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Mariosousa willardiana - bark

Mariosousa willardiana - bark

Mariosousa willardiana - habit

Mariosousa willardiana - leaf pinnules

Mariosousa willardiana - leaf pinnules

Mariosousa willardiana - leaf 

Mariosousa willardiana - stem

Mariosousa willardiana - bark

Mariosousa willardiana - habit

Mariosousa willardiana - peeling bark

Mariosousa willardiana - terminal pinnae pair with gland

Mariosousa willardiana - multiple stems

Mariosousa willardiana - peeling bark

Mariosousa willardiana - leaves and pods

Mariosousa willardiana - immature fruit

Mariosousa willardiana - immature fruit and flower

Mariosousa willardiana - leaf blade

Mariosousa willardiana - pinnae

Mariosousa willardiana - pinnae attachment

Mariosousa willardiana - flower

Mariosousa willardiana - immature fruit and flower

Mariosousa willardiana - seed

Mariosousa willardiana - habit

Name

Mariosousa willardiana (Rose) Seigler & Ebinger

Synonymy and types

Acacia willardiana Rose in Vasey and Rose, Contr. U. S. Natl. Herb. 1:88. 1890. - TYPE: Mexico, Sonora, rocky islands and ledges on the coast of Guaymas, 1-2 April 1890, E.Palmer 164 (holotype: US!).

Prosopis heterophylla Bentham, London J. Bot. 5:82. 1846. - Senegalia heterophylla (Bentham) Britton & Rose, N. Amer. Fl. 23:114. 1928. - TYPE: Mexico, Sonora, Alta, 1830, T.Coulter s.n. (holotype: TCD). (not Acacia heterophylla Willdenow 1806).

Formal description

Tree to 10 m tall. Bark smooth, white to reddish yellow, exfoliating and papery.  Twigs light gray, becoming dark reddish purple, not flexuous, glabrous.  Short shoots absent.  Leaves alternate, 30-400 mm long. Stipules herbaceous, light brown, narrowly linear, to 1.1 x 0.2 mm near the base, glabrous, tardily deciduous.  Petiole adaxially flattened, not grooved, 20-400 mm long, usually glabrous; petiole gland solitary, located between to just below the lower pinna pair, sessile, circular, 0.3-0.7 mm across, doughnut-shaped, glabrous.  Rachis flattened, not grooved, 0-100 mm long, glabrous, glands absent.  Pinnae 1 (rarely 3) pairs per leaf, 16-80 mm long.  Petiolules 2.5-8 mm long. Leaflets 4 to 20 pairs per pinna, opposite, 1-5 mm between leaflets, oblong to elliptic, 3.0-7.5(12.0) x 1.0-2.3 mm, glabrous to rarely lightly pubescent with appressed hairs on both surfaces, lateral veins not obvious, only one vein from the base, base oblique, margins lightly ciliate, apex narrowly acute to acuminate.  Inflorescence a loosely flowered cylindrical spike 30-90 mm long, solitary from the leaf axil, or in short racemose clusters. Peduncle 5-25 x 0.4-0.8 mm, glabrous or nearly so.  Involucre absent.  Floral bracts linear, to 1 mm long, glabrous to lightly pubescent, early deciduous.  Flowers sessile, creamy-white; calyx 5-lobed, 1.3-2.2 mm long, glabrous; corolla 5-lobed, 2.4-3.6 mm long, glabrous; stamen filaments 6-8 mm long, distinct; ovary glabrous, on a stipe to 1 mm long.   Legumes light yellowish-brown, straight, flattened, not constricted between the seeds, oblong, 70- 180 x 8-22 mm, chartaceous, irregularly striate, glabrous, eglandular, dehiscent along both sutures; stipe to 14 mm long; apex obtuse.  Seeds uniseriate, no pulp, dark brown, near circular, strongly flattened, 6-11 mm across, smooth; pleurogram usually absent, when present, U-shaped, about 2 m across.   Flowers: February-June.  Chromosome number: Not determined.

Distribution

Arid hills, rocky slopes and washes in desert scrub vegetation between sea level and 500 m elevation in Sonora, Mexico.

Additional info

A common species at lower elevations in the state of Sonora, Mariosousa willardiana is a very obvious component of the desert scrub of this region because of it nearly white, to yellowish, to almost reddish, papery, exfoliating bark.  Being so obvious, this species is commonly collected; numerous specimens are from the vicinity of Guaymas.  The majority of the specimens lack pinnae, which are early deciduous; only the elongated, flattened petioles persist.  Although flowering is common from February through May, it may occur at other times if moisture is available.

Mariosousa willardiana is easily separated from other members of this group.  The most obvious differences are the elongated, flattened petioles that may reach 400 mm in length, and the leaves with usually only one pair of pinnae, though rarely two or three may be present.  This species has very small, tardily deciduous stipules (1.1 mm long or less), and the fruit valves are papery with more irregular striations than those found in other members of this genus.  The characteristic papery, exfoliating bark is shared only with M. salazari, a species of this group restricted to central Mexico.

Bentham (1846) tentatively assigned this taxon to Prosopis heterophylla based on a single fruiting specimen.  He suggested that the general habit of the plant was more like that of Prosopis than any other genus, and mentioned the almost phyllodinous vertical expansion of the petiole.  Based on flowering material, Vasey and Rose (1890) realized that this taxon was an Acacia, and used the name Acacia willardiana.

Vassal (1972) and Vassal and Guinet (1972) noted that Mariosousa willardiana is a species with authentic phyllodes and suggested that, if he had seen flowering material, Bentham (1846) would have placed this species in the "Juliflorae".  Vassal considered this species to be a member of his section Heterophyllum, subsection Spiciferae, and to have affinities to this predominantly Australian group.  In our view, homology between the phyllodes of that group and the apparent petioles of M. willardiana should be examined more thoroughly.  Further, in most other characteristics, this species differs little from other members of the genus Mariosousa.

Flowering time

February-June.

Representative specimens

MEXICO:

Sonora:

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