Mimosa hirsutissima

Title

Mimosa hirsutissima

Authors

Rupert C. Barneby

Scientific Name

Mimosa hirsutissima Mart.

Description

415. Mimosa hirsutissima Martius, Flora 21(2, Beibl. 4-5): 55 (=Herb. fl. bras. 135). 1838.- Typus infra sub var. hirsutissima indicatur.

Diffuse, trailing or sometimes pliantly humifuse, often functionally herbaceous subshrubs commonly 5-20 dm, sometimes only 2-4 dm, exceptionally attaining 30 dm, either unarmed or armed at some (but often at only few and random) nodes with a hornlike or less often laterally compressed, infrapetiolar aculeus 1-4 mm rarely accompanied by a pair of infrastipular ones, the simple or widely few-branched stems together with lf-stks and peduncles hirsute with coarse or subcapillary, erect, erect-incurved, or exceptionally forwardly subappressed, pallid or rufescent setae to 1.5-7(-9) mm, the olivaceous, sometimes brunnescent foliage subconcolorous, the plane lfts smooth or almost so above, venulose beneath, finely strigulose or subappressed-pilose on both faces to glabrous above or on both faces, but always ciliate with forwardly subappressed setae not retroconfluent into a continuous margin, the globose or plumply ellipsoid, solitary or rarely geminate capitula in an often long succession of lf-axils on a peduncle much longer than subtending lf-stk but sometimes no longer than the whole lf. Stipules firm, lanceolate or linear-attenuate 4-11 x 0.5-2 mm, dorsally (1-)3-5-nerved and either glabrous, puberulent, or setose, always setose-ciliate, persistent. Leafstalks (3-)5-55(-65) mm, the petioles 3-65 mm, nearly always progressively shorter upward along stem, at middle subcompressed laterally and (0.4-)0.5-l mm diam., the interpinnal segments), if present, (1—)3—11 (—15) mm, the ventral groove then bridged between lower pair(s) but spicules 0; pinnae commonly 1-jug. in lower lvs and 2-3-jug. in upper ones, but in some populations (or individual plants) all or almost all 1-jug., in others both 1- and 2-jug., 3 pairs relatively infrequent; rachis of longer pinnae (2-)2.5-7(-8) cm, the interfoliolar segments (l-)2-5.5(-6.5) mm; lfts of longer pinnae mostly (8—) 10—17(—19), exceptionally 5-11-jug., a little decrescent at each end of rachis, otherwise subequilong, the first pair l-2(-2.5) mm distant from subulate green or reddish, sometimes setiform and obscure paraphyllidia 0.3-0.9(-l .2) mm, the blades obliquely lance-oblong, oblong-elliptic or obliquely ovate from semicordate base, abruptly apiculate-mucronulate, those at and near mid-rachis (5—)6—21 x (1.6-)2-7 mm, 2.4-5 times as long as wide, all (4-)5-6-nerved from pulvinule, the midrib displaced to divide blade 1:2-5, either simple or 1-4-branched beyond mid-blade, one primary nerve on each side produced almost to blade apex (the anterior one sometimes weak submarginal), the 2-3 outer posterior nerves successively shorter, the venation all immersed or almost so above, beneath prominulous and often pallid. Peduncles ascending toward meridian (3-)4-10(-12) cm; capitula without filaments 6- 8(-9) mm diam., prior to anthesis moriform but hispid with emergent bracteal setae; bracts linear or linear-spatulate 1.5—2.6(—3.1) x 0.2-0.45 mm,  glabrous 1-nerved dorsally, toward apex setose-ciliate; flowers 4-merous 4-androus, some lower ones staminate and shorter; calyx pallid paleaceous, commonly 1.5-2.5(-2.7) mm, less often 1-1.7 mm, the lobes deeply fimbriate-decompound or setose-dentate; corolla narrowly vaseshaped or subcylindric (1.7-)2.2-3.5(-3.7) mm, the ovate-cymbiform callous, dorsally carinate lobes 0.7-1.3 x 0.5-0.8 mm, externally greenish and usually glabrous but sometimes minutely hispidulous or papillate, internally often anthocyanic; filaments pink, weakly monadelphous through 0.6-1 mm, exserted 4.5-8 mm; ovary glabrous, commonly 4-ovulate. Pods ±2-12 per capitulum, sessile or almost so, in profile narrowly oblong 9-26 x 4-6.5 mm, 2—4-seeded, abruptly cuspidate apically, the shallowly constricted replum 0.3-0.6(-0.7) mm wide, charged on back and sides with setae to (0.7-) 1.3-3.5 mm, the stiffly papery, green or reddish, finally brunnescent valves either glabrous, or puberulent overall, or both puberulent and setulose-scaberulous (the indumentum sometimes variable between articles of one pod), when ripe breaking up into free-falling biconvex, tardily dehiscent articles 3-5 mm long; seeds (few seen ripe) plumply lentiform 3-3.5 x 2.4-2.8 mm, the smooth testa dull brown.

This easily recognized but somewhat polymorphic species was separated by Bentham into an aculeate M. nervosa referred, despite the existence of unarmed varieties, to the armed ser. Pudicae, and a little group of unarmed but otherwise similar species (Mm. hirsutissima, hirsuta and barbigera) which formed the nucleus of unarmed ser. Hirsutae. The random and erratic development of aculei at different nodes of one stem, on different stems of one plant, and from one to another population of like plants growing under similar ecological conditions, make segregation along these lines impossible to maintain. Members of the unarmed group were supposed by Bentham to differ in pubescence, length of leafstalk, and number of pinnae, but these features are now known to vary independently, and the definition of infraspecific categories has become ever more elusive as comparative material has accumulated. With some misgiving I here propose to recognize two main series of forms, one more or less glabrous, but each variable in length of cauline setae, in development of aculei, and in size of leaflets; and beyond these a pubescent variety endemic to the Tocantins-São Francisco watershed in western Bahia and adjoining Goiás, in which the leaf-formula is somewhat modified. The more pubescent var. hirsutissima is very widely but discontinuously dispersed from the Orinoco valley south to Paraguay and, in Brazil, to S. Paulo, whereas the presumably derived, glabrate var. barbigera is confined to middle and high elevations of the Brazilian Planalto.

As redefined herein, M. hirsutissima differs when armed from M. pudica in the paleaceous calyx. When unarmed and with more than one pair of pinnae per leaf it resembles M. procurrens, but this, like M. pudica, has a minute calyx. When unarmed but with all pinnae conjugate, its paleaceous calyx combined with axillary peduncles is characteristic; whereas when armed and with all pinnae conjugate it is infallibly distinguished by relatively wide, mostly 5-6-nerved leaflets.