Monographs Details: Mimosa irwinii 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.
Scientific Name:Mimosa irwinii Barneby
Synonyms:Mimosa speciosissima Taub., Mimosa splendida Barneby
Description:Species Description - Humifuse rosette-shrub with prostrate woody branches to 1 m bearing a terminal rosettelike cluster of ample lvs and many subvertical scapiform peduncles, these all involved at base in a cone of imbricate stipules silky-pilose externally with fine straight orange setae to ±10 mm, the scapes and lf-stks rufescently shaggy with attenuate, flexuously recurved or crumpled flagelli form setae to ± 5-7 mm, the thick-textured lfts glabrous and lustrous on both faces, scabrous- or setose-ciliolate, the massive capitula oblong-ellipsoid. Stipules ovate- or triangular-acuminate 8-15 x 3.5-9 mm, the firmly coriaceous persistent blades rufous-barbate on both faces but the setae readily deciduous when dry, the ventral face then castaneous-striate. Leaf-stalks of all but an occasional juvenile lf (no further described) 1.5-3 dm, the petiole 30-60 x 1.5-2 mm, the longer interpinnal segments 1.3-3.3 mm; a spicule 1-3 mm between each pinna-pair (sometimes concealed by setae); pinnae 9-16-jug., strongly decrescent proximally and less so distally, the rachis of longer ones (4-)6-9.5 cm, the longer interfoliolar segments 2.5-3.5 mm; lfts of longer pinnae 22-29-jug., a little decrescent toward each end of rachis, the first pair directly next to pulvinus (paraphyllidia 0), all in outline narrowly oblong obtuse or subapiculate, the longer ones 9-15 x 2-3 mm, the margin at maturity callous and pallid but no thicker than the blade itself, the venation almost fully immersed. Peduncles 2-3 dm; capitula without filaments 1.8-5.5 x 1.2-2 cm; bracts linear-lanceolate or -oblanceolate 6-7 x 0.5-1 mm, dorsally and marginally densely setose; flowers 4-merous 8-androus, some proximal ones staminate and a little smaller; calyx campanulate, the membranous tube glabrous but at base with a ring of fine straight setae to ±4 mm, the orifice ciliate with erect pappiform setae to ±2 mm; corolla 6-7 mm, the funnelform tube glabrous, the ovate 1-nerved lobes ±1.5 mm, subappressed-setulose overall, penicillately setose at apex; filaments lavender-pink, monadelphous through 1.1-1.5 mm, exserted 12-16 mm; ovary densely barbate, the glabrous style greatly elongate, surpassing the longest filament by 7-10 mm; pod unknown.

Distribution and Ecology - In rocky campo near 1000 m, known only from the type-locality on Chapada dos Veadeiros, near 14°S in e.-centr. Goiás, Brazil.—Fl. III-IV(-?).


Ample radical leaves and scapiform peduncles arising from a thatch of imbricate stipules clad in long rufous setae are features common to M. irwinii and M. speciosissima, and suggest a close common precursor. As shown in the diagnosis, they differ in several significant details. The year’s growth in M. speciosissima starts anew directly from the xylopodium, whereas M. irwinii develops humifuse woody trunks bearing the rosette of foliage and peduncles at apex. Compared with those of M. speciosissima the leaves of M. irwinii are more coarsely divided and the leafstalk bears a spicule between each pinna-pair. While the capitula of M. speciosissima are globose, those of M. irwinii are exceptionally massive, becoming ellipsoid and 2-3 times as long as their diameter. In another direction M. irwinii may be related to the pachycaul treelet M. splendida, which shares the rufous indumentum and spiculate leaf-stalks that are rare in ser. Pachycarpae. This, however, is grossly different in life-form, and in globose capitula that nestle in the rosette of leaves on peduncles less than a third as long.

This unique mimosa is dedicated with affection to my friend and former colleague Howard S. Irwin, whose immensely rich and judiciously selected collection of planaltine mimosae has contributed much of what is novel in this revision.

Distribution:Brazil South America| Goiás Brazil South America|

Narratives:Mimosa irwinii