Monographs Details: Mimosa acroconica 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
Synonyms:Mimosa caliciadenia Barneby
Description:Species Description - Openly branched, prickly and viscid, arborescent shrubs with incurved-ascending branches, attaining 4 m, armed on flowering stems and on back of lf-stks with few scattered, horizontal or widely acroscopic, yellowish aculei ±1-2 mm, the stems in addition densely finely villous with erect shining hairs less than 0.5 mm and hispid with many erect, gland-tipped setae 0.5-1.5 mm and a few scattered, tapering eglandular ones, the foliage olivaceous subconcolorous, the lfts facially glabrous or almost so, minutely ciliolate, the relatively loose capitula solitary and paired in the axils of a succession of coevally expanding lvs, when very young subtended and concealed by a cone of ample papery stipular bracts, the inflorescence paniculate, openly foliate. Stipules caducous from a wide lunate scar, all fallen from developed lvs and from base of peduncles at anthesis, those subtending nascent capitula papery-membranous, ovate-acuminate, dorsally gibbous, 6.5-12 x 4-8 mm, the blade silky-pilosulous but eglandular dorsally, glabrous and finely several-nerved within. Leaves sessile, the short first pair of pinnae directly next to discolored pulvinus, the rachis of lvs below the inflorescence 5-16 cm (upper lvs shorter and simpler, no further described), the longer interpinnal segments 8—12(—15) mm, the ventral sulcus narrow, bridged and minutely spiculate between pinna-pairs; pinnae 5-14-jug., accrescent distally, the rachis of distal pair 2.5-3.5 cm, the longer interfoliolar segments 1-1.5 mm; lfts of distal pinnae ± 17-24(-28)-jug., little graduated, the first pair close beyond minute subulate paraphyllidia, the blades narrowly oblong from shallowly semicordate base, broadly rounded at apex, the largest 5-7.5 x 1.7-2 mm, all almost veinless above, beneath 3—4-nerved from pulvinule, the midrib subcentric, finely 1-2-branched well above midblade, the inner posterior nerve produced nearly to blade apex, the margin plane. Peduncles 2.5-4 cm; capitula plumply ellipsoid or ovoid, prior to anthesis conelike, without filaments to 12 x 9 mm diam., the slenderly clavate receptacle to 7.5 x 1.2 mm; bracts elliptic-oblanceolate 3-5 x 1-1.5 mm, surpassing but scarcely concealing the obovoid obtuse, subglabrous fl-buds, early deciduous, the claw glabrous, the thin-textured blade brownish, dorsally pilosulous; flowers 4-merous 8-androus, many lower ones staminate; calyx shallowly campanulate ±0.5 mm, the membranous tube externally glabrous, the low-deltate lobes randomly minutely setulose-ciliolate; corolla broadly turbinate but contracted at base into a neck nearly 0.5 mm, that of longer flowers 3.5-4.1 mm, the broadly deltate-ovate, pink, 1-nerved lobes ±1-1.2 x 1-1.2 mm; filaments bright pink, monadelphous through ±0.7 mm, exserted 10-11.5 mm; ovary setulose; ovules ±6; pod not known.

Distribution and Ecology - In campo rupestre at ±1000 m, known only from Sa. do Cabral, near 17°40'S in centr. Minas Gerais, Brazil.—Fl. II—III(—?).

Discussion:

Mimosa acroconica is clearly related to M. caliciadenia, of which it has the armament of scattered aculei and the pin-shaped setae concave and resiniferous at tip, but differs in two striking particulars: truly sessile leaves; and extraordinary dilated stipules, the papery sheath of which falls precociously from a lunate scar. The similar bractlike stipules that subtend nascent capitula are imbricate at the tip of inflorescence branchlets into a loose cone of scales much like that seen in some species of Desmodium, but unexpected in Mimosa. It is to this feature that the epithet alludes.

I am much indebted to Dr. J. R. Pirani for the loan and gift of the type material.

Distribution:Brazil South America| Minas Gerais Brazil South America|