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Vachellia rigidula - trunk

Vachellia rigidula

Vachellia rigidula

Vachellia rigidula - habit

Vachellia rigidula

Vachellia rigidula

Vachellia rigidula - habit

Vachellia rigidula

Vachellia rigidula - stem

Name

Vachellia rigidula (Bentham) Seigler & Ebinger, Phytologia 87:  166.  2005.
syn. Acacia rigidula Bentham, London Jour. Bot.  1:  504.  1842.

Synonymy and types

Basionym:  Acacia rigidula Benth., London J. Bot.  1: 504.  1842.  Acaciopsis rigidula (Benth.) Britton & Rose, N. Amer. Fl.  23: 94.  1928. - TYPE:  UNITED STATES.  TEXAS:  T. Drummond 161 (holotype:  K).

Formal description

Shrub or small tree to 4 m tall.  Bark light gray to brown, smooth to shallowly furrowed.  Twigs dark purplish brown to dark gray, slightly flexuous, glabrous to lightly puberulent.  Short shoots commonly present above the stipular spines, to 4 mm long, covered with acuminate stipules and old leaf bases.  Leaves alternate, also commonly clustered on the short shoots, 2-9 mm long.  Stipular spines light gray, symmetrical, terete, straight, thin, to 50(80) x 2.1 mm near the base, glabrous to lightly puberulent.  Petiole adaxially grooved, 2-5(9) mm long, densely puberulent; petiolar gland solitary, mostly located medially on the petiole, sessile, circular, 0.3-0.6(1.1) mm across, apex depressed, glabrous to puberulent.  Rachis absent.  Pinnae 1 pair per leaf, 5-20 mm long.  Petiolules 1-3 mm long.  Leaflets (2)3 to 5 pairs per pinna, mostly opposite, 1.5-5.0 mm between leaflets, oblong to elliptic, terminal leaflets (4)6-13(16) x 3-6(8) mm, lateral leaflets slightly smaller, glabrous to rarely lightly pubescent beneath, lateral veins obvious, 3 to 5 veins from the base, base oblique, margins not ciliate, apex obtuse and usually mucronate.  Inflorescence a loosely flowered elongated spike, 10-35 mm long, solitary or in clusters of 2 to 4(8) on the short shoots.  Peduncles 0-3 x 0.4-0.7 mm, puberulent.  Involucre usually 4-lobed, at the base of the peduncle, the lobe apex ciliate the remainder glabrous to puberulent, persistent.  Floral bracts spatulate, 0.4-0.8 mm long, puberulent, deciduous.  Flowers sessile, white to cream-colored; calyx 4-lobed, 0.4-0.8 mm long, puberulent; corolla 4-lobed, 1.2-1.7 mm long, glabrous or nearly so; stamen filaments 2.7-4.0 mm long, distinct; ovary glabrous to densely pubescent, on a stipe to 0.3 mm long.  Legumes dark reddish brown, straight to slightly curved, flattened, constricted between the seeds, linear, 40-100 x 3-6 mm, coriaceous, reticulately striate, puberulent, eglandular, dehiscent; a chartaceous pericarpic strip lining each valve; stipe to 7 mm long; apex acuminate and sometimes beaked.  Seeds uniseriate, no pulp, dark brown, oblong, strongly flattened, 5.5-7.5 x 3.5-4.5 mm, smooth; pleurogram oblong to U-shaped, 1.1-1.9 mm across. Flowers in February to July. Chromosome number: 2n = 26 (Turner 1959).

Distribution

Open areas, disturbed sites, on rocky slopes and bluffs, along fence rows and along arroyos, particularly at lower elevations from southern Texas (Isely 1973, Turner 1959) and northern Mexico in the states of Coahuila, Jalisco, Michoacán, Nuevo León, Querétaro, San Luis Potosí and Tamaulipas.

Additional info

Commonly forms extensive thickets and extensively collected, Vachellia rigidula is relatively consistent in its morphological characteristics.  Much of the variation found has to do with the difference in size of the primary leaves (those developing on rapidly growing shoots), and the usually smaller leaves that develop from the short shoots.  In most specimens, the terminal pair of leaflets are less than 11 mm long, but may rarely exceed 16 mm.  Also, there are rarely only two pairs of leaflets on a pinna, but three to five pairs are common.  Pubescence of the spines, twigs, petioles, and petiolar glands also varies.  Generally, the spines, twigs and petiolar glands are glabrous to lightly puberulent, but the petioles may be densely puberulent.  Glabrous individuals are rarely found.  No variation, however, was found in the test for cyanogenic glycosides.  Of the more than 100 specimens tested, all gave negative results, even when emulsin was used.

Plants in populations of this species from the area south of San Antonio, Texas (Atascosa Co.), occasionally harbor ants in the spines (Seigler and Conn 1982, Seigler et al. 1982).  This does not appear to represent a highly coadaptive interaction such as exists in other, better known ant acacias, for the ant species is found on a number of unrelated plants.  Neither the ant nor the Vachellia species appear to have undergone any significant modification.

Some confusion has existed as to the correct name for this taxon.  Bentham (1842, 1875) and Standley (1922) used the name A. amentacea and listed A. rigidula as a synonym.  Recently, Turner (1959) considered both A. amentacea (Central Mexico) and A. rigidula (Texas and adjacent Mexico) distinct species.  More recently Isely (1969) suggested that A. rigidula and A. amentacea are conspecific.  It is now apparent that the name A. amentacea has been incorrectly applied (see discussion under V. bilimekii).

Flowering time

February-July.

Representative specimens

MEXICO:

Coahuila:

Jalisco:

Michoacán:

Nuevo León:

Querétaro:

San Luís Potosí:

Tamaulipas:

Veracruz:

UNITED STATES:

Texas:

Atascosa Co.:

Bee Co.:

Brewster Co.:

Calhoun Co.:

Cameron Co.:

Dimmit Co.:

Duval Co.:

Edwards Co.:

Frio Co.:

Goliad Co.:

Hidalgo Co.:

Jackson Co.:

Jim Hogg Co.:

Jim Wells Co.:

Karnes Co.:

Kinney Co.:

LaSalle Co.:

Live Oak Co.:

Maverick Co.:

McMullen Co.:

Nueces Co.:

San Patricio Co.:

Starr Co.:

Uvalde Co.:

Val Verde Co.:

Webb Co.:

Willacy Co.:

Zapata Co.:

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