Monographs Details: Mimosa sceptrum Barneby
Authors:Rupert C. Barneby
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.
Family:Mimosaceae
Scientific Name:Mimosa sceptrum Barneby
Description:Species Description - Robust unarmed, virgately erect subshrubs 10- 14(—?) dm tall with simple stem densely amply foliate to middle or beyond and passing into a long efoliate pseudoraceme of solitary globose capitula maturing in acropetal sequence, the stem and all axes of lvs and inflorescence hispid or when young silky-hispid with spreading-ascending yellowish smooth setae ±0.7-1.4 mm, the brownish-olivaceous subconcolorous lfts loosely strigulose on both faces with fine, antrorsely subappressed setae not more than 0.1 mm diam. at dilated base, the slender, continuously pallid margin ciliate with pluriseriate setae. Stipules erect firm, lanceolate or lance-attenuate (3-)4-9 x 0.5-1.4 mm, dorsally hispid like stem, glabrous within, becoming dry but persistent. Leafstalks including grossly dilated but hard pulvinus 3-9 x 0.6-1 mm, widely shallowly grooved ventrally (spicule 0); pinnae 1-jug., the rachis (3- )3.5-8.5 cm, the longer interfoliolar segments 48 mm; lfts (7-)8-12-jug., decrescent at each end of rachis, the first pair 1-1.5 mm distant from lance-subulate, often unequal paraphyllidia 1-2.2 mm, the blades broadly lance-oblong from semicordate base, abruptly acuminate, those near mid-rachis 15-26 x 4.5-8 mm, 2.8-3.5 times as long as wide, all veinless above or almost so, beneath 3-nerved from pulvinule, the slenderly prominulous midrib dividing blade ±1:2, the inner posterior nerve expiring near mid-blade, the outer one very short. Peduncles 10-25(-28) mm; capitula without filaments 11-13 mm diam., prior to anthesis conelike, densely silky-setose; bracts linear-oblance-attenuate (3-)3.5-5 x 0.4-0.6 mm, proximally embracing the subtended fl, beyond middle silky-barbate dorsally; flowers 4-merous 4-androus, apparently all bisexual; calyx a minute collar or obsolete; corolla narrowly vase-shaped or subtubular 4-5 mm, the membranous ovate, faintly 1-nerved lobes 0.9-1.2 mm, minutely puberulent at tip; filaments pink, free to base, exserted 7.5-10 mm. Pods (not seen fully ripe) packed into a dense hard sphere 18-22 mm diam., sessile, in profile obliquely obovate ±8-9 x 4 mm, broadly cuneate at base, at apex excentrically mucronulate by the style-base, the slender sutures ±0.4 mm wide, not constricted between the two seeds, the firmly papery valves densely barbate overall with ascending pallid setae to ±2 mm; dehiscence not seen, perhaps valvate as in M. dolens; seeds unknown.

Distribution and Ecology - In wet campo and on roadsides, near 500 (?) m, localized on both slopes of Sa. de Amambay in Paraguay and adj. Mato Grosso do Sul, Brazil. -Fl. XII-I.

Discussion:

Mimosa sceptrum resembles M. brevipes and M. custodis in virgate growth-form, indeterminate pseudoraceme of capitula, and minute calyx, but differs from both in fewer larger leaflets and fatter capitula of larger flowers; and is further distinguished from M. brevipes in the seemingly valvate dehiscence of the pod and in the continuous marginal nerve of the less prominently nerved leaflets. These species all occur in close proximity on Sa. de Amambay, and M. sceptrum may perhaps have arisen through introgression between M. brevipes or M. custodis and some sympatric form of M. dolens, which could have contributed the genes for valvately dehiscent fruits borne in a dense sphere, but which is consistently distinguished from M. brevipes and its allies by the determinate pseudoraceme.

Distribution:Brazil South America| Rio Grande do Sul Brazil South America| Paraguay South America| Amambay Paraguay South America|

Narratives:Mimosa sceptrum