Mimosa hexandra Micheli
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Authors
Rupert C. Barneby
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Authority
Barneby, Rupert C. 1991. Sensitivae Censitae. A description of the genus Mimosa Linnaeus (Mimosaceae) in the New World. Mem. New York Bot. Gard. 65: 1-835.
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Family
Mimosaceae
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Scientific Name
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Type
92. Mimosa hexandra M. Micheli, Mem. Soc. Phys. Geneve 30(7): 91, t. 27. 1889.-"Ad ripas fluminis Mbay prope Paraguari, [Balansa] n. 4422."—Holotypus, collected X.1882, G!; isotypi, +B = F Neg. 1415! F! = F Neg. 54886, K! L! NY! P!—M. bimucronata subsp. h
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Synonyms
Mimosa bimucronata var. hexandra (Micheli) J.F.Macbr., Mimosa bimucronata var. genuina Hassl., Mimosa bimucronata f. micheliana Hassl., Acacia, Mimosa hexandra var. tropica S.Moore, Mimosa vepres Lindm., Mimosa hexandra var. vepres (Lindm.) Chodat & Hassl., Mimosa acanthophora Harms, Mimosa bimucronata subsp. sepiaria (Benth.) Hassl., Mimosa bimucronata var. gymnocarpa Hassl., Mimosa coroncoro Killip & Dugand, Mimosa fascifolia Rizzini, Mimosa hexandra Micheli
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Description
Species Description - Microphyllidious arborescent shrubs and trees 2-6 m, the stiff annotinous branches pallid, lenticellate and erratically armed at and sometimes between nodes with brown, straight or apically recurved, commonly puberulent infrastipular aculei 3-5 mm, but some homotinous branchlets unarmed, the stems and all lf- and infiorescence-axes densely finely puberulent with pallid eglandular hairs sometimes mixed with livid granules, the commonly crowded lvs potentially dimorphic, the primary ones solitary, these sometimes replaced late in season by an axillary fascicle of smaller lvs, the concolorous lfts glabrous facially, usually minutely ciliolate, the pseudoracemes of smaller globose capitula either solitary and terminal to branchlets or paniculate, at anthesis leafless except sometimes at very base and projected ±1-2 dm from foliage, but often hysteranthously leafy, the pods then axillary and immersed. Stipules erect subulate or subsetiform 1-4 x 0.4-0.8 mm, 1-nerved, becoming stiff persistent. Leafstalks of primary lvs 1.5—3.5(—4) cm, of fasciculate ones 0.4-1.5 cm, all charged between each pinna-pair with a pallid ascending spicule 0.3-1 mm, the petiole of primary lvs 5-17 mm, at middle 0.4-0.8 mm diam., the longer interpinnal segments 3-7(-11) mm; pinnae of primary lvs 2-4-, of fasciculate ones mostly 2-jug., the rachis of longer ones 8-30 mm; lfts of longer pinnae of primary lvs mostly 8-16-jug., exceptionally to 24-jug., of fasciculate lvs 5-8-jug., in each case either subdecrescent at each end of rachis or accrescent distally (the terminal pair then broadest), narrowly oblong to linear-oblong or (distally) semi-obovate, the longest (3-)4-8(-9.5) x 0.7-2.6 mm, ±3-4(-5) times as long as wide, all abruptly apiculate at apex, smooth above, beneath 3-4(-5)-nerved from pulvinule, the moderately displaced midrib few-branched above mid-blade, the inner posterior nerve produced nearly to blade apex, the outer ones short. Peduncles 1-4 per node, 8-22 mm; capitula without filaments 5-7 mm diam., prior to anthesis moriform; bracts linear-elliptic 0.6-1 mm, 1-nerved, puberulent distally; flowers either all 3-merous or mixed 3- and 4-merous, or (exceptionally) largely 4-merous, commonly diplostemonous (rarely 4-merous 6-androus!); calyx membranous 0.7-1.2 mm, irregularly 3-4-denticulate, the triangular teeth 0.2-0.5 mm, the whole puberulent externally; corolla whitish, narrowly vaseshaped 1.8-2.8 mm, the tube glabrous, the ovate, shallowly concave, apically callous 1 -nerved lobes 0.8-1.1 x 0.65-0.9 mm, usually densely puberulent on each side of glabrous midrib, sometimes silky overall; filaments white, monadelphous for 0.1-0.4 mm, exserted (3-)4-7 mm. Pods 1-12 per capitulum, sessile, linear-oblong straight or subdecurved (25-)30-50(-60) x 8—11(—14) mm, compressed but low-convex over the 6-8 (-9) seeds, the 2-ribbed suture ±0.7 mm wide ventrally, the firm suberous castaneous venulose valves either glabrous or at first puberulent but early glabrate, when ripe consisting of livid exocarp ±0.1 mm thick and crustaceous pallid endocarp to 0.5-0.7 mm thick near the transverse septa, the ripe pod often truly lomentiform, breaking (under light pressure) both through the septa and through the sutures into free-falling closed (indehiscent) subrectangular articles ±4-6 mm long, but the replum at times persisting entire and the fruit a craspedium; seeds transverse, disciform ±5.5 mm diam.
Distribution and Ecology - In low, sometimes temporarily inundated places in semideciduous or deciduous brush-woodland communities, discontinuously dispersed in tropical North and both tropical and subtropical South America: locally abundant in Paraguay, especially along rio Paraguai and its immediate affluents, extending feebly n. into the Pantanal in Mato Grosso, Brazil, and s. into Formosa and Misiones, Argentina; local in the lower S. Francisco valley, downstream from Xique-xique, in Bahia, Brazil and in adj. s. Piauí and w. Pernambuco (lat. ±8°-9°30'S); arid lowlands of n.-e. Colombia (Atlántico) and n.-w. Venezuela (Falcón); Pacific lowlands of isthmus of Tehuantepec in Oaxaca, Mexico.—Fl. s. of the Equator IX-III(-IV), in n. South America mostly VI-IX(-?), in Mexico VII-XI.-Carquejo (Bahia). Map 16, the Mexican extension not shown.
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Discussion
Mimosa hexandra is very closely related to M. bimucronata, but differs usually in all, and always in some combination of these characters: smaller leaves, more deeply cleft puberulent calyx, mostly or wholly trimerous corollas; but especially, as first pointed out by Burkart (1948, 1.e.) in the suberous valves of the pod. While the valves of the pod are always much thicker than those of M. bimucronata, their mode of dehiscence has proved to be less constant than expected by Burkart. In some the whole pod breaks transversely across both valves and replum in the manner of a loment, but in others the valves alone separate into free-falling articles and the replum persists. A study of the distribution of these modes of dehiscence is hampered by lack of ripe fruiting material.
Other close relatives of M. hexandra are M. exalbescens and M. pseudosepiaria, which see for comment.
The fragmented range of M. hexandra poses a problem in interpretation. Its immediately close kindred are all East Brazilian, and it seems reasonable to suppose that it arose as a xeromorphic derivative of M. bimucronata, acquiring in the process a reduced leaf-formula, a mostly trimerous flower, and a modified pod, all relatively advanced characters. It is, however, at present most abundant by far in the upper Paraguai valley, and occurs in Atlantic Brazil only very locally in northern Bahia and states adjoining to the north. Because M. bimucronata is found, but less commonly, in Paraguay, M. hexandra might have arisen there, even by independent mutation. The populations in northern South America and Mexico seem certainly to be derived from south of the Amazon, but whether they migrated thence by savanna pathways during episodes of forest decline or reached their contemporary habitat in xeromorphic scrub-woodland by accidents of long-distance dispersal is impossible to determine.
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Distribution
Brazil South America| Paraguay South America| Colombia South America| Argentina South America| Mexico North America|