. in Emory, Rep. U. S. Mex. Bound. Surv.  2: 62.  1858.   Acaciopsis schottii (Torr.) Britton & Rose, N. Amer. Fl.  23: 96.  1928. - TYPE:  UNITED STATES.  TEXAS:  Brewster Co., near the canon of San Carlos, at the Comanche Crossing of the Río Grande, Sep, C. C. Parry 330 (holotype:  NY; isotype:  GH).

Formal description

Shrub to 3 m tall.  Bark dark gray, smooth.  Twigs dark reddish brown to gray, not flexuous, glabrous to very lightly pubescent with appressed hairs.  Short shoots commonly present above the stipular spines, to 5 mm long, covered with acuminate stipules and old leaf bases.  Leaves alternate, also clustered on the short shoots, 4.8-17.5 mm long.  Stipular spines reddish brown, usually becoming light gray with age, symmetrical, terete, slightly recurved, stout, to 15 (22) x 0.9 mm near the base, usually glabrous.  Petiole adaxially shallowly grooved, 4-10 mm long, lightly puberulent with appressed hairs; petiolar gland solitary, located just below the lower pinna pair, sessile, circular, 0.1-0.4 mm across, doughnut-shaped, glabrous.  Rachis adaxially slightly grooved, 0-8 mm long, lightly puberulent with appressed hairs, a sessile, doughnut-shaped gland 0.1-0.2 mm across between the upper pinna pair.  Pinnae 1 (rarely 2 on primary leaves) pair per leaf, 9-25 mm long, 0-8 mm between pinna pairs.  Petiolules 1.7-4.0 mm long.  Leaflets 6 to 14 pairs per pinna, alternate, 1-4 mm between leaflets, linear and subterete, 2.8-6.1 x 0.3-0.6 mm, glabrous, lateral veins not obvious, only one vein from the base, base cuneate, margins rarely ciliate, apex acute.  Inflorescence a densely flowered globose head, 5.5-7.5 mm across, solitary or in small clusters of 2 to 5 on the short shoots.  Peduncles 20-28 x 0.3-0.5 mm, lightly puberulent with appressed hairs and sometimes glandular.  Involucre bracts 4 to 7, scattered near the middle of the peduncle, lightly puberulent, early deciduous.  Floral bracts spatulate, 0.8-1.2 mm long, puberulent, deciduous.  Flowers sessile, pale yellow; calyx 5-lobed, 0.8-1.2 mm long, glabrous to lightly puberulent on the lobes; corolla 5-lobed, 1.8-2.2 mm long, glabrous or nearly so; stamen filaments 3-4 mm long, distinct, ovary glabrous and sometimes glandular, sessile or on a stipe to 0.2 mm long.  Legumes light reddish-brown, slightly curved, flattened, sometimes constricted between the seeds, linear, 40-90 x 5.5-7.8 mm, chartaceous, reticulately striate, glabrous, glandular with round, purplish-brownish, sessile glands 0.2 mm in diameter, indehiscent; a chartaceous pericarpic strip lining each valve; stipe to 8 mm long; apex acuminate and usually beaked.  Seeds uniseriate, no pulp, mottled gray, oblong, strongly flattened, 5.0-6.8 x 4.2-5.8 mm, smooth; pleurogram oval to U-shaped, 0.6-1.0 mm across. Flowers in April to August. Chromosome number:  Not determined.

Distribution

Locally common between 900 and 1,300 feet in washes and on slopes in open thorn -scrub vegetation in Brewster and Presidio Counties, Texas and probably in adjacent Mexico in the states of Chihuahua and Coahuila.

Additional info

Vachellia schottii is obviously a member of the Vachellia constricta group having a chartaceous pericarpic strip lining each fruit valve, involucral bracts near the middle of the peduncle, and flattened, oblong, mottled seeds with a small central pleurogram.  This taxon is easily distinguished, however, by its linear, subterete, alternate, and widely spaced leaflets that give the foliage a feathery appearance.

The morphological characteristics of the specimens examined are relatively constant, but some variation was observed in leaves and fruits. All of the leaves on the short shoots consistently had a single pair of pinnae, the first formed leaves on developing twigs commonly had two pairs of pinnae, usually with shorter petioles, larger petiolar glands, but slightly shorter leaflets.  In general, fruit curvature and constrictions were variable with some fruits being only slightly curved and others strongly falcate.  Although some fruits lacked constrictions, others were strongly, but usually irregularly, constricted.  Also, though the fruits were usually slightly glutinous, a few were not.  Half of the specimens tested were cyanogenic with an even number of strong and weak reactions among the cyanogenic specimens (Clarke et al. 1990).

Vachellia schottii shows close affinities to V. constricta and V. vernicosa (syn. A. neovernicosa) (Isely 1969).  It has been suggested (Clarke et al. 1990), that hybridization between these taxa may occur in western Coahuila, eastern Chihuahua, and the Trans-Pecos region of Texas where the three species are sympatric.  A PCA of many specimens from the Big Bend Region of Texas suggests that Vachellia schottii hybridizes with V. vernicosa.

Within its very restricted range, Vachellia schottii is a common species, often occurring in large, relatively pure thickets over extensive areas, apparently being quite successful within this small area (Turner 1959).  These areas of thorn scrub are located on limestone or gypsum derived soils, where V. constricta and V. vernicosa are also common.  Vachellia schottii is particularly common in the Big Bend region of Texas, and probably occurs to the south in adjacent Mexico, though the authors have seen no specimens from that region.

Vachellia schottii was named after Arthur Schott, an assistant to Dr. C. C. Parry, the physician on the United States Mexican boundary survey (Emory 1857-1859).  Volume 2 of this work was published in 1858, and is titled "Botany of the Boundary", with John Torrey writing pages 27 to 270.

Flowering time

April-August.

Representative specimens

UNITED STATES:

Texas:

Brewster Co.:

Presidio Co.:

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