Monographs Details: Mimosa pedersenii 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 macrocalyx Micheli, Mimosa axillaris Benth.
Description:Species Description - Slender unarmed microphyllidious shrubs and subshrubs, in open stony places matted or trailing and adventitiously rooting, when crowded taller and sarmentose, according to Hassler attaining 4 m but mostly much smaller, the stems and peduncles pilosulous with spreading-ascending basifixed scaberulous setae to 0.7-1.2 mm, the lfts strongly bicolored, dark brown-olivaceous glabrous and lustrous above, beneath paler dull and puberulent, the subglobose capitula solitary in a long succession of lf-axils both along the primary homotinous stems and on lateral spurs or branchlets. Stipules 2.5-3 x 0.2 mm, 1-nerved, dorsally puberulent, setulose-ciliolate, persistent. Leaf-stalks including pulvinus 1-2 mm, charged ventrally at apex with a slender spicule less than 1 mm (often concealed by pubescence); pinnae of primary cauline lvs 1.5-3.5 cm, the interfoliolar segments 0.5-1.2 mm; lfts of longer pinnae 24-42-jug., subequilong except at very ends of rachis, the blades linear-oblong a little incurved toward apex (hence shallowly sigmoid), deltately acute, the longer ones (2.7-)3-5.5 x 0.6-1 mm, 4.4-5.6 times as long as wide, nerveless or almost so above, beneath l(-2)-nerved from pulvinule, the midrib displaced proximally but subcentric distally, the posterior nerve, when visible, short and weak, the corneous margin ±0.1 mm diam., ciliolate with widely ascending slender setulae ±0.2-0.7 mm. Peduncles 2.5-5 cm; capitula without filaments 4.5-5.5 mm diam.; bracts narrowly elliptic 0.7-1.1 x 0.2-0.4 mm, puberulent dorsally and ciliolate, subpersistent; flowers 4-merous 4-androus, apparently all bisexual; calyx paleaceous-pappiform 1.1-1.8 mm, the brownish tube ±0.5 mm, the lobes usually not deeply decompound; corolla 1.8-2.1 mm, narrowly vase-shaped, the ovate concave, not much thickened lobes 0.5-0.6 x 0.4-0.5 mm, at margin thinly puberulent with minute forwardly appressed hairs; filaments pink, free to base, exserted 3.5-5 mm. Pods numerous in a capituliform cluster, in profile elliptic 1-seeded or undulately oblong-elliptic to 4-seeded, cuneately attenuate at base, including a short terminal cusp 6-10 x 3 mm, the replum 0.3 mm wide, the papery valves densely comose with fine erect-incurved tapering scaberulous setae to ± 1 mm, not seen fully mature but presumably breaking up into free-falling articles.

Distribution and Ecology - In open stony places and about streamside thickets, ±300-500 m, scattered from the lower Paraná-Paraguai watershed in depto. Canendi-yu, Paraguay s. through the Uruguai basin to Misiones, Argentina and w. Rio Grande do Sul, Brazil (lat. 24°30'-29°S) and e. into centr. Paraná (mun. Tibagi).—Fl. IX-II.

Discussion:

The known collections of M. pedersenii have been misidentified either as M. axillaris or M. macrocalyx, between which they are in some respects morphologically intermediate. The species resembles M. (subser. Axillares) axillaris in loosely spreading-ascending scaberulous setae of stems and peduncles, but differs in uniformly very short petioles, in relatively long calyx, and in corollas only residually puberulent on margin of the lobes with minute, forwardly appressed hairs. From M. macrocalyx, of which it has the calyx and almost glabrous corolla, M. pedersenii differs markedly in the character of the cauline and peduncular setae, in somewhat shorter pinnae, and in plumper capitula, without filaments 4.5-5 (not 3.5—4) mm in diam.

I dedicate this mimosa to Troels Myndel Pedersen, distinguished Danish-Argentine student of Amaranthaceae and the flora of subtropical South America, whose many critically chosen and beautifully prepared specimens of Mimosa have contributed much to my knowledge of the genus.

Distribution:Brazil South America| ParanĂ¡ Brazil South America| Rio Grande do Sul Brazil South America| Argentina South America| Misiones Argentina South America| Paraguay South America|