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Name

Mariosousa russelliana (Britton & Rose) Seigler & Ebinger

Synonymy and types

Acacia russelliana (Britton & Rose) Lundell, Contr. Univ. Michigan Herb. 4:7. 1940. - Senegalia russelliana Britton & Rose, N. Amer. Fl. 23:112. 1928. - TYPE: Mexico, Sinaloa, vicinity of San Blas, 22 March 1910, J.N.Rose, P.C.Standley & P.G.Russell 13204 (holotype: US!, isotype: GH!, NY!).

 

Formal description

Shrub or small tree to 8 m tall. Bark dark-gray, shallowly furrowed.  Twigs light brown to greenish brown, not flexuous, glabrous.  Short shoots absent.  Leaves alternate, 60-140 mm long. Stipules herbaceous, light brown, narrowly linear, to 2.5 x 4 mm near the base, glabrous, tardily deciduous.  Petiole adaxially shallowly grooved, 20-50 mm long, glabrous; petiole gland solitary, located near the middle of the petiole to just below the lowest pinna pair, sessile, usually circular, 0.4-1.5 mm across, doughnut-shaped to urn-shaped, glabrous, sometimes absent.  Rachis adaxially shallowly grooved, 30-90 mm long, glabrous, rarely a sessile, doughnut-shaped gland, 0.4-0.9 mm across, between the upper pinna pair.  Pinnae (2)4 to 11 pairs per leaf, 35-70 mm long, 5-12 mm between pinna pairs.  Petiolules 2.0-3.5 mm long.  Leaflets 18 to 34 pairs per pinna, opposite, 0.9-1.8 mm between leaflets, oblong, 4.0-7.5 x 1.3-1.8 mm, glabrous, lateral veins obvious, a midvein and 1 to 3 smaller veins from the base, base oblique, margins sometimes lightly ciliate, apex obtuse to broadly acute.  Inflorescence a loosely flowered cylindrical spike 25-60 mm long, solitary (rarely 2 to 3) from the leaf axil, or rarely in terminal racemose clusters. Peduncle 1-10 x 0.4-0.7 mm, glabrous.  Involucre absent.  Floral bracts linear, to 1 mm long, glabrous, early deciduous.  Flowers sessile, creamy-white; calyx 5-lobed, 1.0-1.4 mm long, glabrous; corolla 5-lobed, 1.7-2.5 mm long, glabrous; stamen filaments 5-7 mm long, distinct; ovary glabrous, on a stipe 2-3 mm across.  Legumes greenish-brown to dark brown, straight, flattened, not constricted between the seeds, oblong, 55-170 x 16-27 mm, cartilagious, trasversely striate, glabrous, eglandular, dehiscent along both sutures; stipe to 15 mm long, apex acute to obtuse, sometimes beaked.  Seeds uniseriate, no pulp, reddish-brown, circular to oval, strongly flattened, 6.8-9.6 x 5.1-8.0 mm, smooth; pleuogram U-shaped, 2-3 mm across.   Flowers: March-August.  Chromosome number: Not determined.

Distribution

Dry, deciduous, tropical forests to thorn -scrub and desert-scrub vegetation, mostly on rocky slopes, from near sea level to about 700 m elevation in southwestern Sonora, and extreme northern Sinaloa, Mexico.

Additional info

A small shrub or understory tree not exceeding 8 m in height, Mariosousa russelliana is a common species of Sonora and parts of Sinaloa.  Occurring at elevations below 700 m, it appears to be a common component of desert and thorn scrub vegetation, and is also found as an understory tree in tropical deciduous forests. Britton and Rose (1928) were the first to recognize this taxon, all previous and subsequent authors considering it to be conspecific with M. coulteri, a species of northeastern Mexico.  It is easily distinguished from this taxon by being essentially glabrous; M. coulteri, in contrast, has leaves that are lightly pubescent beneath with appressed hairs and the petiole and rachis are lightly puberulent, as are many other vegetative parts of the plant.  Also, the perianth of most flowers of M. coulteri are puberulent, being glabrous in M. russelliana.

The only species of this genus found within the range of Mariosousa russelliana are M. willardiana and M. millefolia.  Both taxa are easily separated from M. russelliana; M. willardiana by its extremely long, flattened petioles topped with 1-3 pairs of pinnae, and M. millefolia by the stalked gland with a bulbous apex between the upper pinnae pair, the long, usually persistent stipules, and the missing petiolar gland.

It is quite probable that Mariosousa russelliana occasionally hybridizes with M. willardiana in thorn -scrub forests on the arid, rocky slopes at lower elevations in extreme northern Sinaloa, Mexico.  The few specimens available suggest that the probably hybrid is similar to M. willardiana in being a small tree with exfoliating, papery bark, petioles that commonly exceed 100 mm in length, and pinnae with fewer than 26 pairs of leaflets that are relatively widely spaced.  The proposed hybrid is similar to M. russelliana in having stipules to 2.5 mm long, petioles that are round in cross-section and slightly adaxially grooved, a doughnut-shaped petiolar gland, and leaves with 2-11 pairs of pinnae.  Few specimens are available, and field studies will be necessary to determine the status of these specimens, and potential hybrids.

Flowering time

March-August.

Representative specimens

MEXICO:

Sinaloa:

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